Part II · Support and Movement · Estimated reading time 110 minutes · Prerequisites: Chapter 7, Chapter 9
In This Chapter
- Learning Objectives
- 10.1 How Skeletal Muscles Produce Movement
- 10.2 Lever Systems
- 10.3 Muscle Architecture
- 10.4 Reading a Muscle's Name
- 10.5 The Head and Neck
- 10.6 The Trunk
- 10.7 The Upper Limb
- 10.8 The Lower Limb
- 10.9 Advanced Topic · Gait, Posture, and Compartment Syndrome
- Chapter Summary
- Case File 10 · Resolution
- Systems Integration Case File · Entry 10
- Review
- Key Terms
10. The Muscular System II
Major Muscles, Actions, and the Mechanics of Movement
Case File 10 — "Move, But Do Not Strain"
Amara, week 3. She is home, and today is her first session of cardiac rehabilitation. The exercise physiologist takes a history, measures her, and writes a prescription that is oddly specific about what she may not do.
| Amara — rehabilitation intake | Value | Note |
|---|---|---|
| Resting heart rate | 68 beats/min | On metoprolol 25 mg twice daily |
| Resting blood pressure | 124/76 mm Hg | Down from 168/98 at presentation |
| Left ventricular ejection fraction | 48% | Hypokinetic lateral wall; mid-lateral segments akinetic, ~14% of LV |
| Six-minute walk distance | 380 m | Predicted for age and sex ~520 m |
| Respiratory rate at rest | 24 breaths/min | Still elevated; visible neck muscle use |
| Prescribed | Treadmill or cycle, 20–30 min, 5 days/week; RPE 11–13 of 20; 5-minute warm-up and 5-minute cool-down, both mandatory | "Large muscles, rhythmic, moderate" |
| Restricted | No breath-holding. No heavy isometric holds. No loads above about 40% of maximum for now. No overhead pressing |
Toby, week 3 after anterior cruciate ligament reconstruction. He has been in a brace, partially weight-bearing, and is starting quadriceps activation work.
| Toby — post-operative assessment | Left (operated) | Right |
|---|---|---|
| Thigh circumference, 15 cm above patella | 46.5 cm | 52.0 cm |
| Straight-leg raise | 12° extension lag | Full |
| Knee effusion | Moderate | None |
| Quadriceps activation (superimposed burst) | 68% of uninvolved side | — |
| Visible wasting | Marked, greatest over the medial thigh just above the knee | — |
Three questions to hold on to.
- Why is large-muscle rhythmic exercise safe for a heart with a fresh scar, while heavy isometric lifting is not? Both raise cardiac work.
- Why does a physiologist care about the calf muscles when the failing organ is the heart?
- Why did Toby's quadriceps waste 5.5 cm of circumference in three weeks — and why is the wasting worst over the vastus medialis specifically?
Learning Objectives
By the end of this chapter you should be able to:
- Distinguish origin from insertion and predict which attachment moves in a given action.
- Assign the four functional roles — agonist, antagonist, synergist, fixator — to muscles in a described movement, and explain reciprocal inhibition.
- Classify a joint–muscle arrangement as a first-, second-, or third-class lever and compute mechanical advantage from moment arms.
- Explain why almost all human levers are third class, and state precisely what that design trades away and what it buys.
- Compare parallel, fusiform, pennate, convergent, and circular muscle architecture, and use physiological cross-sectional area to predict force versus excursion.
- Decode an unfamiliar muscle name using the seven naming conventions.
- Name the principal muscles of facial expression, mastication, the tongue, swallowing, and the neck, with their cranial nerve supply, and explain accessory muscle use in dyspnoea.
- Describe the erector spinae group, the four layers of the abdominal wall and their fiber directions, the diaphragm and intercostals, and the pelvic floor.
- Explain intra-abdominal pressure and analyse the four phases of the Valsalva manoeuvre hemodynamically.
- Name the shoulder girdle movers, the four rotator cuff muscles and their actions, the arm and forearm compartments, and the intrinsic hand muscles with their innervation.
- Name the gluteal group, quadriceps, hamstrings, adductors, and the four leg compartments with their nerves and actions.
- Explain the skeletal muscle pump quantitatively and relate it to venous return, oedema, and exercise tolerance.
- Describe the phases of the gait cycle and state which muscles act, and in which contraction mode, in each.
- Explain compartment syndrome as an anatomy-driven emergency, including why pulses are preserved until late.
- Predict the functional consequences of injury to the long thoracic, axillary, radial, common fibular, and superior gluteal nerves.
10.1 How Skeletal Muscles Produce Movement
A muscle spans at least one joint and attaches to bone at both ends. When it shortens, it pulls both attachments toward each other — but only one of them usually moves.
- The origin is conventionally the attachment on the more stationary or more proximal bone.
- The insertion is the attachment on the more mobile or more distal bone.
The convention is useful and slightly dishonest, because which end moves depends on what is fixed. The brachialis originating on the humerus and inserting on the ulna flexes the elbow when your arm hangs free — insertion moves. During a pull-up, the ulna is fixed to the bar and the humerus moves toward it — origin moves. This is a reversed action, and it is the reason strength gained in one direction does not automatically transfer to the other.
Muscles never act alone. Any real movement assigns four roles.
| Role | Definition | Example: flexing the elbow with a weight |
|---|---|---|
| Agonist (prime mover) | The muscle chiefly responsible for the action | Brachialis and biceps brachii |
| Antagonist | Opposes the agonist; must relax or lengthen under control | Triceps brachii |
| Synergist | Assists the agonist, adds force, or cancels an unwanted action | Brachioradialis adds force; the wrist extensors stabilize so the finger flexors do not curl the fingers |
| Fixator | Stabilizes the origin so the agonist has a firm base to pull from | Rotator cuff and scapular stabilizers hold the shoulder still |
The synergist role that students under-appreciate is cancelling unwanted actions. Biceps brachii both flexes the elbow and supinates the forearm. If you want flexion without supination, pronator teres must contract simultaneously to cancel the rotation. Two muscles fire, one net movement results, and neither muscle is doing what its name suggests. Almost all movement is like this.
FOUR ROLES IN ONE MOVEMENT — flexing the loaded elbow
══════════════════════════════════════════════════════════════════
FIXATOR ┌── scapula held by trapezius,
stabilizes the origin │ rhomboids, serratus anterior
║ │
╔════╩═════╗
║ SCAPULA ║
╚════╤═════╝
│
┌─────┴──────┐ ANTAGONIST ── TRICEPS
│ HUMERUS │ lengthens under control (eccentric)
│ │ and is ACTIVELY INHIBITED
│ ▓▓▓▓▓ ────┼──► AGONIST ── BRACHIALIS + BICEPS
│ ░░░░░ ────┼──► SYNERGIST ── BRACHIORADIALIS (adds force)
└─────┬──────┘ SYNERGIST ── PRONATOR TERES (cancels the
│ biceps' unwanted supination)
╔═════╧═════╗
║ ULNA ║ ◄── insertion; this bone MOVES
╚═══════════╝
══════════════════════════════════════════════════════════════════
RECIPROCAL INHIBITION — the spinal circuit that makes it automatic
MUSCLE SPINDLE in biceps ──► Ia AFFERENT ──┬──► α-motor neuron
(stretch receptor) │ to BICEPS (+)
│ EXCITE
└──► Ia INHIBITORY
INTERNEURON
│ (−)
▼
α-motor neuron
to TRICEPS
INHIBIT
ONE afferent volley EXCITES the agonist and INHIBITS the
antagonist through a single inhibitory interneuron.
Cost: one extra synapse (~0.5 ms). Benefit: the brain never
has to send a separate "relax" command.
▲ Remove the inhibitory interneuron and you get TETANUS TOXIN
(§9.4): agonist and antagonist contract together, violently.
Figure 10.1 — The four functional roles in one movement, and the reciprocal inhibition circuit that coordinates them.
Described: A schematic of elbow flexion against a load. The scapula is held steady by trapezius, rhomboids, and serratus anterior acting as fixators, giving the arm muscles a stable base. The humerus carries the agonists, brachialis and biceps brachii, which pull the ulna upward; the ulna is the insertion and is the bone that moves. Brachioradialis acts as a synergist adding force, and pronator teres acts as a second kind of synergist, cancelling the unwanted supination that biceps would otherwise produce. Triceps brachii is the antagonist; it lengthens under control and is at the same time actively inhibited. The lower panel shows the reciprocal inhibition circuit: a muscle spindle in the biceps sends a Ia afferent into the spinal cord, where it branches. One branch excites the alpha motor neuron supplying the biceps. The other branch excites a Ia inhibitory interneuron, which inhibits the alpha motor neuron supplying the triceps. A single afferent volley therefore excites the agonist and inhibits the antagonist at a cost of one extra synapse and about half a millisecond, so the brain never has to issue a separate command to relax. A closing note observes that removing the inhibitory interneuron is exactly what tetanus toxin does, producing violent simultaneous contraction of agonist and antagonist.
Reciprocal inhibition is why you do not have to think about relaxing your triceps when you bend your elbow. It is hard-wired at spinal level, and it can be voluntarily overridden — which is exactly what co-contraction is. Deliberately tightening agonist and antagonist together stiffens a joint, and the nervous system does this whenever stability matters more than efficiency: on ice, on an unstable ankle, and — importantly for Toby — around a knee whose ligament has just been reconstructed. Co-contraction is protective and metabolically expensive, and reducing it is one of the measurable adaptations of early strength training (§9.8).
10.2 Lever Systems
A joint is a fulcrum, a bone is a rigid bar, muscle supplies the effort, and gravity or an external object supplies the load. Every movement in the body is therefore a lever problem, and levers obey arithmetic.
Mechanical advantage (MA) = effort arm ÷ load arm, where each arm is the perpendicular distance from the fulcrum to the line of action.
- MA greater than 1 — a power lever. Less muscle force needed than the load, but the load moves less far and more slowly than the muscle shortens.
- MA less than 1 — a speed lever. More muscle force needed than the load, but the load moves farther and faster than the muscle shortens.
═══ FIRST CLASS ═══ E — F — L ═══ "see-saw" ══════════════════════════
Effort and load on OPPOSITE sides of the fulcrum.
MA can be >1 or <1.
EFFORT (neck extensors) LOAD (weight of face)
↓ ↓
══════════════╤═══════▲═══════════════════
F = atlanto-occipital joint
BODY EXAMPLES nodding the head · triceps extending the elbow
(olecranon = effort, elbow = fulcrum, hand = load)
═══ SECOND CLASS ═══ F — L — E ═══ "wheelbarrow" ═════════════════════
LOAD in the middle. MA is ALWAYS >1 → always a POWER lever.
Rare in the body.
LOAD (body weight EFFORT (Achilles pull
through the tibia) on the calcaneus)
↓ ↓
▲══════════════════════════════════════
F = metatarsophalangeal joints (ball of the foot)
NUMBERS MTP → ankle ≈ 12 cm (load arm)
MTP → calcaneal insertion ≈ 17 cm (effort arm)
MA = 17/12 ≈ 1.4 → the calf lifts 1.4× its own force
BODY EXAMPLE rising onto tiptoe
═══ THIRD CLASS ═══ F — E — L ═══ "tweezers" ═════════════════════════
EFFORT in the middle. MA is ALWAYS <1 → always a SPEED lever.
THE OVERWHELMING MAJORITY OF HUMAN MUSCLES.
EFFORT (biceps on radial tuberosity) LOAD (10 kg in hand)
↓ ↓
▲═════════╤════════════════════════════════════════
F = elbow joint
WORKED EXAMPLE — the biceps at the elbow
───────────────────────────────────────────────────────────
Effort arm (elbow → radial tuberosity) ≈ 4 cm
Load arm (elbow → centre of the hand) ≈ 35 cm
MA = 4/35 = 0.114 (a 8.75-fold force DISADVANTAGE)
To hold 10 kg (98 N) in the hand:
Muscle force = 98 N × 35/4 = 858 N ≈ 87 kg of pull
And the ELBOW JOINT must carry 858 + 98 = 956 N of compression.
WHAT YOU BUY:
Biceps shortens 1 cm → the hand travels 8.75 cm
Biceps shortens at 20 cm/s → the hand moves at 175 cm/s
───────────────────────────────────────────────────────────
═══ THE TRADE ═══════════════════════════════════════════════════════
Muscle can shorten only ~30–40% of its length, and its shortening
velocity is limited by myosin ATPase (§9.6). A third-class lever
converts a SHORT, SLOW, STRONG muscle movement into a LONG, FAST,
WEAK limb movement. The cost is paid in muscle force and in JOINT
COMPRESSION — which is why joints wear out and why muscles are so
much stronger than the loads they visibly lift.
Figure 10.2 — The three lever classes with body examples, and the numerical cost of a third-class lever.
Described: Three lever diagrams. A first-class lever places the fulcrum between effort and load, like a see-saw, and its mechanical advantage may be greater or less than one; body examples are nodding the head, where the neck extensors provide effort behind the atlanto-occipital fulcrum and the weight of the face is the load in front, and the triceps extending the elbow, where the olecranon is the effort point, the elbow the fulcrum, and the hand the load. A second-class lever places the load between fulcrum and effort, like a wheelbarrow, and always has a mechanical advantage greater than one, making it a power lever; the body example is rising onto tiptoe, where the fulcrum is the ball of the foot at the metatarsophalangeal joints, body weight passes down the tibia to the ankle about 12 centimeters away, and the Achilles tendon pulls on the calcaneus about 17 centimeters away, giving a mechanical advantage of about 1.4. A third-class lever places the effort between fulcrum and load, like tweezers, always has a mechanical advantage less than one, and describes the overwhelming majority of human muscles. The worked example is the biceps at the elbow: the effort arm from elbow to radial tuberosity is about 4 centimeters and the load arm from elbow to the centre of the hand about 35 centimeters, giving a mechanical advantage of 0.114, an eight-and-three-quarter-fold force disadvantage. Holding 10 kilograms, or 98 newtons, in the hand therefore requires about 858 newtons of muscle force, roughly 87 kilograms of pull, and loads the elbow joint with about 956 newtons of compression. In exchange, one centimeter of biceps shortening moves the hand 8.75 centimeters, and a muscle shortening at 20 centimeters per second moves the hand at 175 centimeters per second. The closing panel states the trade: muscle can shorten only thirty to forty percent of its length and its velocity is capped by myosin ATPase rate, so a third-class lever converts short, slow, strong muscle movement into long, fast, weak limb movement, at a cost paid in muscle force and joint compression.
That last line is worth dwelling on. The human body is built almost entirely out of levers that multiply the force a muscle must produce. This looks like bad engineering until you ask what the alternative would be. To move your hand at 175 cm/s using a power lever, the muscle would have to shorten at 175 cm/s — which no myosin can do — and it would have to be long enough to shorten several tens of centimetres. Your arm would be the size of your leg and it would still be slow. Instead, evolution chose short, thick, strong muscles close to the joint and paid for speed with force.
The bill arrives at the joint. Compression forces at the hip during walking reach roughly 3 times body weight, at the knee during stair descent 3.5 times, and at the patellofemoral joint during a deep squat 7 times — almost none of which comes from the external load. It comes from the muscles. This is the mechanical reason osteoarthritis is a disease of joints rather than of muscles (Chapter 7), and why the patella exists at all: by holding the quadriceps tendon away from the knee's axis, the patella increases the extensor moment arm by 30–50%, reducing the force the quadriceps must generate for the same knee torque.
Predict This
A surgeon repairing a ruptured distal biceps tendon reattaches it 1 cm closer to the elbow joint than its natural site — moving the effort arm from 4 cm to 3 cm.
Before reading on, predict two things: what happens to the force the biceps must generate to hold the same weight, and what happens to the speed at which the hand can move for the same muscle shortening velocity.
(Answers: muscle force required rises by one third — from 858 N to 1,143 N for a 10 kg load — because MA falls from 0.114 to 0.086. Hand speed for a given muscle velocity rises by the same proportion. The patient will be measurably weaker in flexion and marginally faster, and joint compression rises. This is why tendon reattachment position is not a matter of surgical convenience.)
10.3 Muscle Architecture
Two muscles of identical mass can differ threefold in the force they produce and threefold in the distance they can move. The difference is architecture — how the fascicles are arranged relative to the tendon.
The governing quantity is physiological cross-sectional area (PCSA): the total cross-sectional area of all the fibers, measured perpendicular to their own long axis.
PCSA = (muscle volume × cos θ) ÷ fiber length, where θ is the pennation angle.
Force is proportional to PCSA at a specific tension of roughly 22–35 newtons per square centimetre of fiber. Excursion — how far the muscle can move a bone — is proportional to fiber length, because a fiber can shorten only about 30–40% of its own length whatever its architecture. For a fixed volume of muscle, those two quantities trade off directly against each other.
PARALLEL / STRAP FUSIFORM (SPINDLE) CONVERGENT (TRIANGULAR)
║║║║║║║║║ ╲║║║║║║╱ ╲╲╲│╱╱╱
║║║║║║║║║ ══╲║║║╱══ ══╲╲│╱╱══
║║║║║║║║║ ╲║╱ ╲│╱
══════════ ▼
fibers run the belly wider than the broad origin →
full length, tendons; still single insertion.
parallel to pull essentially parallel DIRECTION of pull
LONG fibers, SARTORIUS 40 cm fibers can be VARIED by
SMALL PCSA RECTUS ABDOMINIS selecting fascicles
→ BIG excursion, BICEPS BRACHII PECTORALIS MAJOR
LOW force TEMPORALIS
UNIPENNATE BIPENNATE MULTIPENNATE CIRCULAR
╲╲╲╲╲╲ ╲╲╲│╱╱╱ ╲╲│╱╱ ╲╲│╱╱ ╭───╮
══════════ ═══╪═══ ══╪══╤══╪══ │ ○ │
fibers on ONE fibers BOTH several tendons ╰───╯
side of tendon sides with pennate sphincters
EXT. DIG. LONG. RECTUS FEMORIS fascicles ORBICULARIS
FIB. LONGUS TIB. ANTERIOR DELTOID ORIS / OCULI
SUBSCAPULARIS EXT. ANAL SPH.
═══ WHY PENNATION MULTIPLIES FORCE ═══════════════════════════════════
Angling short fibers onto a tendon lets you PACK MORE FIBERS into
the same volume. Each fiber pulls at angle θ, so only cos θ of its
force reaches the tendon — but you gained far more fibers than you
lost to the cosine. At θ = 20°, cos θ = 0.94: you lose 6% per fiber
and may gain 200–300% more fibers.
PENALTY: fibers are SHORT, so EXCURSION and VELOCITY are small.
═══ REAL NUMBERS ═════════════════════════════════════════════════════
MUSCLE ARCHITECTURE FIBER LEN PENNATION PCSA MAX FORCE
─────────────────────────────────────────────────────────────────────
Sartorius parallel ~40 cm 0° 1.9 cm² ~ 55 N
Biceps brachii fusiform ~13 cm 0° 4.6 cm² ~130 N
Vastus lateralis unipennate ~9.5 cm 5° 35 cm² ~980 N
Gastroc. (med.) bipennate ~5 cm 17° 21 cm² ~590 N
SOLEUS multipennate ~4 cm 25° 50 cm² ~1400 N
─────────────────────────────────────────────────────────────────────
Soleus and sartorius have similar MASS. Soleus makes ~25× the force;
sartorius moves ~10× as far. Neither is "better"; they answer
different questions.
Figure 10.3 — Muscle fascicle architecture, and the force-versus-excursion trade it determines.
Described: Six architectural patterns with their consequences. Parallel or strap muscles have fibers running the full length of the muscle parallel to the line of pull, giving long fibers, small physiological cross-sectional area, large excursion, and low force; examples are sartorius and rectus abdominis. Fusiform muscles are similar but with a belly wider than their tendons; biceps brachii is the example. Convergent muscles have a broad origin narrowing to a single insertion, so the direction of pull can be varied by selecting which fascicles contract; pectoralis major and temporalis are examples. Unipennate muscles have short fibers attaching to one side of a tendon, as in extensor digitorum longus and fibularis longus. Bipennate muscles have fibers on both sides of a central tendon, as in rectus femoris and tibialis anterior. Multipennate muscles have several tendons with pennate fascicles between them, as in deltoid and subscapularis. Circular muscles form sphincters, as in orbicularis oris, orbicularis oculi, and the external anal sphincter. Pennation multiplies force because angling short fibers onto a tendon packs far more fibers into the same volume; each fiber contributes only the cosine of its pennation angle, so at twenty degrees only six percent of each fiber's force is lost while two to three times as many fibers may be gained. The penalty is that short fibers give small excursion and low velocity. A table of real values follows: sartorius is parallel with 40 centimeter fibers, no pennation, 1.9 square centimeters of physiological cross-sectional area, and about 55 newtons of force; biceps brachii is fusiform with 13 centimeter fibers, 4.6 square centimeters, and about 130 newtons; vastus lateralis is unipennate with 9.5 centimeter fibers at five degrees, 35 square centimeters, and about 980 newtons; the medial gastrocnemius is bipennate with 5 centimeter fibers at seventeen degrees, 21 square centimeters, and about 590 newtons; and soleus is multipennate with 4 centimeter fibers at twenty-five degrees, 50 square centimeters, and about 1,400 newtons. Soleus and sartorius have similar mass, yet soleus produces roughly twenty-five times the force while sartorius moves roughly ten times as far.
Architecture is predictive, which makes it a clean instance of Thread 1. Tell me a muscle's fiber length and pennation angle and I can tell you what it is for. Long parallel fibers means a muscle built to move a bone a long way — the sartorius, which must flex, abduct, and laterally rotate the hip through a huge arc. Short steeply pennate fibers means a muscle built to hold a large load through a small range — the soleus, holding your entire body upright over your ankles for sixteen hours a day.
It also predicts injury. Biarticular muscles with long fibers crossing two joints — the hamstrings, rectus femoris, gastrocnemius — can be stretched at one joint while shortening at the other, and they are stretched most while contracting hardest. That combination is eccentric loading at long length (§9.6), which is where muscle tears. Hamstring strains happen in late swing phase of sprinting, when the hip is flexed and the knee is extending: the muscle is being lengthened at both ends while contracting maximally to decelerate the shank.
Histology · The Myotendinous Junction
Where does a muscle actually attach to its tendon? Not by any simple butt-joint: a cell membrane in tension would tear immediately. Instead, at the myotendinous junction the sarcolemma is thrown into deep, finger-like folds that interdigitate with collagen fibrils of the tendon, increasing the contact area by 10 to 20 times.
The geometry does something specific. It converts a tensile load — pulling a membrane straight apart, which membranes tolerate very poorly — into a shear load distributed across a large folded surface, which they tolerate well. The terminal actin filaments of the last sarcomere insert into a dense subsarcolemmal plaque containing vinculin, talin, and α-actinin, which connects through integrins to laminin and type IV collagen in the basal lamina, which connects to the type I collagen of the tendon proper.
Two clinical facts fall directly out of this histology. First, the myotendinous junction is still the weakest point of the muscle–tendon unit despite all the folding, and it is where most strains occur — a "pulled hamstring" is usually a myotendinous junction injury, not a mid-belly tear. Second, the junction is highly adaptive: eccentric training increases the folding and the density of the anchoring proteins, which is part of why eccentric loading protects against strain injury.
Note the parallel with §9.2. Inside the fiber, force leaves through costameres and dystrophin; at the end of the fiber, it leaves through this folded junction. Both are solutions to the same problem — how do you attach something that pulls, to something else, without tearing?
Check Your Understanding 10.3
- Two muscles have identical volume. One has 4 cm fibers at 25° pennation, the other 20 cm parallel fibers. Which produces more force, which moves a bone further, and roughly by what factor?
- Why does the patella increase quadriceps effectiveness without the quadriceps getting any stronger?
- A surgeon must replace a damaged thumb flexor with a transferred muscle. What two architectural properties must the donor match, and why?
Show answers
- For a fixed volume, PCSA is inversely proportional to fiber length, so the 4 cm muscle has about five times the PCSA and produces roughly five times the force, reduced by cos 25° (0.91) to about 4.5 times. The 20 cm parallel muscle shortens about 30–40% of 20 cm, or 6–8 cm, against 1.2–1.6 cm for the short-fibered muscle — roughly five times the excursion. Force and excursion trade off directly, and the product (work) is similar.
- Because it increases the moment arm. The patella holds the quadriceps tendon away from the knee's axis of rotation, raising the effort arm by 30–50%. Torque equals force times moment arm, so the same muscle force generates 30–50% more extension torque. Nothing about the muscle changed; the lever did. This is also why patellectomy leaves patients with a permanent 15–30% extension torque deficit.
- Fiber length and PCSA. Fiber length must match because it determines excursion — a donor with short fibers cannot move the thumb through its required range no matter how strong it is, and a donor with fibers that are too long will be slack through part of the range. PCSA must be adequate because it determines force. Tendon transfer surgery is, in practice, an applied exercise in muscle architecture, which is why the architectural properties of human muscles have been catalogued in such detail.
10.4 Reading a Muscle's Name
There are roughly 640 named skeletal muscles, and nobody memorizes them as a list. They are memorized as descriptions, because almost every name is a compressed statement of what the muscle looks like, where it is, or what it does. Learn the seven conventions and most names decode themselves.
| Convention | Clue words | Example |
|---|---|---|
| 1 · Location | brachii (arm), femoris (thigh), abdominis, oris (mouth), dorsi (back), carpi (wrist), pollicis (thumb), hallucis (big toe) | Tibialis anterior — on the tibia, at the front |
| 2 · Shape | deltoid (triangle), trapezius (trapezoid), rhomboid, serratus (saw-toothed), quadratus (square), teres (round), gracilis (slender), platysma (flat plate) | Rhomboid major — a large rhomboid |
| 3 · Relative size | maximus, medius, minimus, major, minor, longus, brevis, vastus (huge) | Gluteus maximus — the largest of the gluteals |
| 4 · Fiber direction | rectus (straight, parallel to midline), transversus (across), oblique (diagonal), circularis | Rectus abdominis — straight, up the belly |
| 5 · Number of origins | biceps (2), triceps (3), quadriceps (4) | Triceps brachii — three heads, in the arm |
| 6 · Attachment points | named origin-then-insertion | Sternocleidomastoid — from sternum and clavicle to mastoid process |
| 7 · Action | flexor, extensor, abductor, adductor, levator, depressor, supinator, pronator, tensor, erector, constrictor | Extensor digitorum longus — the long extender of the digits |
Most names use two, three, or four conventions at once. Decode them left to right, and the muscle assembles itself.
Worked examples.
- Extensor carpi radialis longus. Action: extensor. Target region: carpi, the wrist. Position: radialis, the radial (thumb) side. Size: longus, the longer of a pair. So: a long muscle on the thumb side of the forearm that extends the wrist — and, because it pulls on the radial side, it also abducts the wrist. Where must it be? On the posterior forearm, since extensors live there. Which nerve? The radial nerve, which supplies the posterior compartment. You have derived four facts from a name.
- Flexor digitorum profundus. Flexes the digits; profundus means deep, so it is deeper than a superficialis that must also exist. Being deeper, it must reach further — and it does: FDP inserts on the distal phalanges, FDS on the middle. If you cannot bend the last joint of your finger, FDP or its tendon is the problem.
- Quadratus lumborum. A square muscle in the lumbar region. Running between the twelfth rib and the iliac crest, its only possible actions are to pull them toward each other — lateral flexion of the trunk — or, acting on a fixed pelvis, to depress the twelfth rib, which it does during forced expiration.
- Levator labii superioris. The lifter of the upper lip. Facial expression, therefore facial nerve.
- Semimembranosus and semitendinosus. Semi means half; one is half-membrane (a broad flat tendon), one is half-tendon (a long cord-like tendon). Both are posterior thigh muscles, and both names describe what they feel like behind the knee — the tendinosus is the prominent cord you can pinch on the medial side.
- Serratus anterior. Saw-toothed, at the front. Its digitations arise from ribs 1–9 laterally and insert on the medial border of the scapula — so it must pull the scapula forward around the chest wall: protraction, and, because of where it grips, upward rotation.
Check Your Understanding 10.4
- Decode flexor hallucis longus completely, and state where in the leg you expect to find it.
- Adductor magnus and gluteus maximus both use a size term. What is the difference in what each term is comparing?
- Predict the action of a muscle called depressor anguli oris, and name its cranial nerve.
Show answers
- Action: flexor. Target: hallucis, the great toe. Size: longus, the longer of a pair, so a brevis also exists. It flexes the great toe, and since flexors of the toes lie in the posterior (plantar-flexing) compartment, it must be in the deep posterior compartment of the leg, with its tendon running behind the medial malleolus and along the sole to the distal phalanx of the great toe. Its nerve is therefore the tibial nerve. It is also, as a consequence of that path, an important supporter of the medial longitudinal arch.
- Maximus compares gluteus maximus with the other gluteals — medius and minimus — so it is ranking members of a named group. Magnus compares adductor magnus with adductor longus and adductor brevis, again ranking within a group, but the group is defined by action rather than by region. In both cases the size word is relative to siblings, never absolute, which is why pectoralis minor is small only compared with pectoralis major.
- It depresses (depressor) the angle (anguli) of the mouth (oris) — the muscle that turns the corners of your mouth down, producing a frown. It is a muscle of facial expression, so it inserts into skin rather than bone and is supplied by the facial nerve, CN VII.
10.5 The Head and Neck
Muscles of facial expression
These are unique in the body: they insert into skin and fascia rather than into bone. Contraction therefore moves skin, not joints, which is exactly what an expression is. All are supplied by the facial nerve (CN VII), and all develop from the second pharyngeal arch — which is why one nerve serves a group of muscles scattered across the whole face.
| Muscle | Action | Everyday name |
|---|---|---|
| Occipitofrontalis (frontal belly) | Raises eyebrows, wrinkles forehead | Surprise |
| Orbicularis oculi | Closes the eye; blinking and forceful closure | The eye's sphincter |
| Orbicularis oris | Closes and protrudes the lips | Kissing, whistling, speech |
| Zygomaticus major and minor | Draws the mouth corner up and laterally | Smiling |
| Buccinator | Compresses the cheek against the teeth | Trumpeter's muscle; keeps food on the grinding surfaces |
| Levator labii superioris | Elevates the upper lip | Sneering |
| Depressor anguli oris | Draws the mouth corner down | Frowning |
| Mentalis | Protrudes the lower lip; wrinkles the chin | Doubt, pouting |
| Platysma | Tenses the neck skin; depresses the mandible | Tension, shaving |
Buccinator deserves a second look because it is the only one with a mechanical rather than an expressive job. Without it, food falls into the space between cheek and teeth on every chew. Facial nerve palsy therefore produces not just a drooping face but food pocketing, drooling, and an inability to close the eye — and the last of those is the dangerous one, since the cornea dries and ulcerates.
Muscles of mastication
Four muscles, all supplied by the mandibular division of the trigeminal nerve (CN V₃), all from the first pharyngeal arch.
| Muscle | Action |
|---|---|
| Masseter | Elevates the mandible; the most powerful muscle in the body per unit weight |
| Temporalis | Elevates and retracts the mandible |
| Medial pterygoid | Elevates and protracts; with its partner, grinds side to side |
| Lateral pterygoid | Protracts and depresses; the only one that opens the jaw |
Molar bite forces of 500–800 newtons are ordinary, and much higher values have been recorded. Note the asymmetry: three muscles close the jaw and one opens it, because closing must overcome food and opening must overcome only gravity. Anatomy is a statement about the job.
The tongue and swallowing
The tongue has intrinsic muscles, entirely within it, which change its shape, and extrinsic muscles, anchored to bone, which change its position. All are supplied by the hypoglossal nerve (CN XII) except palatoglossus (CN X).
- Genioglossus — from the mandible, fanning back into the tongue; protrudes the tongue. It is the muscle that pulls the tongue forward, away from the posterior pharyngeal wall, and it is therefore the principal muscle keeping the upper airway open during sleep. Loss of genioglossus tone is central to obstructive sleep apnoea, which Amara will be diagnosed with in Chapter 22.
- Hyoglossus depresses the tongue; styloglossus retracts and elevates it.
Swallowing recruits two strap groups working against each other. The suprahyoid muscles (digastric, mylohyoid, stylohyoid, geniohyoid) elevate the hyoid and larynx — you can feel your larynx rise when you swallow, and that elevation is what tucks it under the epiglottis and protects the airway. The infrahyoid muscles (sternohyoid, sternothyroid, thyrohyoid, omohyoid) then depress it again. The three pharyngeal constrictors (CN X) then strip the bolus downward in sequence.
The neck, and the accessory muscles of breathing
| Muscle | Attachments | Action | Nerve |
|---|---|---|---|
| Sternocleidomastoid | Sternum + clavicle → mastoid process | Both: flexes the neck (and extends the head on the neck). One side: rotates the head to the opposite side and tilts it to the same side | Accessory (CN XI) + C2–C3 |
| Scalenes (anterior, middle, posterior) | Cervical transverse processes → ribs 1 and 2 | Elevate ribs 1–2; lateral flexion of the neck | Ventral rami C3–C8 |
| Splenius capitis / cervicis | Spinous processes → mastoid, occiput | Extend and rotate head to the same side | Dorsal rami |
| Longus colli / capitis | Anterior vertebral bodies | Flex the neck | Ventral rami |
The sternocleidomastoid's rotation is the one to reason through rather than memorize: it runs from front-and-medial to back-and-lateral, so pulling it swings the face away from that side. Test it by asking a patient to turn their head against your hand and watching the muscle on the opposite side stand out.
Clinical Connection · Accessory Muscle Use — Amara's Respiratory Rate of 24
Quiet breathing is done almost entirely by the diaphragm, with a small contribution from the external intercostals; it costs about 2–3% of resting oxygen consumption. When the work of breathing rises — because the lungs are stiff with fluid, because the airways are narrowed, or because ventilation must increase — the body recruits accessory muscles: sternocleidomastoid and scalenes to elevate the upper ribs and sternum, pectoralis minor and serratus anterior to pull the ribcage up when the arms are braced, and the abdominal wall for forced expiration.
This is why patients in respiratory distress instinctively adopt the tripod position, leaning forward on braced arms: fixing the shoulder girdle reverses the origin and insertion of pectoralis minor and serratus anterior, so that muscles which normally move the scapula on the ribs instead move the ribs on the scapula. The reversed action of §10.1, used clinically.
Accessory muscle use is one of the most useful signs at the bedside because it is visible from the doorway, it appears before oxygen saturation falls, and it quantifies effort rather than outcome. Amara's respiratory rate of 24 with visible scalene and sternocleidomastoid contraction is the muscular readout of a stiff, congested lung — pulmonary venous pressure raised because a ventricle with 14% of its wall akinetic does not empty well, so fluid accumulates in the interstitium, so the lung is harder to inflate (Chapter 22).
There is a cost. Accessory muscle work can raise the oxygen consumption of breathing from 2–3% of total to 15–25%, and those muscles are competing for the same limited cardiac output as everything else. In severe heart failure this becomes its own vicious loop: the harder the respiratory muscles work, the more of the failing heart's output they consume. It is one reason mechanical ventilation helps a failing heart even when oxygenation is adequate.
10.6 The Trunk
Before descending into regions, orient yourself. The next three sections work through the maps below; the exhaustive origin, insertion, action, and innervation lists live in Appendix D · Muscle Reference Tables. The tables in this chapter are deliberately selective — the point here is the reasoning, not the catalogue, and every table below is a sample chosen to make a mechanical argument rather than a complete inventory.
ANTERIOR SUPERFICIAL MUSCLES — schematic, subject facing you
═════════════════════════════════════════════════════════════
╭───────────────╮
FRONTALIS ────┤ ○ ○ ├──── TEMPORALIS
ORBICULARIS │ ▽ │
OCULI/ORIS ───┤ ═════ ├──── MASSETER
╰───┬───────┬───╯
STERNOCLEIDO────────┤ ╲ ╱ ├──────── TRAPEZIUS (upper)
MASTOID │ ╲ ╱ │
╔══════╧═══╪═══╧══════╗
DELTOID ─────╢▓▓▓ │ ▓▓▓ ╟───── DELTOID
║ ▓▓ PECTORALIS ▓▓ ║
BICEPS ──────╢ ▓▓ MAJOR ▓▓ ╟───── SERRATUS
BRACHII ║ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓ ║ ANTERIOR
║ ┌─┬─────────┬─┐ ║ (the "boxer's
BRACHIO──────╢ │ │ RECTUS │ │ ╟─── muscle", seen
RADIALIS ║ │ │ ABDOMIN-│ │ ◄──╫──── as digitations
║ │ ├─────────┤ │ ║ under the arm)
FLEXOR ──────╢ │ │ IS │ │ ║
COMPART- ║ │ ├─────────┤ │◄───╫──── EXTERNAL
MENT ║ │ │ │ │ ║ OBLIQUE
║ └─┴────┬────┴─┘ ║ (fibers run
THENAR ──────╢ ╱ │ ╲ ║ INFERO-
╚════╱════╪════╲══════╝ MEDIALLY)
TENSOR FASCIAE ╱ INGUINAL ╲ ← "hands in
LATAE ─────────┤ LIGAMENT ├──── ADDUCTOR pockets"
│ │ GROUP
SARTORIUS ──────┤ ╲ ╱ ├──── (longus, brevis,
(crosses │ ╲ RECTUS │ magnus, gracilis,
the thigh) │ FEMORIS │ pectineus)
QUADRICEPS ─────┤ VL │ VM ├──── VASTUS MEDIALIS
│ │ │ (bulges just above
╰──────┬──────╯ the knee — the first
TIBIALIS ──────────────┤ muscle to waste)
ANTERIOR ╱ ╲
FIBULARIS ───────────┤ ├──── GASTROCNEMIUS
LONGUS ╰─┬─╯ (medial + lateral heads)
╱ ╲
EXTENSOR ────────────┤ ├──── retinacula
DIGITORUM LONGUS ╰───╯
Figure 10.4 — Anterior superficial musculature, schematic.
Described: A schematic front view of the body's superficial muscles. On the head, frontalis lies over the forehead, orbicularis oculi around the eyes and orbicularis oris around the mouth, with temporalis above the ear and masseter over the angle of the jaw. In the neck, sternocleidomastoid runs obliquely down to the sternum and clavicle, with the upper trapezius behind it. Across the shoulder, deltoid caps the joint on each side; pectoralis major fans across the chest from sternum and clavicle to the humerus. Serratus anterior appears as finger-like digitations on the lateral chest wall beneath the arm. In the arm, biceps brachii lies anteriorly, continuing to brachioradialis and the flexor compartment of the forearm and then the thenar muscles of the thumb. On the abdomen, rectus abdominis runs vertically in paired columns divided by tendinous intersections, flanked by external oblique whose fibers run inferomedially, in the direction the hands take when placed in front pockets. The inguinal ligament marks the lower border of the abdominal wall. In the thigh, tensor fasciae latae lies laterally, the adductor group medially, and sartorius crosses obliquely from the lateral hip to the medial knee; the quadriceps occupies the front, with rectus femoris centrally, vastus lateralis laterally, and vastus medialis bulging just above the knee on the medial side — the first part of the muscle to waste after knee injury. In the leg, tibialis anterior lies just lateral to the tibial shaft, fibularis longus on the lateral side, extensor digitorum longus between them, and the two heads of gastrocnemius bulge posteriorly; retinacula bind the tendons at the ankle.
POSTERIOR SUPERFICIAL MUSCLES — schematic, subject facing away
══════════════════════════════════════════════════════════════
╭───────────────╮
OCCIPITALIS ─────┤ ├──── SPLENIUS CAPITIS
╰───┬───────┬───╯
╔═══════╧═══════╧═══════╗
TRAPEZIUS ────╢ ╲╲╲ upper ╱╱╱ ╟──── LEVATOR SCAPULAE
(whole ║ ╲╲╲ ╱╱╱ ║ (deep to trapezius)
diamond) ║ ╲══ middle ══╱ ║
DELTOID ──────╢ ▓▓ ┌─────────┐ ▓▓ ╟──── RHOMBOIDS
(posterior) ║ ▓▓ │ ◄ SCAP- │ ▓▓ ║ (deep to trapezius;
║ │ ULA ► │ ║ retract scapula)
TRICEPS ──────╢ └─────────┘ ╟──── INFRASPINATUS
BRACHII ║ lower trapezius ║ TERES MINOR/MAJOR
║ ╲╲╲╲╲╲╲╲╲╲╲╲╲╲╲ ║ (rotator cuff below)
EXTENSOR ─────╢ LATISSIMUS DORSI ║
COMPARTMENT ║ ╱╱╱╱╱╱╱╱╱╱╱╱╱╱╱ ║
║ ║ ERECTOR ║ ║ ◄── ERECTOR SPINAE
║ ║ SPINAE ║ ║ runs the whole
╚═══╧═════════╧═════════╝ column in a groove
│ ╱─────╲ │ beside the spinous
GLUTEUS ──────────┤ ╱ GLUTEUS╲ ├──────── processes
MEDIUS │╱ MAXIMUS ╲│
(deep, above) ├────────────┤
│ │
HAMSTRINGS ───────┤ BF │ ST ├──── SEMITENDINOSUS +
(biceps femoris, │ │ SM │ SEMIMEMBRANOSUS
semitendinosus, ╰──────┬─────╯ (medial)
semimembranosus) ╱ ╲
╱ ╲
GASTROCNEMIUS ────────┤ ├──── SOLEUS
(2 heads) │ │ (deep to gastrocnemius,
╰──┬──╯ wider, visible either side)
CALCANEAL (ACHILLES) TENDON
│
calcaneus
Figure 10.5 — Posterior superficial musculature, schematic.
Described: A schematic back view. Occipitalis and splenius capitis lie on the back of the head and upper neck. Trapezius forms a large diamond from the occiput and cervical and thoracic spinous processes to the clavicle, acromion, and scapular spine; its upper fibers elevate and upwardly rotate the scapula, its middle fibers retract it, and its lower fibers depress and upwardly rotate it. Levator scapulae and the rhomboids lie deep to trapezius and retract and downwardly rotate the scapula. The posterior deltoid caps the shoulder, with infraspinatus, teres minor, and teres major below the scapular spine. Triceps brachii runs down the back of the arm to the extensor compartment of the forearm. Latissimus dorsi sweeps from the lower thoracic and lumbar spine, sacrum, and iliac crest upward and laterally to the humerus. The erector spinae runs the whole length of the vertebral column in the groove beside the spinous processes. In the buttock, gluteus maximus forms the bulk, with gluteus medius deep and superior to it. The hamstrings run down the back of the thigh: biceps femoris laterally, semitendinosus and semimembranosus medially. In the calf, the two heads of gastrocnemius overlie the broader soleus, which is visible on either side of them; both converge into the calcaneal, or Achilles, tendon inserting on the calcaneus.
The vertebral column: the erector spinae
The erector spinae is not one muscle but three vertical columns on each side, running from the sacrum and iliac crest to the ribs, transverse processes, and skull. From lateral to medial: iliocostalis, longissimus, spinalis — "I Love Spine."
Deeper still lies the transversospinalis group — semispinalis, multifidus, and rotatores — running obliquely from transverse process to spinous process across one to six segments. These are short, deep, densely spindle-rich muscles that are less about producing movement than about controlling it segment by segment. Multifidus in particular is a postural stabilizer, and it atrophies rapidly and selectively after back injury, often failing to recover spontaneously even when pain resolves — a specific target of rehabilitation.
The whole group works largely eccentrically. When you bend forward, the erector spinae does not lift you; gravity lowers you while the erector spinae pays out under control. At full flexion it goes electrically silent — the flexion–relaxation phenomenon — and the load transfers to the posterior ligaments and thoracolumbar fascia. That instant of silence is when most lifting injuries happen.
The abdominal wall: four layers, four fiber directions
THE ABDOMINAL WALL IN CROSS-SECTION — one side, skin at the top
═══════════════════════════════════════════════════════════════════
skin / fat
─────────────────────────────────────────────────────────────────
1 EXTERNAL OBLIQUE fibers run INFERO-MEDIALLY ╲╲╲╲╲╲╲╲
largest, most superficial "hands in pockets"
─────────────────────────────────────────────────────────────────
2 INTERNAL OBLIQUE fibers run SUPERO-MEDIALLY ╱╱╱╱╱╱╱╱
at RIGHT ANGLES to layer 1
─────────────────────────────────────────────────────────────────
3 TRANSVERSUS ABDOMINIS fibers run HORIZONTALLY ════════
deepest; the PRIMARY generator of intra-abdominal pressure
(activates ~30 ms BEFORE limb movement in healthy people —
a feed-forward postural command, delayed in low-back pain)
─────────────────────────────────────────────────────────────────
4 RECTUS ABDOMINIS fibers run VERTICALLY ║║║║║║║║
paired straps inside the RECTUS SHEATH, crossed by
3–4 TENDINOUS INTERSECTIONS (the "six-pack" divisions)
─────────────────────────────────────────────────────────────────
transversalis fascia · extraperitoneal fat · PERITONEUM
═══ WHY CROSS-PLYING IS STRONG ═══════════════════════════════════
╲╲╲ + ╱╱╱ + ═══ + ║║║ = fibers oriented in FOUR
directions. A load applied along ANY axis finds fibers aligned
to resist it. A single-direction sheet would fail in shear.
This is plywood, and it is the same reason plywood exists.
The three flat muscles' APONEUROSES interweave at the midline
LINEA ALBA, so the two sides are one continuous mechanical unit.
═══ THE INGUINAL CANAL ═══════════════════════════════════════════
A ~4 cm oblique tunnel above the medial half of the inguinal
ligament. DEEP RING (transversalis fascia) → SUPERFICIAL RING
(external oblique aponeurosis). Contents: spermatic cord (male)
or round ligament of the uterus (female), plus the ilioinguinal
nerve. It exists because the TESTIS descended through the wall.
INDIRECT HERNIA through the DEEP RING,
LATERAL to the inferior epigastric vessels.
Congenital (patent processus vaginalis).
DIRECT HERNIA through HESSELBACH'S TRIANGLE,
MEDIAL to the inferior epigastric vessels.
Acquired weakness. Older patients.
Figure 10.6 — The four layers of the abdominal wall, their fiber directions, and the inguinal canal.
Described: A cross-section through one side of the abdominal wall from skin inward. The first and most superficial muscular layer is external oblique, whose fibers run inferomedially, in the direction the hands take when slipped into front pockets. The second layer is internal oblique, whose fibers run superomedially, at right angles to the first. The third and deepest flat layer is transversus abdominis, whose fibers run horizontally; it is the primary generator of intra-abdominal pressure and, in healthy people, activates about thirty milliseconds before a limb movement as a feed-forward postural command, a timing that is delayed in people with low back pain. The fourth layer is rectus abdominis, a pair of vertical straps enclosed in the rectus sheath and crossed by three or four tendinous intersections. Beneath all of these lie transversalis fascia, extraperitoneal fat, and the peritoneum. The cross-plied arrangement gives fibers in four different directions, so a load along any axis meets fibers aligned to resist it, whereas a single-direction sheet would fail in shear; the aponeuroses of the three flat muscles interweave at the midline linea alba so that both sides act as one mechanical unit. The inguinal canal is a four-centimeter oblique tunnel above the medial half of the inguinal ligament, running from the deep ring in transversalis fascia to the superficial ring in the external oblique aponeurosis, transmitting the spermatic cord in males or the round ligament of the uterus in females together with the ilioinguinal nerve; it exists because the testis descended through the abdominal wall. Indirect hernias pass through the deep ring lateral to the inferior epigastric vessels and are congenital; direct hernias push through Hesselbach's triangle medial to those vessels and result from acquired weakness in older patients.
The abdominal muscles produce four actions: flexion of the trunk (rectus abdominis), lateral flexion (obliques and quadratus lumborum on one side), rotation (external oblique of one side with internal oblique of the opposite side, because their fibers are then in line), and — the action that matters most and is taught least — compression of the abdominal contents.
Clinical Connection · Hernia and the Inguinal Canal
A hernia is the protrusion of viscera through a wall that should contain them, and the inguinal region accounts for about 75% of all abdominal wall hernias. The anatomy tells you why.
The inguinal canal is a hole in a pressurized container. Every cough, laugh, lift, and strain raises intra-abdominal pressure against it. The body's defences are mechanical and rather elegant: the canal runs obliquely, so rising pressure presses its walls together rather than forcing them apart, and contraction of the internal oblique and transversus abdominis pulls the arching lower fibers downward over the deep ring — a "shutter mechanism" that closes the door precisely when pressure rises.
Those defences fail in predictable ways. In an indirect hernia, the processus vaginalis — the peritoneal outpouching that preceded the descending testis — failed to obliterate, leaving a ready-made sac at the deep ring; this is why indirect hernias are commoner in young people and in males, and why they follow the path of testicular descent into the scrotum. In a direct hernia, the posterior wall of the canal weakens with age and chronic straining, and bowel pushes straight through Hesselbach's triangle — medial to the inferior epigastric vessels, which is the landmark that distinguishes the two at operation.
The dangerous complication is strangulation: bowel trapped in the neck of the sac first obstructs, then has its venous drainage occluded, then swells, then loses arterial inflow, and infarcts. That sequence — venous before arterial, because veins are thin-walled and low-pressure — is the same logic that governs compartment syndrome in §10.9, and it is a general principle worth carrying: compression kills outflow before inflow.
The thorax: diaphragm and intercostals
The diaphragm is the principal muscle of breathing and one of the strangest muscles in the body: a dome of skeletal muscle with a central tendon, separating thorax from abdomen. Its fibers run from the xiphoid, the lower six costal cartilages, and the upper lumbar vertebrae (as the crura) and converge on that central tendon.
When it contracts, the dome descends, increasing thoracic volume vertically. Descent is 1.5 cm in quiet breathing and up to 7–10 cm in deep breathing, and it accounts for roughly 70% of tidal volume. Three structures pass through it: the inferior vena cava at T8, the oesophagus at T10 (with the vagus nerves), and the aorta at T12 (with the thoracic duct and azygos vein).
Its nerve supply is the phrenic nerve, C3–C5 — "C3, 4, 5 keeps the diaphragm alive" — which seems absurd for a muscle sitting at the bottom of the ribcage until you learn where it came from (see the Development sidebar).
The external intercostals run inferomedially, in the same "hands in pockets" direction as external oblique, and elevate the ribs during inspiration. The internal intercostals run perpendicular to them and depress the ribs during forced expiration. The neurovascular bundle — vein, artery, nerve from superior to inferior — runs in the costal groove on the inferior margin of each rib, which is why a chest drain is inserted just above a rib, never below it.
The pelvic floor
The floor of the abdominopelvic container is a muscular sling, and it is under load whenever the container is pressurized.
- The pelvic diaphragm = levator ani (pubococcygeus, puborectalis, iliococcygeus) + coccygeus. Its job is to support the pelvic viscera against intra-abdominal pressure and to maintain continence.
- Puborectalis forms a sling around the anorectal junction, pulling it forward to maintain an anorectal angle of about 90°. That kink, not the sphincter alone, is what maintains continence of solid stool; defaecation requires puborectalis to relax and the angle to open, which is why squatting posture assists it.
- The external urethral and anal sphincters are skeletal muscle and voluntarily controlled.
- Innervation is the pudendal nerve, S2–S4 — "S2, 3, 4 keeps the pee off the floor."
Clinical Connection · Pelvic Floor Dysfunction
The pelvic floor must do two opposite things: hold tension continuously against gravity and intra-abdominal pressure, and relax completely and voluntarily at socially acceptable moments. Failure of either produces disease.
Too little support. Weakened or denervated levator ani leads to stress urinary incontinence — leakage on cough, laugh, or lift, precisely because those are the moments when intra-abdominal pressure spikes and the floor fails to counter it — and to pelvic organ prolapse. The commonest causes are vaginal delivery (direct stretch injury to levator ani and traction on the pudendal nerve), chronic straining, chronic cough, obesity, and the loss of oestrogen's support of connective tissue at menopause, which is why this returns in Chapter 27. Pelvic floor muscle training is genuinely effective and works exactly as resistance training works anywhere: neural adaptation first, hypertrophy later (§9.8).
Too little relaxation. A hypertonic, non-relaxing pelvic floor produces obstructed defaecation, incomplete bladder emptying, and chronic pelvic pain. Here strengthening exercises make things worse, and treatment aims at down-training — an unusually clear demonstration that "weak" and "dysfunctional" are not synonyms.
There is a direct link to this chapter's Case File. Every Valsalva manoeuvre loads the pelvic floor with the full force of intra-abdominal pressure. Repeated heavy straining is a well-recognized contributor to both prolapse and hernia, which means that the advice given to Amara — do not strain, do not hold your breath — protects three separate structures at once: her heart, her abdominal wall, and her pelvic floor.
Intra-abdominal pressure and the Valsalva manoeuvre
Now assemble the container. Close the glottis. Contract the diaphragm downward, the abdominal wall inward, and the pelvic floor upward. The abdominal contents are fluid and essentially incompressible, so pressure rises — from a resting 0–5 mm Hg to 100–200 mm Hg or more during a maximal effort.
That pressurized cylinder does real mechanical work. It stiffens the trunk, giving the limb muscles a rigid base to pull against, and it transfers part of the compressive load from the lumbar spine to the abdominal contents — which is why weightlifters brace, and why lifting belts work by increasing intra-abdominal pressure rather than by supporting the back directly.
It also does something to the circulation, and that is the heart of Case File 10.
THE VALSALVA MANOEUVRE — forced expiration against a closed glottis
══════════════════════════════════════════════════════════════════════
INTRATHORACIC PRESSURE
+40 ┤ ┌──────────────────────┐
│ │ STRAIN │
0 ┼────────┘ └────────────────────────
│
ARTERIAL PRESSURE (systolic)
200 ┤ ╱╲ ◄── PHASE IV
│ PHASE I ╱╲ ╱ ╲ OVERSHOOT
160 ┤ ╱ ╲ PHASE II ╱ ╲
│ ╱ ╲___ ╱ ╲___
120 ┼───────╱ ╲___ ___╱ ╱ ────
│ ╲______╱ │
80 ┤ ▲ ╲╱ ◄── PHASE III
│ │ transient DROP
└───┬────────┬──────────┬─────────┬────────┬────────────►
0s 2s 5s 10s 12s TIME
↑ strain begins ↑ strain released
HEART RATE ──────↗↗↗ rises through phase II ↗↗↗──────↘↘ falls
(phase IV
bradycardia)
══ WHAT IS HAPPENING ════════════════════════════════════════════════
PHASE I (0–2 s) Raised intrathoracic pressure SQUEEZES the aorta
and thoracic vessels → BP RISES transiently.
Mechanical, not reflex.
PHASE II (2–10 s) Raised pressure COMPRESSES the great veins →
VENOUS RETURN FALLS → preload falls → stroke
volume falls → BP FALLS. Baroreceptors unload →
SYMPATHETIC surge → tachycardia + vasoconstriction
→ BP partially recovers late in phase II.
PHASE III (~1 s) Release. Intrathoracic pressure falls, the aorta
is no longer squeezed → transient BP DROP.
PHASE IV Venous return SURGES into a heart that is now
fast and a circulation that is now CONSTRICTED
→ BP OVERSHOOTS above baseline → baroreflex →
REFLEX BRADYCARDIA.
══ WHY THIS MATTERS TO A DAMAGED HEART ══════════════════════════════
• Phases I and IV are AFTERLOAD SPIKES. Direct measurements during
maximal weightlifting have recorded arterial pressures above
300/200 mm Hg, and in one classic study peaks of 480/350 mm Hg.
• Wall stress σ = (P × r) / 2h (Laplace). Raising P raises the
tension every surviving myocyte must generate — and wall tension
is a PRINCIPAL DETERMINANT of myocardial oxygen demand.
• Phase II drops CORONARY PERFUSION PRESSURE at the exact moment
demand is highest.
• A 3-week-old infarct scar is still remodelling; a sudden pressure
spike risks expansion of the thinned, non-contractile segment.
Figure 10.7 — The four phases of the Valsalva manoeuvre and their hemodynamic consequences.
Described: Three time-aligned traces across about twelve seconds. The top trace shows intrathoracic pressure jumping to about plus forty millimetres of mercury when the strain begins at two seconds and returning to zero when it is released at ten seconds. The middle trace shows systolic arterial pressure passing through four phases. In phase I, during the first one to two seconds of strain, raised intrathoracic pressure mechanically squeezes the aorta and thoracic vessels and arterial pressure rises transiently. In phase II, from about two to ten seconds, the same raised pressure compresses the great veins, venous return falls, preload and stroke volume fall, and arterial pressure falls; baroreceptors unload, producing a sympathetic surge with tachycardia and vasoconstriction so that pressure partially recovers late in the phase. In phase III, lasting about a second after release, intrathoracic pressure falls, the aorta is no longer compressed, and arterial pressure drops transiently. In phase IV, venous return surges into a heart that is now beating fast and a circulation that is now constricted, so arterial pressure overshoots above baseline and the baroreflex produces reflex bradycardia. The bottom trace shows heart rate rising through phase II and falling in phase IV. A closing panel explains why this matters to a damaged heart: phases I and IV are afterload spikes, and direct measurements during maximal weightlifting have recorded arterial pressures above 300 over 200 millimetres of mercury, with one classic study reporting peaks of 480 over 350; by the law of Laplace, wall stress equals pressure times radius divided by twice the wall thickness, so raising pressure raises the tension every surviving myocyte must generate, and wall tension is a principal determinant of myocardial oxygen demand; phase II lowers coronary perfusion pressure at the moment demand is highest; and a three-week-old infarct scar is still remodelling, so a sudden pressure spike risks expansion of the thinned, non-contractile segment.
Exercise & Sport · The Valsalva Manoeuvre — Useful, and Dangerous to the Wrong Person
For a healthy lifter, the Valsalva manoeuvre is not a mistake. Bracing raises intra-abdominal pressure, stiffens the trunk, protects the lumbar spine, and measurably increases the load that can be lifted safely. Almost every heavy lift in every sport is performed with a partial Valsalva, and coaching people out of it entirely tends to produce worse spinal mechanics rather than better.
The physiological price is paid in blood pressure. Intra-arterial recordings during maximal resistance exercise show systolic pressures routinely above 250 mm Hg and, in the classic measurements of double-leg press to failure, transient peaks approaching 480/350 mm Hg. In a young athlete with compliant arteries and a normal heart, these excursions last seconds and are tolerated. The relevant consequences in a healthy population are things like transient post-lift dizziness and, occasionally, exercise-induced syncope during phase II.
Three groups should not be doing this, and all three follow from Figure 10.7 rather than from convention:
- Recent myocardial infarction or unstable coronary disease. The afterload spike raises myocardial oxygen demand through wall tension while phase II lowers coronary perfusion pressure. Supply falls as demand rises, in a heart whose supply is already the limiting factor. This is Amara.
- Uncontrolled hypertension, aortic aneurysm, or recent intracranial or ocular surgery. Pressure transmitted to a weakened vessel or a healing wound is the whole risk.
- Proliferative retinopathy — pressure spikes into fragile new vessels.
The rehabilitation rule follows directly and is not arbitrary: exhale through the exertion, never hold the breath; keep loads moderate (about 30–50% of maximum for 10–15 repetitions, RPE 11–13); avoid sustained isometric holds and overhead pressing. Note the word sustained. Modern cardiac rehabilitation does include resistance training, typically from about week 3–5 after an uncomplicated infarct, because muscle strength predicts function and independence. What is prohibited is not resistance work but the breath-holding and the maximal effort, because those are what produce the pressure excursion.
Check Your Understanding 10.6
- Why does the four-layer arrangement of the abdominal wall resist tearing better than a single thick sheet of the same total mass?
- During phase II of a Valsalva manoeuvre, blood pressure falls even though the person is straining hard. Explain the mechanism, and name the reflex that partially corrects it.
- A patient has a chest drain inserted immediately below the fifth rib and develops bleeding. What was injured and why?
Show answers
- Because the four layers run their fibers in four different directions — inferomedial, superomedial, horizontal, and vertical. Collagen and muscle fibers resist tension along their own axis and resist very little across it, so a single-direction sheet is strong in one direction and fails in shear or in oblique loading. Cross-plying guarantees that a load along any axis finds fibers aligned to take it. This is exactly the principle of plywood, and it is also why the aponeuroses interweave at the linea alba rather than simply meeting there.
- Raised intrathoracic pressure compresses the great veins, which are thin-walled and low-pressure, so venous return falls. Less preload means less end-diastolic volume, so by the Frank–Starling relationship stroke volume falls, and mean arterial pressure falls with it. The baroreflex detects the fall through carotid sinus and aortic arch receptors and responds with sympathetic activation — tachycardia, increased contractility, and vasoconstriction — which partially restores pressure by the end of phase II.
- The intercostal neurovascular bundle, which runs in the costal groove along the inferior margin of each rib in the order vein, artery, nerve from superior to inferior. A drain placed immediately below a rib passes directly through it. Drains and needles are therefore placed just above the rib below the target space, where the bundle is not.
10.7 The Upper Limb
The upper limb is built for reach and manipulation, and every anatomical decision reflects that. The shoulder trades stability for range; the elbow and forearm add rotation; the hand adds precision. The cost is that the shoulder is the most commonly dislocated joint in the body.
The shoulder girdle: moving the platform
Before the arm can move, the scapula must move. The glenohumeral joint contributes about 120° of the 180° of full abduction; the remaining 60° comes from upward rotation of the scapula, in a 2:1 ratio called scapulohumeral rhythm.
| Muscle | Attachments | Action on scapula | Nerve |
|---|---|---|---|
| Trapezius | Occiput, C7–T12 spines → clavicle, acromion, scapular spine | Upper: elevates + upwardly rotates. Middle: retracts. Lower: depresses + upwardly rotates | Accessory (CN XI) |
| Serratus anterior | Ribs 1–9 → medial border of scapula | Protracts and upwardly rotates; holds the scapula against the chest wall | Long thoracic (C5–C7) |
| Rhomboid major/minor | C7–T5 spines → medial border | Retracts + downwardly rotates | Dorsal scapular |
| Levator scapulae | C1–C4 transverse → superior angle | Elevates + downwardly rotates | Dorsal scapular + C3–C4 |
| Pectoralis minor | Ribs 3–5 → coracoid process | Depresses + protracts; elevates ribs if scapula fixed | Medial pectoral |
Injury to the long thoracic nerve paralyses serratus anterior and produces a winged scapula: the medial border lifts away from the chest wall, most obviously when the patient pushes against a wall. Because serratus anterior is also an upward rotator, the patient cannot raise the arm fully overhead. One nerve, one muscle, two deficits, both predictable from the attachments.
The rotator cuff
THE ROTATOR CUFF — SITS — posterior view of the right scapula
═══════════════════════════════════════════════════════════════════
ACROMION CORACO-ACROMIAL ARCH
┌──────────┐ (acromion + coracoacromial
│▒▒▒▒▒▒▒▒▒▒│ ligament + coracoid)
SUPRA- │▒▒▒▒▒▒▒▒▒▒│ ▼▼▼▼▼▼▼
SPINATUS ──►│▓▓▓▓▓▓▓▓▓▓├────── ══════════ ← subacromial
(above the │▓▓▓▓▓▓▓▓▓▓│ space 7–14 mm, holding the
spine) ├══════════┤ SUBACROMIAL BURSA and the
│ │ supraspinatus tendon
SCAPULAR │ SPINE │ │
SPINE ─────►│══════════│ ▼
│▓▓▓▓▓▓▓▓▓▓│ ┌─────────────┐
INFRA- ───►│▓▓▓▓▓▓▓▓▓▓├─────►│ GREATER │
SPINATUS │▓▓▓▓▓▓▓▓▓▓│ │ TUBERCLE │ ← S, I, T
(below) │ │ │ of humerus │ insert here
TERES ───►│▒▒▒▒▒▒▒▒▒▒├─────►└─────────────┘
MINOR └──────────┘ ┌─────────────┐
│ LESSER │ ← SUBSCAPULARIS
SUBSCAPULARIS lies on the │ TUBERCLE │ inserts here
ANTERIOR (rib) surface ──────►└─────────────┘ (the only one)
═══ WHAT EACH DOES ════════════════════════════════════════════════
S SUPRASPINATUS initiates abduction 0–15°; then COMPRESSES the
head into the glenoid [suprascapular n.]
I INFRASPINATUS lateral (external) rotation [suprascapular n.]
T TERES MINOR lateral (external) rotation [axillary n.]
S SUBSCAPULARIS medial (internal) rotation [subscapular nn.]
═══ THE REAL JOB ══════════════════════════════════════════════════
The glenoid fossa holds only about ONE THIRD of the humeral head —
"a golf ball on a tee." DELTOID pulls the head UPWARD (a shear
force). The cuff generates a COMPRESSIVE force that centres the
head in the socket, converting deltoid's shear into rotation.
The cuff is a DYNAMIC LIGAMENT, not primarily a mover.
▲ Tear the cuff and deltoid contraction drives the head UP into
the coracoacromial arch instead of rotating it. The patient
shrugs instead of abducting.
▲ The supraspinatus tendon has a relatively avascular "critical
zone" ~1 cm from its insertion, and it is squeezed between the
head and the arch on every overhead movement. Which is why
~95% of cuff tears begin there.
Figure 10.8 — The rotator cuff: attachments, actions, and why the supraspinatus tendon fails first.
Described: A posterior view of the right scapula with the four rotator cuff muscles. Supraspinatus lies in the fossa above the scapular spine; infraspinatus lies below it; teres minor lies below infraspinatus along the lateral border. All three pass laterally to insert on the greater tubercle of the humerus. Subscapularis lies on the anterior, rib-facing surface of the scapula and is the only cuff muscle inserting on the lesser tubercle. Above the joint, the acromion, coracoacromial ligament, and coracoid process form the coracoacromial arch, beneath which lies the subacromial space of seven to fourteen millimeters containing the subacromial bursa and the supraspinatus tendon. Supraspinatus initiates abduction through the first fifteen degrees and then compresses the humeral head into the glenoid, supplied by the suprascapular nerve; infraspinatus laterally rotates, also suprascapular; teres minor laterally rotates, supplied by the axillary nerve; subscapularis medially rotates, supplied by the upper and lower subscapular nerves. The cuff's real job is stabilization: the glenoid fossa holds only about one third of the humeral head, and deltoid pulls the head upward as a shear force, so the cuff generates a compressive force that centres the head and converts that shear into rotation, acting as a dynamic ligament rather than a prime mover. If the cuff is torn, deltoid contraction drives the head upward into the coracoacromial arch and the patient shrugs instead of abducting. The supraspinatus tendon has a relatively avascular critical zone about one centimeter from its insertion and is compressed between the humeral head and the arch during overhead movement, which is why about ninety-five percent of cuff tears begin there.
Arm, forearm, and hand
| Region | Compartment | Muscles | Principal actions | Nerve |
|---|---|---|---|---|
| Arm | Anterior | Biceps brachii, brachialis, coracobrachialis | Elbow flexion; biceps also supinates | Musculocutaneous (C5–C6) |
| Arm | Posterior | Triceps brachii (3 heads), anconeus | Elbow extension | Radial (C6–C8) |
| Forearm | Anterior (flexor–pronator) | Superficial: pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, flexor carpi ulnaris. Deep: flexor digitorum profundus, flexor pollicis longus, pronator quadratus | Wrist and finger flexion; pronation | Median, except flexor carpi ulnaris and the medial half of flexor digitorum profundus (ulnar) |
| Forearm | Posterior (extensor–supinator) | Extensor carpi radialis longus/brevis, extensor digitorum, extensor carpi ulnaris, supinator, abductor pollicis longus, extensor pollicis longus/brevis | Wrist and finger extension; supination | Radial (deep branch / posterior interosseous) |
| Hand | Thenar | Abductor pollicis brevis, flexor pollicis brevis, opponens pollicis | Thumb opposition — the human hand's defining movement | Median |
| Hand | Hypothenar | Abductor, flexor, opponens digiti minimi | Little finger control | Ulnar |
| Hand | Intrinsic | Lumbricals (4), palmar interossei (PAD — adduct), dorsal interossei (DAB — abduct), adductor pollicis | Flex metacarpophalangeal joints while extending interphalangeal joints; spread and close the fingers | Lumbricals 1–2 median, all the rest ulnar |
The full origin, insertion, action, and innervation lists for every muscle named above are in Appendix D. Three points from this table are worth carrying:
- The superficial flexors share a common origin on the medial epicondyle, and the superficial extensors share one on the lateral epicondyle. Overuse of either group inflames its shared tendon: medial epicondylitis (golfer's elbow) and lateral epicondylitis (tennis elbow).
- Brachialis, not biceps, is the workhorse of elbow flexion. It has a larger PCSA, crosses only the elbow, and works in every forearm position, whereas biceps loses effectiveness in pronation. Biceps is the more powerful supinator — which is why right-handed screw threads tighten clockwise.
- The hand is powered from the forearm and controlled from within. Gross grip comes from long flexors whose bellies are in the forearm — keeping the hand light — while precision comes from the intrinsic muscles. Ulnar nerve injury spares grip strength and destroys dexterity, which is why the claw hand deformity is so functionally devastating despite normal-looking bulk.
Clinical Connection · Rotator Cuff Tear
Cuff tears are common — present in roughly 25% of people over 60 and over 50% over 80, often without symptoms — and their presentation is entirely predictable from Figure 10.8.
Mechanism. Most are degenerative rather than traumatic: repeated compression of the supraspinatus tendon in the subacromial space, superimposed on the tendon's relatively avascular critical zone, in a tissue whose collagen turnover slows with age. Overhead work and overhead sport accelerate it. A minority are acute, from a fall on an outstretched hand or a forced abduction.
Presentation. Pain over the lateral deltoid, worse at night and worse lying on the shoulder. A painful arc between roughly 60° and 120° of abduction, which is exactly the range in which the tendon is compressed under the arch. Weakness of abduction and of external rotation. In a large tear, the shrug sign: without the cuff's compressive force, deltoid contraction drives the humeral head superiorly instead of rotating it, so the patient elevates the whole shoulder girdle rather than abducting the arm. The empty can test — resisted elevation with the arm internally rotated — isolates supraspinatus. The drop arm test, in which a passively abducted arm cannot be lowered under control, is the eccentric version and indicates a large tear.
Why it does not simply heal. Tendon is poorly vascularized, the torn ends retract under the resting tension of the muscle, the defect fills with disorganized scar rather than aligned collagen, and — over months — the muscle undergoes fatty infiltration and atrophy that is largely irreversible even after successful repair. This is why the timing of surgery matters and why the degree of fatty infiltration on preoperative MRI predicts outcome better than the size of the tear.
Why rehabilitation can work without surgery. The cuff is a dynamic stabilizer working alongside the scapular stabilizers. Strengthening the remaining cuff and, crucially, restoring scapular upward rotation by training serratus anterior and lower trapezius increases the subacromial space and reduces impingement. Many degenerative tears become asymptomatic without ever being repaired.
Imaging · Ultrasound of a Muscle Tear, MRI of a Cuff
Ultrasound is the ideal first study for a suspected muscle tear, for reasons that come straight from §1.8: it is real-time, portable, harmless, and — uniquely — dynamic. The sonographer can ask the patient to contract the muscle and watch the torn ends separate, which no static image can show. Normal muscle has a characteristic "starry sky" appearance in cross-section — hypoechoic fascicles speckled with bright perimysial septa — and in longitudinal section a feathery pennate pattern with the echogenic central tendon visible. A tear appears as a hypoechoic or anechoic gap filled with haematoma, with retracted muscle ends and disruption of the pennate lines. Ultrasound also grades severity: grade 1 is oedema without architectural disruption, grade 2 is partial fiber disruption, grade 3 is complete rupture with retraction. Its limitations are equally predictable — it cannot see through bone, and it is highly operator-dependent.
MRI is the standard for the rotator cuff, because the structures of interest are deep, sit under bone, and require fine soft-tissue contrast. Fluid-sensitive sequences show tendon oedema and fluid tracking into the subacromial-subdeltoid bursa; a full-thickness tear appears as a fluid-filled gap traversing the tendon from articular to bursal surface. Two measurements drive management, and both are muscle rather than tendon findings: the degree of fatty infiltration of the supraspinatus and infraspinatus bellies, graded on sagittal images, and the tangent sign, in which the supraspinatus belly has atrophied below a line drawn across the top of the scapular fossa. Both are direct visual measurements of the tissue changes described in §9.8 — disuse atrophy and fatty replacement — and both predict whether repaired tendon will have a functioning muscle to pull on.
Thread 1 · Structure Determines Function
Run the shoulder backwards as a design exercise. Requirement: place the hand anywhere in a large volume of space. Consequence: the joint must have enormous range, which means the socket must be shallow, which means bony stability is impossible. Something else must supply stability, and whatever supplies it must not restrict range — so it must be active rather than passive. That is the rotator cuff: four muscles arranged to pull the head into the socket from four directions at once.
Now read the pathology out of the same design. The stabilizer is a tendon squeezed under a bony arch in the one position the joint is most used. It has a poorly vascularized zone. It cannot be rested, because it works during almost every arm movement. Predict the commonest shoulder disorder in people over fifty, and you predict rotator cuff disease before you have read a word of orthopaedics.
10.8 The Lower Limb
The lower limb is built for the opposite specification: weight bearing, propulsion, and stability. Its joints are deeper, its ligaments stronger, its muscles larger, and its range smaller.
The gluteal group and the hip
| Muscle | Action | Nerve |
|---|---|---|
| Gluteus maximus | Powerful hip extension and lateral rotation. Largely silent in level walking; recruited for stairs, rising from a chair, running, and lifting | Inferior gluteal (L5–S2) |
| Gluteus medius and minimus | Hip abduction and medial rotation. Critically, they stabilize the pelvis in single-leg stance | Superior gluteal (L4–S1) |
| Tensor fasciae latae | Tenses the iliotibial band; assists abduction and flexion | Superior gluteal |
| Iliopsoas (psoas major + iliacus) | The principal hip flexor; also stabilizes the lumbar spine | Ventral rami L1–L3; femoral n. |
| Deep lateral rotators (piriformis, obturator internus/externus, gemelli, quadratus femoris) | Lateral rotation; fine control of the femoral head in the acetabulum | Various sacral plexus branches |
The gluteus medius is the muscle whose failure is most visible. In single-leg stance it must generate enough torque to keep the pelvis level against the entire weight of the body acting at the opposite side. When it is weak or its nerve is damaged, the unsupported side of the pelvis drops — a positive Trendelenburg sign — and the patient compensates by leaning the trunk over the stance leg. Every step of walking is a single-leg stance, so this is not an academic finding.
Thigh, and the biarticular problem
| Group | Muscles | Actions | Nerve |
|---|---|---|---|
| Quadriceps femoris | Rectus femoris (from the AIIS — also flexes the hip), vastus lateralis, vastus medialis, vastus intermedius | Knee extension | Femoral (L2–L4) |
| Hamstrings | Biceps femoris (long head), semitendinosus, semimembranosus | Hip extension + knee flexion | Sciatic — tibial division (L5–S2) |
| Biceps femoris short head | Knee flexion only (does not cross the hip) | Sciatic — common fibular division | |
| Adductors | Adductor longus, brevis, magnus, gracilis, pectineus | Hip adduction; adductor magnus also extends | Obturator (L2–L4); magnus's hamstring part by the tibial n. |
All four quadriceps converge into the quadriceps tendon, which envelops the patella and continues as the patellar ligament to the tibial tuberosity. The vastus medialis has a distinct distal portion — the vastus medialis obliquus (VMO) — whose fibers run at 50–55° to the femoral shaft and insert on the medial border of the patella. Its role is to oppose the lateral pull of vastus lateralis and hold the patella in its groove, and it is the most distal and most visible part of the quadriceps mass just above the knee.
Hold on to that. It is the answer to Case File question 3.
Exercise & Sport · Muscle Imbalance and Injury
Muscles work in pairs and chains, and injury is often better predicted by ratios than by absolute strength.
Hamstring-to-quadriceps ratio. A conventional ratio below about 0.6 (concentric hamstring peak torque divided by concentric quadriceps peak torque) is associated with hamstring strain and with ACL injury. The mechanism is mechanical: the hamstrings pull the tibia posteriorly and therefore protect the anterior cruciate ligament against the anterior drawer force that the quadriceps generates. A quadriceps-dominant athlete loads the ACL harder on every landing and cutting movement — which is directly relevant to how Toby tore his in the first place, and to why his rehabilitation programme trains hamstrings as hard as it trains quadriceps.
Eccentric hamstring strength is the more useful measurement still, because hamstring strains occur during eccentric action in terminal swing (§10.3). Programmes built around the Nordic hamstring curl reduce hamstring injury rates by roughly half in sprinting sports, and the mechanism is the one from §9.6: eccentric training adds sarcomeres in series, shifting the length–tension curve so that the muscle's optimum sits at a longer length, so it is no longer operating on the steep descending limb at the moment of peak load.
Scapular and hip stabilizers. Weak lower trapezius and serratus anterior produce insufficient scapular upward rotation and narrow the subacromial space, contributing to impingement (§10.7). Weak gluteus medius allows the femur to adduct and internally rotate on landing, increasing the dynamic knee valgus that loads the ACL and the patellofemoral joint. In both cases the painful structure is distal to the weak muscle, which is why treating the site of pain so often fails.
The general principle. A joint is stabilized by the balance of forces across it, not by any single muscle. Training a prime mover while ignoring its antagonists and stabilizers reliably produces a stronger, less stable joint — which is a fair one-sentence description of a great deal of avoidable sports injury.
The leg: four compartments
The leg below the knee is divided by tough fascial septa into four compartments. Compartment membership predicts nerve, blood supply, action, and — as §10.9 shows — pathology.
| Compartment | Muscles | Action | Nerve | Artery |
|---|---|---|---|---|
| Anterior | Tibialis anterior, extensor digitorum longus, extensor hallucis longus, fibularis tertius | Dorsiflexion, toe extension, inversion | Deep fibular | Anterior tibial |
| Lateral | Fibularis longus, fibularis brevis | Eversion, assists plantarflexion | Superficial fibular | Branches of fibular |
| Posterior, superficial | Gastrocnemius (2 heads, crosses the knee), soleus, plantaris | Plantarflexion | Tibial | Posterior tibial |
| Posterior, deep | Tibialis posterior, flexor digitorum longus, flexor hallucis longus, popliteus | Inversion, toe flexion; popliteus unlocks the knee | Tibial | Posterior tibial, fibular |
Compartment membership is the organizing fact; for the individual attachments of each muscle listed here, see Appendix D.
The intrinsic foot muscles lie in four plantar layers, with extensor digitorum brevis on the dorsum. Their job is not to move the toes much — it is to stiffen the foot into a rigid lever at push-off, to support the arches alongside the plantar aponeurosis, and to supply an enormous volume of proprioceptive information. They are supplied by the medial and lateral plantar nerves from the tibial nerve.
Clinical Connection · Foot Drop and the Common Fibular Nerve
The common fibular nerve winds around the neck of the fibula immediately beneath the skin, with essentially no soft tissue padding it against the bone. It is the most commonly injured nerve in the lower limb, and it is injured by exactly the things that anatomical position predicts: a plaster cast or brace pressing on the fibular neck, prolonged leg crossing, squatting or kneeling for hours, a fibular neck fracture, prolonged immobility on an operating table, and substantial weight loss removing what little padding existed.
The nerve divides into a deep branch (anterior compartment — dorsiflexors) and a superficial branch (lateral compartment — evertors). A lesion at the fibular neck takes both.
The consequences follow directly:
- Foot drop. With the dorsiflexors paralysed, the foot hangs in plantarflexion. Two gait abnormalities result, and both are visible: foot slap at initial contact, because tibialis anterior can no longer eccentrically lower the forefoot after heel strike, and a steppage gait, in which the patient exaggerates hip and knee flexion to lift the toes clear during swing.
- Loss of eversion, from the lateral compartment, which destabilizes the ankle and predisposes to inversion sprain.
- Sensory loss over the dorsum of the foot and the lateral leg. Inversion is preserved, since tibialis posterior is in the posterior compartment on the tibial nerve.
Note what a plantar-flexed foot means functionally. Toe clearance during swing is only about 1 to 1.5 centimeters in normal walking; that margin is why a few degrees of lost dorsiflexion causes tripping. Treatment is an ankle-foot orthosis that mechanically substitutes for the missing eccentric control, and removal of the compressive cause — and prognosis is good if the axons are merely compressed rather than severed.
The skeletal muscle pump
Predict This
Stand perfectly still and the column of blood between your right atrium and your ankle is about 120 cm tall, so the pressure in an ankle vein is roughly 90 mm Hg — higher than the pressure in many arteries elsewhere in the body.
Before reading on, predict what happens to that pressure after ten ordinary walking steps, and name the two structures that make your answer possible.
(Answer: it falls to about 20–30 mm Hg, and it does so within roughly ten steps. The two structures are the calf muscles, which compress the deep veins running inside their fascial compartments, and the venous valves, which rectify that compression into one-way proximal flow. Without the valves, squeezing the vein would simply move blood in both directions and accomplish nothing.)
THE CALF MUSCLE PUMP — the "peripheral heart"
══════════════════════════════════════════════════════════════════
STANDING STILL WALKING
─────────────────── ──────────────────────────────
Hydrostatic column from the Each calf contraction squeezes
right atrium to the ankle the deep veins; VALVES force
≈ 120 cm of blood blood PROXIMALLY only.
↓ ↓
ANKLE VENOUS PRESSURE AMBULATORY VENOUS PRESSURE
≈ 90 mm Hg falls to 20–30 mm Hg
within about 10 steps
▲ vein CONTRACTION RELAXATION
│ ┌──────────┐ ┌──────────┐
═══╪═══ valve OPEN │ ▲▲▲▲▲ │ valve │ ▲ │ valve
│ │ ═╪═ OPEN │ open │ ══╪══ │ CLOSED
┌──┴──┐ │ │ │ above │ │ │ above
│ │ ← deep vein │ MUSCLE │ │ MUSCLE │
│ │ inside the │ SQUEEZES │ │ RELAXES │
│ │ compartment │ ══╪══ │ valve │ ═╪═ │ valve
└──┬──┘ │ │ │ CLOSED │ │ OPEN │ open
│ │ ▼ │ below │ ▲ │ below
═══╪═══ valve └──────────┘ └──────────┘
│ (prevents blood ejected vein refills
▼ backflow) UPWARD from below
and from
superficial veins
via perforators
═══ NUMBERS THAT MATTER ═════════════════════════════════════════
Calf venous volume ~100–150 mL
Ejection fraction per contraction ~60–70%
Venous return needed at rest ~5 L/min
Venous return needed in hard exercise ~25 L/min
Fall in ankle venous pressure on walking 90 → 20–30 mm Hg
═══ WHEN IT FAILS ═══════════════════════════════════════════════
Valve incompetence → pressure does NOT fall on walking →
AMBULATORY VENOUS HYPERTENSION → capillary filtration exceeds
reabsorption → oedema → skin changes → VENOUS ULCER
(classically above the medial malleolus).
Immobility → stasis → one arm of VIRCHOW'S TRIAD → DVT.
Standing still at attention → no pump → venous pooling →
falling preload → falling cardiac output → SYNCOPE.
Figure 10.9 — The skeletal muscle pump: how contraction and valves convert a static column of blood into flow.
Described: A comparison of standing still with walking. Standing still, the hydrostatic column of blood from the right atrium to the ankle is about 120 centimeters tall, so ankle venous pressure is about 90 millimetres of mercury. Walking, each calf contraction squeezes the deep veins that lie within the muscle compartments, and one-way bicuspid valves force blood proximally only, so ambulatory venous pressure falls to 20 to 30 millimetres of mercury within about ten steps. Two panels show the cycle. During contraction, the muscle squeezes the vein, the valve above opens and the valve below closes, and blood is ejected upward. During relaxation, the muscle releases, the valve above closes and the valve below opens, and the vein refills from below and from the superficial veins through perforating veins. Key numbers follow: the calf holds roughly 100 to 150 millilitres of venous blood; each contraction ejects about 60 to 70 percent of it; venous return must be about 5 litres per minute at rest and about 25 litres per minute in hard exercise; and walking drops ankle venous pressure from about 90 to 20 or 30 millimetres of mercury. When the pump fails, the consequences are specific. Valve incompetence means the pressure does not fall on walking, producing ambulatory venous hypertension, so capillary filtration exceeds reabsorption, giving oedema, skin changes, and eventually a venous ulcer classically above the medial malleolus. Immobility produces stasis, one arm of Virchow's triad, and deep vein thrombosis. Standing still at attention removes the pump entirely, causing venous pooling, falling preload, falling cardiac output, and syncope.
Exercise & Sport · The Muscle Pump and Venous Return
Cardiac output cannot exceed venous return. That sentence is the whole of this sidebar, and it is the reason a cardiac rehabilitation physiologist cares about calf muscles.
At rest, lying down, venous return is comfortable — the pressure gradient from peripheral veins (about 15 mm Hg) to the right atrium (about 0–5 mm Hg) is small but sufficient, and the hydrostatic problem does not exist. Stand up, and roughly 500–700 mL of blood shifts into the veins of the legs and splanchnic circulation within seconds. Preload falls, stroke volume falls, and the baroreflex must compensate. Now exercise upright, and venous return must rise five-fold to 25 L/min while gravity is still working against it.
Four mechanisms supply that, and the muscle pump is the largest.
- The skeletal muscle pump. Rhythmic contraction of the calf and thigh muscles compresses deep veins, with valves making the flow one-way. Each calf contraction ejects 60–70% of 100–150 mL. This is genuinely a second pump in series with the heart, and it does work the heart would otherwise have to do.
- The respiratory pump. Inspiration lowers intrathoracic pressure and raises intra-abdominal pressure, so blood is drawn from abdomen to thorax on every breath. Deeper, faster breathing during exercise amplifies it — and note that this is exactly the mechanism a Valsalva manoeuvre reverses (§10.6): sustained positive intrathoracic pressure turns the respiratory pump off and impedes venous return instead.
- Sympathetic venoconstriction, which shifts blood out of the venous reservoir (which holds about 65% of total blood volume) into the effective circulation.
- Reduced venous compliance and increased pressure gradient from the vasodilated muscle beds.
Two practical consequences for Amara. First, rhythmic large-muscle exercise recruits the muscle pump automatically, so preload rises smoothly as demand rises — the peripheral pump and the central pump scale together. Second, and this is why her cool-down is not optional: stopping suddenly removes the muscle pump while the arterioles of the working muscle are still dilated. Blood pools, venous return collapses, preload falls, and cardiac output falls with it. In a healthy person that produces post-exercise dizziness. In a patient with a fixed akinetic segment and limited contractile reserve, it can produce symptomatic hypotension and, in a heart with a scar border zone, arrhythmia. The five-minute cool-down is a prescription for the calf muscles.
Development · Myotomes and Why the Diaphragm Is Supplied From the Neck
In the fourth week of development, paraxial mesoderm segments into somites on either side of the neural tube. Each somite's myotome gives rise to skeletal muscle, and each is invaded by the nerve of its own spinal level — and that nerve keeps its muscle for life, no matter where the muscle subsequently travels.
Every myotome splits into two:
- Epaxial (dorsal) — supplied by the dorsal ramus. Becomes the deep intrinsic back muscles only: erector spinae, transversospinalis.
- Hypaxial (ventral) — supplied by the ventral ramus. Becomes everything else: the body wall, the limbs, the diaphragm.
This explains three things that otherwise look arbitrary.
Why the diaphragm is C3–C5. It develops largely from the septum transversum, which forms at cervical level and then descends with the growing thorax to reach the level of the lower ribs. Its nerve, the phrenic, is dragged down with it — which is why a muscle at the bottom of the thorax answers to the neck. It also explains referred pain: irritation of the diaphragm, from blood, infection, or gallbladder disease, is felt at the tip of the shoulder, because the brain interprets a C3–C5 signal as coming from the C3–C5 dermatome, which is over the shoulder.
Why limb muscles have plexuses. Limb buds form opposite specific segments and are invaded by several ventral rami at once; as the bud grows and rotates, the fibers intermingle to form the brachial and lumbosacral plexuses. This is why a single peripheral nerve carries fibers from several spinal levels, and why a root lesion and a nerve lesion produce different patterns — the distinction that dominates Chapter 13.
Why myotome testing works. Because the segmental assignment is preserved, a single movement can test a single root: C5 shoulder abduction, C6 elbow flexion and wrist extension, C7 elbow extension, C8 finger flexion, T1 finger abduction, L2–L3 hip flexion, L4 knee extension and ankle dorsiflexion, L5 great toe extension, S1 plantarflexion and ankle eversion. Six movements localize a lesion in the lower limb to a single vertebral level — which is a remarkable amount of clinical power derived from a four-week-old embryo.
Check Your Understanding 10.8
- A patient's pelvis drops on the left when he stands on his right leg. Which muscle is weak, on which side, and which nerve would you suspect?
- Why does a soldier standing rigidly at attention faint, while a soldier marching does not?
- Gastrocnemius crosses two joints and soleus crosses one. Predict how each behaves when the knee is bent, and what test this makes possible.
Show answers
- The right gluteus medius (and minimus) — the muscles of the stance leg, which must hold the pelvis level against body weight acting on the opposite side. This is a positive Trendelenburg sign, and the nerve is the superior gluteal (L4–S1). Note the common error: the weakness is on the side the patient is standing on, not the side that drops.
- Standing still removes the skeletal muscle pump. Blood pools in the leg veins under a 90 mm Hg hydrostatic column, venous return falls, preload falls, stroke volume falls, and cerebral perfusion eventually fails — often with a vasovagal reflex superimposed. Marching contracts the calf and thigh rhythmically, and one-way valves convert that into upward flow, sustaining venous return. The traditional advice to keep flexing the calves while standing at attention is physiologically exact.
- Gastrocnemius originates above the knee on the femoral condyles, so bending the knee shortens it and drops it onto the ascending limb of its length–tension curve, greatly reducing its contribution. Soleus originates below the knee and is unaffected. This makes the Silfverskiöld test possible: if ankle dorsiflexion is limited with the knee straight but improves with the knee bent, the tight structure is gastrocnemius; if it is limited in both positions, it is soleus or the joint itself. Two muscles, one tendon, distinguished by knee position alone.
10.9 Advanced Topic · Gait, Posture, and Compartment Syndrome
The gait cycle
Walking is the most-performed voluntary movement in human life — roughly 5,000 to 10,000 cycles a day — and it is, mechanically, a controlled fall repeated indefinitely. One gait cycle runs from initial contact of one foot to the next initial contact of the same foot.
ONE GAIT CYCLE — right limb — 0% to 100%
═══════════════════════════════════════════════════════════════════════
STANCE PHASE (60%) SWING PHASE (40%)
┌──────────────────────────────────┬──────────────────────────────┐
│ IC │ LOADING │ MID- │ TERMINAL │ PRE- │ INITIAL│ MID- │TERMINAL│
│ 0% │ RESPONSE│ STANCE│ STANCE │SWING │ SWING │SWING │ SWING │
│ │ 0–12% │12–31% │ 31–50% │50–62%│ 62–75% │75–87%│87–100% │
│heel │ foot │ body │ heel off │ toe │ clear │ │ decel- │
│strike flat │ passes│ │ off │ ground │ │ erate │
└─────┴─────────┴───────┴──────────┴──────┴────────┴──────┴────────┘
▲DOUBLE SUPPORT (≈10%) ▲DOUBLE SUPPORT (≈10%)
both feet down both feet down
▲ In RUNNING, double support disappears and is replaced by a
FLIGHT phase with neither foot down.
MUSCLE ACTIVATION (█ = active; E = eccentric, C = concentric)
═══════════════════════════════════════════════════════════════════════
TIBIALIS ANTERIOR ██E██│ │ │ │███C████████████
lowers the holds the foot up
forefoot (no for TOE CLEARANCE
slap) (only 1–1.5 cm!)
─────────────────────────────────────────────────────────────────────
QUADRICEPS ██E███│ │ │ │ │██E███
absorbs the stabilise
15° knee flexion for contact
─────────────────────────────────────────────────────────────────────
GLUT. MAXIMUS ████│ │ │ │ │███████
+ HAMSTRINGS: extend the hip, decelerate the
control trunk swinging shank (E)
─────────────────────────────────────────────────────────────────────
GLUTEUS MEDIUS ███████████████████████████│ │ │
ACTIVE THROUGHOUT SINGLE SUPPORT — keeps the
opposite side of the pelvis from dropping
─────────────────────────────────────────────────────────────────────
SOLEUS / GASTROC. │ ██E████████│███C█████│ │ │
controls the PUSH-OFF: the single
forward fall largest power burst in
of the tibia the cycle, ~2.5–4 W/kg
─────────────────────────────────────────────────────────────────────
ILIOPSOAS │ │███C███│ │
initiates swing
═══════════════════════════════════════════════════════════════════════
TYPICAL VALUES cadence ~110 steps/min · step length ~0.7 m ·
stride ~1.4 m · speed ~1.3 m/s · stance:swing 60:40
NOTE how much of this is ECCENTRIC. Walking is mostly controlled
lowering, punctuated by one concentric burst at push-off.
Figure 10.10 — The gait cycle: phases, timings, and which muscles act in which contraction mode.
Described: One complete gait cycle of the right limb from zero to one hundred percent, divided into a stance phase occupying the first sixty percent and a swing phase occupying the last forty. Stance subdivides into initial contact at zero percent, loading response from zero to twelve percent, midstance from twelve to thirty-one percent, terminal stance from thirty-one to fifty percent, and pre-swing from fifty to sixty-two percent. Swing subdivides into initial swing from sixty-two to seventy-five percent, midswing from seventy-five to eighty-seven, and terminal swing from eighty-seven to one hundred. Two periods of double support, each about ten percent of the cycle, occur at the beginning and end of stance; in running these disappear and are replaced by a flight phase with neither foot on the ground. Muscle activation is shown as bars. Tibialis anterior works eccentrically after heel strike to lower the forefoot without a slap, and concentrically through swing to hold the foot up for toe clearance, a margin of only one to one and a half centimeters. The quadriceps works eccentrically during loading response to absorb about fifteen degrees of knee flexion, and again in terminal swing to stabilize for contact. Gluteus maximus with the hamstrings extends the hip and controls the trunk in early stance, and the hamstrings work eccentrically in terminal swing to decelerate the swinging shank. Gluteus medius is active throughout single support, preventing the opposite side of the pelvis from dropping. Soleus and gastrocnemius work eccentrically through midstance to control the forward fall of the tibia and then concentrically in terminal stance for push-off, the single largest power burst of the cycle at about two and a half to four watts per kilogram. Iliopsoas fires concentrically to initiate swing. Typical values are a cadence of about 110 steps per minute, a step length of 0.7 metres, a stride of 1.4 metres, and a walking speed of about 1.3 metres per second. The figure notes how much of gait is eccentric: walking is mostly controlled lowering, punctuated by one concentric burst at push-off.
Two things about that figure are worth stating explicitly.
Gait is cheap because it is mostly passive. The inverted-pendulum mechanics of stance exchange potential and kinetic energy, and elastic storage in the Achilles tendon and plantar aponeurosis returns a substantial fraction of the energy of each step. Muscles mostly control the exchange eccentrically rather than driving it. Net metabolic cost of walking at a comfortable speed is only about 2 to 4 mL of oxygen per kilogram per minute above rest — which is why walking is the ideal medium for cardiac rehabilitation: it is rhythmic, it uses the largest muscles in the body, it recruits the muscle pump, and its intensity is finely and continuously adjustable.
Gluteus maximus is nearly silent in level walking. The largest muscle in the body barely participates in the movement people assume it exists for. It is recruited for stairs, for rising from a chair, for running, and for lifting — which is why "can you get out of a chair without using your arms?" is a better functional test than "can you walk?"
Posture and the antigravity muscles
Standing still is not passive, but it is remarkably close to it. The body's line of gravity falls just anterior to the ankle, just posterior to the hip and knee, and near the vertebral bodies. That geometry means:
- The knee is stabilized largely by ligaments; the quadriceps is nearly silent in quiet standing.
- The hip is held by the iliofemoral ligament; gluteus maximus is nearly silent.
- The ankle cannot be passively stabilized, because the line of gravity is in front of it and the body would topple forward. Soleus is therefore tonically active — the single most important antigravity muscle in the body, and one of the most type-I-rich muscles you have.
- The erector spinae and the deep neck extensors fire intermittently to keep the trunk and head balanced.
Standing is thus a low-cost, continuously corrected sway around an equilibrium, monitored by vision, the vestibular apparatus, and — most importantly for balance — proprioceptors in the foot, ankle, and neck. That control loop is a negative feedback system in the strict sense of §1.5, and it is the loop that degrades in aging.
Aging · Sarcopenia's Functional Consequences, Falls, and Grip Strength
Chapter 9 described sarcopenia cellularly: 3–8% mass loss per decade after 30, strength falling faster than mass, power falling fastest, preferential type II atrophy, and motor unit remodelling with fiber-type grouping. This chapter is where those cells become a life.
The functional thresholds are specific. Rising from a chair without using the arms requires roughly 40–50% of maximum knee extensor torque relative to body weight. Climbing a standard stair requires similar. Crossing a road at the pedestrian-signal speed requires about 1.2 m/s of gait speed, and gait speed below 0.8 m/s is one of the standard diagnostic criteria for sarcopenia. Because power declines faster than strength, and because recovering a stumble requires generating force in under 200 milliseconds, an older adult can lose the ability to catch themselves long before they lose the ability to walk.
Falls. Roughly one in three adults over 65 falls each year. The chain from muscle to fracture runs through this chapter: type II atrophy reduces the rate of force development, so the protective step is too slow; gluteus medius weakness reduces lateral pelvic control, so lateral balance fails first; reduced ankle dorsiflexor strength reduces toe clearance, and the margin was only 1–1.5 cm to begin with; and reduced proprioception degrades the feedback loop. Then the osteoporosis of Chapter 6 converts a fall into a hip fracture. Amara's mother Adwoa, 78, sits at exactly this intersection, and it is why her assessment includes a gait speed and a chair-rise test rather than only a DEXA scan.
Grip strength as a mortality predictor. This finding is initially bizarre and, on reflection, not. In large prospective cohorts, each 5 kg lower grip strength is associated with roughly a 16% higher risk of all-cause mortality, and grip predicts cardiovascular death and disability better than systolic blood pressure does. Grip is not causal. It is a cheap, reproducible integrated readout of total muscle mass, neural drive, nutritional status, inflammatory burden, and general physiological reserve — the same reasoning that makes skin the readout of circulatory status in §1.3. One dynamometer measures the whole organism.
And it is treatable. Progressive resistance training produces hypertrophy and large strength gains into the tenth decade; power training with moderate loads moved quickly improves function more than heavy slow training; and combining it with adequate protein overcomes much of the anabolic resistance of older muscle. Sarcopenia is the rare geriatric syndrome with a genuinely effective intervention that costs almost nothing.
Compartment syndrome
The fascia that organizes muscles into compartments — the same septa that made the table in §10.8 so predictive — is tough, and it does not stretch acutely. That single material property creates one of the true surgical emergencies.
The mechanism. Bleeding, oedema, or swelling inside a closed fascial compartment raises the pressure within it. Normal intracompartmental pressure is 0–8 mm Hg. As it rises, the first vessels to collapse are the ones with the lowest internal pressure: veins. Venous outflow obstruction raises capillary hydrostatic pressure, which drives more fluid out of the capillaries into the compartment, which raises the pressure further. That is positive feedback with no built-in brake (§1.5), and it is why the condition escalates over hours.
Why the pulse is preserved. This is the fact that costs limbs. Capillary perfusion fails when compartment pressure approaches capillary pressure — roughly 20–30 mm Hg. A major artery, by contrast, carries a mean pressure of 90 mm Hg or more and stays open until compartment pressure approaches that. Tissue can therefore be dying while a distal pulse remains perfectly palpable. Waiting for a pulse to disappear is waiting for the muscle to be dead.
The diagnostic criterion is therefore a pressure, not a pulse: ΔP = diastolic blood pressure − compartment pressure, with a value below 30 mm Hg taken as the threshold for fasciotomy — surgically opening the fascia to decompress the compartment.
The clinical signs are the classic "P"s, but their order matters:
| Sign | When it appears | Note |
|---|---|---|
| Pain out of proportion to the injury | Earliest | The most reliable early sign |
| Pain on passive stretch of the muscles in that compartment | Early | Stretch pulls on ischaemic muscle; dorsiflexing the toes to test the deep posterior compartment |
| Paraesthesia in the nerve traversing the compartment | Early-intermediate | Nerve is more sensitive to ischaemia than muscle |
| Pressure — a tense, wood-hard compartment | Intermediate | |
| Paresis / paralysis | Late | Muscle already injured |
| Pulselessness / pallor | Very late | The limb is usually already lost |
The clock. Nerve dysfunction begins at 2–4 hours of ischaemia; irreversible muscle necrosis begins at 4–6 hours; by 8–12 hours the damage is largely fixed. The anterior compartment of the leg is the commonest site, both for acute compartment syndrome after tibial fracture and for chronic exertional compartment syndrome in runners, in whom exercise-induced muscle swelling of up to 20% of volume transiently exceeds what the fascia allows.
And the loop closes back to Chapter 9. Necrotic muscle releases myoglobin, potassium, phosphate, and creatine kinase — rhabdomyolysis (§9.7). Hyperkalaemia threatens the heart; myoglobin threatens the kidney. A missed compartment syndrome kills through the kidney and the heart, not through the leg.
Thread 2 · Homeostasis Is the Master Concept
Compartment syndrome, the Valsalva manoeuvre, and the muscle pump are the same physics with different signs.
In each, a pressure inside a container determines whether blood can flow. Raise pressure in a fascial compartment and you stop capillary perfusion while the arterial pulse persists. Raise pressure in the thorax and you stop venous return while arterial pressure briefly rises. Raise and lower pressure rhythmically in the calf and you create flow, because valves rectify the oscillation into one-way movement.
The regulated variable behind all three is tissue perfusion, and the lesson is that perfusion depends on a pressure gradient, not on a pressure. This is why a normal blood pressure does not guarantee a perfused muscle, why a palpable pulse does not exclude ischaemia, and why the treatment for a pressurized compartment is not to raise the blood pressure but to open the container.
Check Your Understanding 10.9
- Why is "pain on passive stretch" an earlier and more useful sign of compartment syndrome than "pulselessness"?
- In which phases of the gait cycle is tibialis anterior active, and in which contraction mode in each? What happens in each phase if it is paralysed?
- Quiet standing costs remarkably little energy. Which muscle is the exception, and why is it the exception?
Show answers
- Because they mark opposite ends of the process. Passive stretch pulls on muscle that is already ischaemic and mechanically distressed, so it hurts as soon as capillary perfusion fails, which happens when compartment pressure reaches only 20–30 mm Hg. A major artery stays open until compartment pressure approaches mean arterial pressure of 90 mm Hg or more, so pulselessness appears only after several hours of ischaemia, by which time the muscle is necrotic. Using pulselessness as your trigger means operating on a dead limb.
- Tibialis anterior is active twice. In loading response (0–12%) it works eccentrically, paying out to lower the forefoot to the ground after heel strike; paralysis here produces the audible foot slap. Throughout swing (62–100%) it works concentrically to hold the ankle dorsiflexed for toe clearance; paralysis here means the toes catch, and the patient compensates with a steppage gait, exaggerating hip and knee flexion to lift the foot over the ground.
- Soleus. The body's line of gravity falls just anterior to the ankle joint, so gravity continuously tends to topple the body forward over the feet. The knee and hip are stabilized passively by ligaments because the line of gravity falls behind them, but there is no passive structure behind the ankle to resist forward rotation. Soleus must therefore be tonically active — which is why it is one of the most type-I-rich, fatigue-resistant, multipennate muscles in the body, built for exactly this job.
Chapter Summary
§10.1 Muscles attach at an origin and an insertion, and either end may move — the reversed action. Every movement assigns four roles: agonist, antagonist, synergist (which adds force or cancels an unwanted action), and fixator. Reciprocal inhibition, a single spinal interneuron, inhibits the antagonist automatically whenever the agonist is excited; voluntary override of it is co-contraction, which buys stability at the cost of efficiency.
§10.2 Every joint is a lever, and mechanical advantage is effort arm over load arm. First-class levers put the fulcrum in the middle, second-class put the load in the middle and always favour force, third-class put the effort in the middle and always favour speed and distance. Almost all human muscles are third-class: the biceps at the elbow has a mechanical advantage of about 0.11, so holding 10 kg requires 858 N of muscle force and loads the elbow with 956 N — the price paid for moving the hand 8.75 times faster than the muscle shortens.
§10.3 Architecture determines force and excursion. Force is proportional to physiological cross-sectional area, excursion to fiber length, and for a fixed muscle volume the two trade off directly. Pennation packs more, shorter fibers onto a tendon, buying large force at the cost of small excursion — soleus makes about 25 times the force of the similarly sized sartorius, which moves about 10 times as far.
§10.4 Muscle names encode location, shape, size, fiber direction, number of heads, attachments, and action, usually several at once. Decoding a name yields the muscle's position, action, compartment, and often its nerve.
§10.5 The facial muscles insert into skin and are supplied by CN VII; the four muscles of mastication by CN V₃; the tongue by CN XII. Suprahyoid and infrahyoid muscles raise and lower the larynx for swallowing. Sternocleidomastoid and the scalenes rotate and flex the neck and serve as accessory muscles of respiration — the visible sign in Amara's respiratory rate of 24, and a cost that can rise from 3% to 25% of total oxygen consumption.
§10.6 The erector spinae works mostly eccentrically and falls silent at full flexion. The abdominal wall's four layers run their fibers in four directions — a cross-plied plywood that resists load along any axis — and its deepest layer, transversus abdominis, is the primary generator of intra-abdominal pressure. The diaphragm (C3–C5) supplies 70% of tidal volume; the pelvic floor (S2–S4) supports the container from below. The Valsalva manoeuvre has four hemodynamic phases: a mechanical pressure rise, a fall as venous return is obstructed with a compensating sympathetic surge, a transient fall on release, and an overshoot with reflex bradycardia.
§10.7 The scapula must rotate for the arm to reach overhead (scapulohumeral rhythm, 2:1). The rotator cuff — supraspinatus, infraspinatus, teres minor, subscapularis — is a dynamic ligament that compresses the humeral head into a shallow socket, converting deltoid's shear into rotation. The forearm divides into anterior flexor-pronator (median, mostly) and posterior extensor-supinator (radial) compartments; hand power comes from the forearm and precision from the intrinsics (mostly ulnar).
§10.8 Gluteus medius stabilizes the pelvis in single-leg stance; its failure is the Trendelenburg sign. The quadriceps extends the knee through the patella, which raises the extensor moment arm 30–50%; the hamstrings are biarticular and are injured eccentrically at long length. The leg's four compartments predict nerve, action, and pathology. The skeletal muscle pump drops ankle venous pressure from 90 to 20–30 mm Hg within ten steps and is a genuine second pump in series with the heart.
§10.9 Gait is 60% stance and 40% swing, is mostly eccentric control punctuated by one concentric push-off, and is metabolically cheap. Soleus is the principal antigravity muscle because the line of gravity falls anterior to the ankle. Compartment syndrome is pressure in a closed fascial space exceeding capillary pressure while arterial pulses persist; the criterion is ΔP under 30 mm Hg, the clock is 4–6 hours, and the systemic danger is rhabdomyolysis.
The Three Threads in Chapter 10
Structure → Function. Architecture is predictive in both directions. Long parallel fibers mean a muscle built for excursion; short pennate fibers mean one built for force. A shallow socket means the stabilizer must be muscular; a muscular stabilizer squeezed under a bony arch means degenerative tendon disease by age 60. Fiber direction in the abdominal wall means resistance to shear. You can now read a muscle's job from its shape and its failure mode from its geometry.
Homeostasis. Pressure in a container governed three separate stories: the Valsalva manoeuvre obstructing venous return, the muscle pump generating it, and compartment syndrome abolishing capillary perfusion while pulses persist. In all three the regulated variable is tissue perfusion, and perfusion depends on a gradient, never on a pressure alone.
Integration. The exercise prescription written for Amara is a cardiovascular decision made entirely in muscular terms. It specifies which muscles (large, rhythmic, distal — for the muscle pump), how they contract (alternating, never sustained — so their own vessels are not occluded), how she breathes (never against a closed glottis — so intrathoracic pressure never obstructs venous return or spikes afterload), and how she stops (gradually — so the pump is not removed while the arterioles are still open). Four muscular specifications, one cardiac purpose.
Case File 10 · Resolution
Question 1 — Why is large-muscle rhythmic exercise safe for a damaged heart when heavy isometric lifting is not? Both raise cardiac work.
They do — but they raise different cardiac work, and the heart pays very differently for the two.
Rhythmic large-muscle exercise imposes a volume load. Walking or cycling alternately contracts and relaxes very large muscle masses. Three consequences follow, and all three are favourable:
- Working muscle dilates its own arterioles. Local metabolites — adenosine, potassium, carbon dioxide, hydrogen ions — produce functional hyperaemia, and because the contraction is intermittent, blood actually flows during the relaxation phases. Total peripheral resistance falls, by as much as three- to fourfold at high intensity.
- Blood pressure therefore rises only modestly. Cardiac output may rise from 5 to 12 or 15 L/min while systolic pressure climbs gradually from about 120 to 160–180 mm Hg, and diastolic pressure stays flat or falls slightly. That last point is decisive: the left coronary artery fills during diastole, so diastolic pressure is coronary perfusion pressure. Supply rises with demand.
- The muscle pump assists filling (see question 2), so preload rises smoothly and stroke volume is supported without a large rise in filling pressure.
The clinical index of demand is the rate–pressure product (heart rate × systolic pressure). For a given patient the ischaemic threshold is reasonably reproducible — often around 20,000 to 25,000. Rhythmic exercise raises the rate–pressure product gradually and predictably, which means it can be titrated, monitored, and stopped. Amara's prescription — RPE 11–13 of 20, 20–30 minutes, with mandatory warm-up and cool-down — is a way of keeping her below her threshold while still delivering a training stimulus.
Heavy isometric work imposes a pressure load, and every element of that is unfavourable.
- The muscle occludes its own blood supply. Sustained contraction above roughly 20% of maximum voluntary force raises intramuscular pressure above capillary perfusion pressure. The muscle cannot vasodilate its way out, so total peripheral resistance does not fall.
- The exercise pressor reflex fires hard. Ischaemic muscle accumulates metabolites, which stimulate group III and IV afferents; together with central command this drives a powerful sympathetic pressor response — a rise in mean arterial pressure of 25–30 mm Hg from a three-minute handgrip at only 30% of maximum, with little change in cardiac output. The rise is almost entirely pressure, not flow.
- Valsalva multiplies it. Heavy lifting is almost always performed with breath-holding (§10.6), so phases I and IV superimpose sharp arterial pressure spikes; intra-arterial recordings during maximal resistance exercise reach above 300/200 mm Hg.
Now apply the law of Laplace: wall stress σ = P·r / 2h. Raising arterial pressure raises the tension every surviving myocyte must develop, and wall tension is one of the three principal determinants of myocardial oxygen demand (with heart rate and contractility). So demand rises steeply — while, during phase II, aortic pressure and therefore coronary perfusion pressure fall. Supply and demand move in opposite directions simultaneously, in a heart whose supply is the limiting factor.
There is a second, purely mechanical concern at three weeks. Her infarct is in the granulation-to-collagen transition: the necrotic muscle has been cleared and replaced by tissue whose tensile strength is still well below that of mature scar. A sudden pressure spike applied to a thinned, akinetic, non-contractile segment risks infarct expansion — stretching and thinning of that segment, which worsens ventricular geometry permanently.
Note carefully what is not being said. Resistance training is not forbidden after a myocardial infarction; contemporary cardiac rehabilitation introduces it at around week 3–5, because muscle strength predicts independence and because sarcopenia is its own risk. What is forbidden is the combination that produces the pressure excursion: maximal effort plus breath-holding plus sustained isometric holding. Hence the prescription's precise wording — exhale through the effort, moderate loads, no sustained holds.
Question 2 — Why does a physiologist care about the calf muscles when the failing organ is the heart?
Because cardiac output cannot exceed venous return, and in an upright human the calf is what generates venous return.
Standing still, the hydrostatic column from right atrium to ankle is about 120 cm of blood, so ankle venous pressure is about 90 mm Hg. Blood pools: 500–700 mL shifts into the legs and splanchnic veins within seconds of standing. Ten steps of walking drop ambulatory venous pressure to 20–30 mm Hg, because each calf contraction squeezes the deep veins inside their fascial compartments and one-way valves rectify that squeeze into upward flow. The calf holds 100–150 mL and ejects 60–70% of it per contraction. It is, quite literally, a second pump arranged in series with the heart, and it does work the heart would otherwise have to do.
For Amara, three consequences follow:
- It supplies the preload her exercise requires. Venous return must rise roughly threefold during her session. The muscle pump, the respiratory pump, and sympathetic venoconstriction supply it. A heart with reduced contractile reserve depends more on adequate filling, not less, because a larger share of its stroke volume must come from the Frank–Starling relationship rather than from increased contractility.
- It lowers venous and capillary pressure in the legs, reducing filtration and therefore peripheral oedema — a symptom she is at risk of as her ventricle stiffens.
- Removing it suddenly is dangerous, which is why the cool-down is mandatory. Stop walking abruptly and the pump switches off while the muscle arterioles are still dilated. Blood pools, venous return collapses, preload falls, and cardiac output falls with it. In a healthy person this is post-exercise dizziness. In a patient with an akinetic segment, limited reserve, and a scar border zone capable of supporting re-entrant arrhythmia, it is a genuine hazard. The five minutes of gradually decreasing walking at the end of her session exist for the calf muscles, not for the heart.
There is also a nuance worth stating, because it is where students over-generalize. More preload is not always better. A stiff ventricle — which is what a scarred one becomes — sits on the steep part of its diastolic pressure–volume curve, so a rapid preload surge raises left atrial and pulmonary capillary pressure sharply and produces breathlessness. This is precisely why her programme is rhythmic and graded rather than intense and intermittent: it lets preload rise smoothly rather than in surges.
Question 3 — Why did Toby's quadriceps waste 5.5 cm in three weeks, and why worst over the vastus medialis?
Three mechanisms, stacked.
1 · Disuse atrophy is fast, and faster than most people expect. Muscle protein synthesis falls measurably within 6 hours of immobilization and drops 30–50% within days, while degradation via the ubiquitin–proteasome system (atrogin-1, MuRF-1) rises. Cross-sectional area falls by roughly 0.5% per day over the first two weeks. Crucially, strength falls faster than size — up to 20–25% in two weeks — because neural drive is lost alongside tissue. And the loss is not even across fiber types: type II fibers atrophy preferentially, so power falls more than either strength or size (§9.8). A 5.5 cm circumference difference at three weeks is entirely consistent with the published rates.
2 · Arthrogenic muscle inhibition. This is the mechanism that distinguishes a post-surgical knee from a limb that has simply been rested, and it is the reason Toby's quadriceps activation measures 68% of the uninvolved side. Abnormal afferent input from an injured, swollen joint reflexively inhibits the quadriceps motor neuron pool at spinal level. It is not weakness from disuse and it is not a lack of effort — the motor neurons cannot be fully recruited.
The evidence is unusually clean: infusing saline into a normal knee to simulate an effusion reduces quadriceps activation in direct proportion to the volume infused, with measurable inhibition from as little as 20–30 mL. Toby has a moderate effusion. His 12° extension lag on straight-leg raise is the visible consequence: he can flex his hip, but he cannot generate the terminal extension torque needed to keep the knee straight while doing it, because the last few degrees of extension demand the most quadriceps force of the whole range and his available drive is capped.
3 · Why the vastus medialis specifically. Two reasons, and both are mechanistic rather than anecdotal.
- The VMO is preferentially inhibited by joint effusion. Experimental effusion studies find a dose–response gradient: volumes around 20–30 mL are enough to inhibit vastus medialis, while substantially larger volumes are needed before vastus lateralis and rectus femoris are affected to the same degree. Whatever the exact mechanism, the vastus medialis motor neuron pool is the most susceptible.
- The VMO's working range is exactly the range Toby is not using. Its oblique fibers, running at 50–55° to the femoral shaft, are most active in the final 15° of knee extension — which is the range restricted by a post-operative brace, avoided because of the extension lag, and never reached in the partial-weight-bearing, flexed-knee gait he has adopted. A muscle region that is not loaded in its working range atrophies fastest in that region.
Add to those a third, purely observational factor: the VMO is the most distal and most superficial part of the quadriceps mass, so its loss is the most visible, and it sits at the level where thigh circumference is conventionally measured.
Why this matters, and what follows from it. The whole picture is a rehabilitation problem defined by §9.8's size principle. If Toby can only produce 20–30% of his maximum force, then by the size principle he is recruiting only type I and the lowest-threshold type IIa units — and the type II fibers, which are the ones atrophying fastest, are never being reached at all. Voluntary exercise alone cannot solve this.
The interventions used clinically all attack that specific problem. Reducing the effusion — compression, elevation, cryotherapy, aspiration if needed — directly lifts the afferent inhibition. Neuromuscular electrical stimulation bypasses the inhibited motor neuron pool entirely and recruits axons directly, and because electrical recruitment is governed by axon diameter rather than by motor neuron size, it reaches type II fibers at low force levels (§9.8) — the one intervention that violates the size principle usefully. Quadriceps setting and straight-leg raises target terminal extension where the VMO works. And blood flow restriction training allows a hypertrophic stimulus at 20–30% of maximum load by producing local metabolic stress, which is the workaround for a patient who cannot tolerate heavy loading.
Note the symmetry with Amara's problem. Both patients need to load muscle. Both have a constraint that forbids the obvious method — her heart cannot tolerate the pressure, his knee cannot tolerate the load or generate the drive. In both cases the solution comes from knowing exactly which physiological variable is the limit, and finding a route around that specific variable rather than around exercise in general.
Systems Integration Case File · Entry 10
Entry 10 — The prescription as physiology
Amara's file now contains an exercise prescription with four explicit restrictions. Toby's contains a 5.5 cm circumference deficit and a 68% activation deficit. Add the muscular system — this time as an organ system in action, not as a tissue.
Your entry:
1 · ADD. In two or three sentences, state what skeletal muscle contributes to Amara's circulation that her heart cannot supply for itself. Use numbers.
2 · CONNECT. Link the muscular system to at least two systems already in your file, stating the direction of causation each time. Then explain, in one sentence each, why the four restrictions on her prescription exist — no breath-holding, no heavy isometrics, moderate loads, mandatory cool-down.
3 · PREDICT. Amara's exercise tolerance will be reassessed at 12 weeks. Predict whether her six-minute walk distance will improve mainly because her heart changed or because her muscle changed, and justify it.
Model responses — read only after writing your own
1 · ADD. Skeletal muscle is a second pump in series with the heart. Standing still, ankle venous pressure is about 90 mm Hg from a 120 cm hydrostatic column; ten steps of walking drop it to 20–30 mm Hg, because each calf contraction ejects 60–70% of the 100–150 mL held in the deep veins and one-way valves rectify that into upward flow. Since cardiac output cannot exceed venous return, and since venous return must rise from about 5 to 12–15 L/min during her session, the muscle pump is supplying preload that a ventricle with an akinetic 14% segment could not generate for itself.
2 · CONNECT. Muscular → cardiovascular: the muscle pump raises venous return, which raises preload, which raises stroke volume through the Frank–Starling relationship; and working muscle dilates its own arterioles, lowering total peripheral resistance so that cardiac output can rise without a large pressure rise. Cardiovascular → muscular: her reduced cardiac output limits oxygen delivery to working muscle, which is a principal reason her six-minute walk distance is 380 m against a predicted 520 m. Muscular → respiratory: her accessory muscles (scalenes, sternocleidomastoid) are recruited at a respiratory rate of 24, raising the oxygen cost of breathing from 2–3% toward 15–25% of total consumption — muscle competing with muscle for the same limited output. Muscular → endocrine/metabolic: skeletal muscle takes up about 80% of a glucose load, so training it improves the insulin resistance that drove her coronary disease.
The four restrictions: No breath-holding, because raised intrathoracic pressure obstructs venous return in phase II and spikes arterial pressure in phases I and IV, moving supply and demand in opposite directions. No heavy isometrics, because sustained contraction above ~20% of maximum occludes the muscle's own vessels, so resistance does not fall and the response is pure pressure. Moderate loads, because the rate–pressure product must stay below her ischaemic threshold and moderate loads keep the rise gradual and titratable. Mandatory cool-down, because stopping abruptly removes the muscle pump while the arterioles are still dilated, collapsing venous return and preload.
3 · PREDICT. Mainly because her muscle changed. Her akinetic segment is scar and cannot recover contractile function (§9.9), so ejection fraction will improve only modestly, from remodelling and from recovery of any stunned but viable myocardium. The large, reliable adaptations of a 12-week endurance programme are peripheral: increased mitochondrial volume (40–50%), increased capillary density (20–30%), greater oxygen extraction, and improved economy — plus reduced sympathetic tone, a lower heart rate at any given workload, and therefore a lower rate–pressure product for the same walking speed. This is one of the most useful and least intuitive facts in cardiac rehabilitation: most of the benefit is delivered by the muscles, not the heart, and it is available to a patient whose ejection fraction never improves at all.
Review
Level 1 · Recall
9.1 In the movement of elbow flexion, the triceps brachii acts as the:
a) agonist b) antagonist c) synergist d) fixator
Answer
b — antagonist. It opposes the movement and lengthens under control while being actively inhibited by the reciprocal inhibition circuit. It becomes the agonist for the opposite movement, elbow extension — roles are assigned by movement, not by muscle.
9.2 Almost all levers in the human body are third class, which means they:
a) favour force over speed b) favour speed and range over force c) have the fulcrum in the middle d) have a mechanical advantage greater than 1
Answer
b. A third-class lever has the effort between the fulcrum and the load, so the effort arm is always shorter than the load arm and mechanical advantage is always less than 1. The muscle must produce more force than the load, and in exchange the load moves further and faster than the muscle shortens. Option c describes a first-class lever; option a and d describe second-class.
9.3 Which muscle initiates abduction of the arm through the first 15 degrees?
a) deltoid b) supraspinatus c) infraspinatus d) serratus anterior
Answer
b — supraspinatus. Deltoid's line of pull at 0° is almost parallel to the humeral shaft, so it produces mainly upward shear rather than rotation; supraspinatus initiates the movement and then continues as a compressive stabilizer. Infraspinatus externally rotates. Serratus anterior upwardly rotates the scapula, which matters above 90°.
9.4 The four layers of the abdominal wall run their fibers in different directions primarily to:
a) allow rotation of the trunk b) resist load applied along any axis c) create the linea alba d) permit the inguinal canal to form
Answer
b. Fibers resist tension along their own axis and very little across it, so a single-direction sheet fails in shear or oblique loading. Cross-plying in four directions guarantees that any load finds fibers aligned to resist it — the plywood principle. Rotation (a) is a genuine consequence of the oblique arrangement but not the primary structural reason.
9.5 Injury to the common fibular nerve at the neck of the fibula produces:
a) loss of plantarflexion and inversion b) foot drop with loss of eversion c) inability to extend the knee d) a positive Trendelenburg sign
Answer
b. The common fibular nerve supplies the anterior compartment (dorsiflexors, via the deep branch) and the lateral compartment (evertors, via the superficial branch). Plantarflexion and inversion are preserved because they are tibial nerve territory. Knee extension is femoral nerve; Trendelenburg is superior gluteal.
9.6 During phase II of a Valsalva manoeuvre, arterial blood pressure falls because:
a) the heart slows b) raised intrathoracic pressure impedes venous return c) peripheral resistance falls d) the aorta is compressed
Answer
b. Sustained positive intrathoracic pressure compresses the thin-walled, low-pressure great veins, so venous return and therefore preload fall; stroke volume falls by the Frank–Starling relationship. Aortic compression (d) is what raises pressure in phase I. Heart rate actually rises during phase II as the baroreflex compensates, and peripheral resistance rises rather than falls.
9.7 Which muscle is tonically active during quiet standing?
a) quadriceps femoris b) gluteus maximus c) soleus d) rectus abdominis
Answer
c — soleus. The body's line of gravity falls just anterior to the ankle, so gravity continuously tends to rotate the body forward over the feet and only muscle can resist it. At the knee and hip the line of gravity falls in positions where ligaments provide passive stability, so quadriceps and gluteus maximus are nearly silent in quiet standing.
9.8 The earliest reliable clinical sign of acute compartment syndrome is:
a) absent distal pulse b) pallor c) pain on passive stretch d) paralysis
Answer
c — pain on passive stretch, together with pain out of proportion to the injury. Capillary perfusion fails at compartment pressures of only 20–30 mm Hg, whereas a major artery stays open until pressure approaches mean arterial pressure, so pulses and colour are preserved until the muscle is already necrotic. Paralysis is also late. Waiting for a, b, or d means waiting too long.
Level 2 · Comprehension
9.9 Explain, with numbers, why the biceps must generate roughly 850 N to hold 10 kg in the hand, and state what the body gains in exchange.
Model answer
The elbow is a third-class lever. The effort arm — the perpendicular distance from the elbow axis to the biceps insertion on the radial tuberosity — is about 4 cm. The load arm, from the elbow to the centre of the hand, is about 35 cm. Mechanical advantage is 4/35 = 0.114.
Torque must balance: muscle force × 4 cm = load × 35 cm. A 10 kg mass weighs 98 N, so muscle force = 98 × 35/4 = 858 N, roughly 87 kg of pull. The elbow joint must also carry the sum, about 956 N of compression.
What the body gains is displacement and velocity, multiplied by the same factor of 8.75. One centimetre of biceps shortening moves the hand 8.75 cm; a muscle shortening at 20 cm/s moves the hand at 175 cm/s. Since muscle can shorten only about 30–40% of its length and its velocity is capped by myosin ATPase rate (§9.6), this gearing is the only way to obtain fast, wide-ranging limb movement from short, slow, strong muscle. The bill is paid in muscle force and joint compression, which is why joints, not muscles, wear out.
9.10 Two muscles have the same mass. One is multipennate with 4 cm fibers, the other is parallel with 20 cm fibers. Explain, using PCSA, how they differ and why neither design is superior.
Model answer
For a fixed muscle volume, PCSA = volume ÷ fiber length (corrected by the cosine of the pennation angle). Shortening the fibers fivefold therefore increases PCSA roughly fivefold. Since force is proportional to PCSA at about 22–35 N/cm², the multipennate muscle produces roughly five times the force, reduced slightly by the cosine of its pennation angle — at 25°, cos θ = 0.91, so about 4.5 times.
Excursion is proportional to fiber length, because a fiber shortens about 30–40% of its own length regardless of arrangement. The parallel muscle can shorten 6–8 cm; the pennate muscle only 1.2–1.6 cm. Maximum shortening velocity scales the same way.
Neither is superior because they answer different questions. Soleus must hold body weight over the ankles through a small range for sixteen hours a day: short pennate fibers and huge PCSA. Sartorius must carry the hip and knee through a very large arc: long parallel fibers and small PCSA. The product of force and excursion — the work available — is similar; only its distribution differs.
9.11 Why does a rotator cuff tear cause the patient to shrug instead of abduct?
Model answer
The glenoid holds only about a third of the humeral head, so the joint has almost no bony stability. Deltoid's line of pull at low angles of abduction is nearly parallel to the humeral shaft, so most of its force is upward shear on the humeral head rather than rotation about the joint centre. The rotator cuff supplies a compressive force that holds the head centred in the glenoid, converting that shear into rotation — it functions as a dynamic ligament.
Remove the cuff, particularly supraspinatus, and deltoid contraction drives the humeral head superiorly into the coracoacromial arch instead of rotating the arm. Unable to abduct, the patient recruits the scapular elevators — upper trapezius and levator scapulae — and lifts the whole shoulder girdle. That substitution is the shrug sign, and it is a direct readout of a force-couple that has lost one of its members.
9.12 Explain why walking is metabolically cheap while standing still on one leg is not.
Model answer
Walking exploits passive mechanics. The stance limb behaves as an inverted pendulum, exchanging potential and kinetic energy so that much of the body's energy is recycled rather than regenerated; elastic storage in the Achilles tendon and plantar aponeurosis returns a further substantial fraction. Most muscle activity in gait is therefore eccentric control of an ongoing fall rather than concentric work, and eccentric contraction is metabolically cheap (§9.6). One concentric burst — the plantarflexor push-off — supplies most of the positive power.
Single-leg standing has no such exchange. There is no fall to control and no elastic return; the hip abductors, especially gluteus medius, must generate a large sustained torque to keep the pelvis level against body weight acting at the opposite side, and the ankle and foot musculature must continuously correct sway. Sustained isometric work with no elastic recovery and no alternating relaxation phase — which also means impaired blood flow to the working muscle — is expensive.
Level 3 · Clinical Application
9.13 A 34-year-old sustains a tibial shaft fracture. Six hours later he has severe pain requiring escalating analgesia, agonizing pain when his toes are passively extended, numbness between his first and second toes, and a palpable dorsalis pedis pulse. Compartment pressure is 52 mm Hg; his blood pressure is 130/78. Analyse.
Model answer
This is acute compartment syndrome of the anterior compartment of the leg, and it needs a fasciotomy now.
The pressure criterion. ΔP = diastolic pressure − compartment pressure = 78 − 52 = 26 mm Hg, which is below the threshold of 30 mm Hg. That single calculation is sufficient.
Why the pulse is present and irrelevant. Capillary perfusion fails when compartment pressure approaches capillary pressure, around 20–30 mm Hg. The dorsalis pedis artery carries a mean pressure near 90 mm Hg and remains patent at 52 mm Hg of external pressure. The muscle is ischaemic; the artery is not occluded. A present pulse excludes nothing.
Localizing the compartment. Passive extension of the toes stretches the flexors, which lie posteriorly; passive flexion would stretch the anterior compartment. Numbness in the first web space is the sensory territory of the deep fibular nerve, which runs in the anterior compartment — so the anterior compartment is at least involved, and in practice all four are released. (The examiner's point is that you must know which movement stretches which compartment, and which nerve traverses it.)
The clock and the systemic risk. Nerve dysfunction begins at 2–4 hours and irreversible muscle necrosis at 4–6 hours; at six hours he is at the edge. Necrotic muscle releases myoglobin, potassium, phosphate, and creatine kinase — rhabdomyolysis (§9.7) — so the threats are hyperkalaemic cardiac arrest and myoglobinuric acute kidney injury, not simply a stiff leg. Elevating the limb above heart level is contraindicated, because it lowers arterial inflow pressure without lowering compartment pressure and therefore worsens the gradient.
9.14 A 68-year-old woman describes leakage of urine when she coughs, sneezes, or lifts her grandchild, but never at night. She has had three vaginal deliveries. Explain the mechanism, and explain why pelvic floor muscle training works.
Model answer
This is stress urinary incontinence, and the word "stress" refers to mechanical stress, not emotional stress. Coughing, sneezing, and lifting are precisely the manoeuvres that raise intra-abdominal pressure — the diaphragm descends, the abdominal wall contracts, and pressure in the abdominopelvic container rises steeply (§10.6).
That pressure is transmitted to the bladder and to the urethra. Continence depends on urethral closure pressure exceeding bladder pressure, and on the pelvic floor providing a firm backstop against which the urethra is compressed. Vaginal delivery causes direct stretch injury to levator ani and traction injury to the pudendal nerve (S2–S4), and the loss of oestrogen at menopause reduces the quality of the supporting connective tissue (Chapter 27). With a weakened floor, the pressure rise is transmitted to the bladder but the urethra is not adequately supported or compressed, and leakage occurs.
Nocturnal continence is preserved because lying down abolishes both the gravitational load and the intra-abdominal pressure spikes — a useful confirmation that the problem is mechanical rather than a bladder storage disorder.
Pelvic floor muscle training works because levator ani is skeletal muscle and responds to progressive resistance exactly as any skeletal muscle does (§9.8): first neural adaptation — better recruitment, better timing, and specifically the learned ability to pre-contract before a cough (the "knack"), which is a feed-forward strategy like transversus abdominis's 30 ms lead — and then hypertrophy over weeks to months. Cure or major improvement is achieved in a majority of women with a supervised programme, which places it among the more effective conservative treatments in medicine.
9.15 A 24-year-old marathon runner — Nia — develops aching pain over the anterior shin that begins predictably at 20 minutes of running, becomes severe enough to stop her, and resolves within 20 minutes of rest. She has transient numbness between her first and second toes during episodes, and examination between episodes is entirely normal. Explain.
Model answer
This is chronic exertional compartment syndrome of the anterior compartment, and every feature of the history is anatomical.
During exercise, working muscle swells — blood flow rises many-fold and metabolite accumulation draws water in — with volume increases of up to about 20%. In a compartment bounded by inextensible fascia, that volume increase raises intracompartmental pressure. When the pressure approaches capillary perfusion pressure (20–30 mm Hg), perfusion of the muscle inside falls just as its demand is highest, producing ischaemic pain.
The reproducibility is the clue: pain begins at a predictable exercise duration, because that is how long it takes the muscle to swell to the critical volume, and resolves predictably with rest as the swelling subsides. Numbness in the first web space localizes it precisely: that is the sensory territory of the deep fibular nerve, which traverses the anterior compartment, and nerve is more sensitive to ischaemia than muscle. A normal examination between episodes is expected and is what distinguishes this from every structural diagnosis.
The differential is chiefly medial tibial stress syndrome and tibial stress fracture, but neither produces the sharply reproducible onset-and-offset timing or the web-space numbness. Diagnosis is by measuring intracompartmental pressure before and after a provocative run. Treatment is activity modification and gait retraining — forefoot striking substantially reduces anterior compartment demand, since it reduces the eccentric dorsiflexor work of lowering the forefoot — and, if that fails, elective fasciotomy. Note the contrast with the acute form: same physics, but reversible, and never an emergency.
Level 4 · Integration and Synthesis
9.16 Amara's programme forbids breath-holding but permits moderate resistance training. Toby's programme uses electrical stimulation rather than voluntary exercise. Explain how the same underlying principle — matching the stimulus to the specific physiological constraint — produces two such different prescriptions, and name the constraint in each case.
Model answer
Both patients need muscle loading, and in both the obvious method is blocked by one identifiable variable. The skill is naming that variable rather than avoiding exercise generally.
Amara's constraint is myocardial oxygen supply–demand balance. Her limiting variable is the rate–pressure product, and specifically the pressure term, because wall stress (σ = P·r / 2h) is a principal determinant of oxygen demand while her supply is fixed by a narrowed artery and 14% of her ventricle is already scar. Anything that spikes arterial pressure is forbidden; anything that raises flow gradually is encouraged. Hence: rhythmic large-muscle work, which lowers peripheral resistance and keeps diastolic pressure (and therefore coronary perfusion pressure) intact; moderate loads and exhalation through the effort, which prevents the Valsalva pressure excursion; and a graded cool-down, which prevents the muscle pump from being withdrawn abruptly. Moderate resistance training is permitted because it is the breath-holding and maximal effort, not the resistance itself, that produce the pressure spike.
Toby's constraint is neural drive, not muscle capacity. His quadriceps activation is 68% of the uninvolved side because of arthrogenic muscle inhibition — the joint effusion is reflexively suppressing his quadriceps motor neuron pool. By the size principle (§9.8), a patient who can voluntarily produce only 20–30% of maximum force recruits only type I and low-threshold type IIa units, so the type II fibers, which are the ones atrophying fastest, are never loaded at all. Voluntary exercise cannot reach them. Neuromuscular electrical stimulation bypasses the motor neuron pool entirely, exciting axons directly, and because electrical recruitment depends on axon diameter rather than motor neuron size, it reaches large fast axons early — violating the size principle in exactly the direction the patient needs. Blood flow restriction training achieves a similar end by a different route, creating a hypertrophic stimulus at 20–30% of maximum load.
The shared principle. In both cases, the correct question is not "how much exercise?" but "which physiological variable is the ceiling, and what route exists around that variable?" For Amara the ceiling is a pressure; for Toby it is a recruitment threshold. Naming it converts a vague restriction into a precise prescription.
9.17 Construct the complete causal chain from Amara's akinetic left ventricular segment to the visible use of her sternocleidomastoid muscles at rest. Name every system involved and the direction of causation at each step.
Model answer
Muscular (cardiac) → 14% of the left ventricular wall is scar and cannot contract (§9.9), so stroke volume and ejection fraction fall to 48%; the scar is also stiffer than muscle, so the ventricle resists filling.
→ Cardiovascular. A ventricle that empties incompletely and fills stiffly requires a higher left ventricular end-diastolic pressure for any given volume. That pressure is transmitted backwards to the left atrium and then into the pulmonary veins.
→ Respiratory (mechanical). Raised pulmonary capillary hydrostatic pressure shifts the Starling balance across the pulmonary capillary wall toward filtration, so fluid accumulates in the pulmonary interstitium. Interstitial oedema reduces lung compliance: the lung becomes stiffer and each breath costs more work. Small airway narrowing raises resistance as well.
→ Nervous. Pulmonary J-receptors and irritant receptors, together with chemoreceptor input and the sensation of increased effort, drive an increase in respiratory rate to 24 breaths per minute.
→ Muscular (skeletal), the visible endpoint. The diaphragm alone can no longer generate the required minute ventilation against a stiffer lung, so accessory muscles are recruited — sternocleidomastoid and scalenes elevating the upper ribs and sternum. That recruitment is visible from the doorway.
And the loop closes. Accessory muscle work raises the oxygen cost of breathing from 2–3% of total consumption toward 15–25%. Those muscles are perfused by the same cardiac output that is already inadequate, so respiratory muscle work competes with the rest of the body — and with the heart itself — for a limited supply. A visible neck muscle is therefore a cardiac sign, exactly as cool pale skin was in Chapter 1: five systems, one finding.
9.18 Argue for or against: "Muscles should be studied by region, as surgeons learn them, not by system." Use at least four specific examples from this chapter.
Model answer
A strong answer argues that both are necessary and they answer different questions, then shows where each fails.
Where regional organization wins:
- Compartments are regional facts with systemic consequences. Knowing that the anterior compartment of the leg contains the dorsiflexors, the deep fibular nerve, and the anterior tibial artery lets you predict foot drop, first-web-space numbness, and compartment syndrome from a single anatomical grouping (§10.8, §10.9).
- The inguinal canal cannot be understood except regionally: its walls are contributed by three different muscle layers, and the distinction between direct and indirect hernia depends on its relationship to a blood vessel, not to a muscle (§10.6).
- The rotator cuff is a regional force-couple. Its function is a property of four muscles acting together against a fifth (deltoid), and no muscle-by-muscle account explains the shrug sign (§10.7).
- Gait is regional in the extreme: it is the coordinated behaviour of muscles from the trunk to the toes, and the diagnostic value of a steppage gait comes from recognizing the whole pattern (§10.9).
Where regional organization fails:
- Segmental innervation is not regional but developmental. The diaphragm sits at the bottom of the thorax and answers to C3–C5, which makes sense only in terms of the septum transversum's descent — and it explains why diaphragmatic irritation refers pain to the shoulder tip.
- Muscle architecture generalizes across regions. PCSA and fiber length predict force and excursion identically in the soleus, the subscapularis, and the levator ani, and that principle drives tendon transfer surgery anywhere in the body.
- The muscle pump and intra-abdominal pressure are system-level phenomena. Neither can be seen by studying the calf or the abdominal wall as regions; both only appear when muscle is treated as an organ system interacting with the circulation.
- Rehabilitation reasoning is systemic. The size principle governs what Toby's quadriceps can be trained to do and what Amara's whole body can tolerate. It has no regional address.
The synthesis: regional anatomy answers what will be injured together and what will fail together; systemic physiology answers why it works and how to train or treat it. A surgeon needs the first and cannot operate safely without it; a clinician managing an infarct or a rehabilitation programme needs the second. The error is not choosing one but assuming that mastering either makes the other unnecessary.
Concept Map to Complete
Copy onto blank paper and fill in every bracket from memory, then check.
A MUSCLE ACTING ON A JOINT
│
┌────────────────────────────┼────────────────────────────┐
FUNCTIONAL ROLE LEVER CLASS ARCHITECTURE
┌────┴────┐ ┌───────┴───────┐ ┌────┴────┐
[ _______ ] prime 1st: [ _ ]-[ _ ]-[ _ ] force ∝ [ ____ ]
[ _______ ] opposes 2nd: [ _ ]-[ _ ]-[ _ ] excursion ∝
[ _______ ] assists / always MA [ >1 / <1 ] [ ____________ ]
cancels 3rd: [ _ ]-[ _ ]-[ _ ] pennation:
[ _______ ] stabilises always MA [ >1 / <1 ] gain [ ______ ],
MOST HUMAN MUSCLES lose [ ______ ]
│
RECIPROCAL INHIBITION: Ia afferent → excites [ ________ ]
→ via [ ______________ ]
inhibits [ __________ ]
═══════════ PRESSURE IN A CONTAINER ═══════════
VALSALVA MUSCLE PUMP COMPARTMENT SYNDROME
phase I [ __ ] ankle pressure normal pressure [ __ ] mm Hg
phase II [ __ ] standing [ __ ] ΔP = [ ______ ] − [ ______ ]
phase III[ __ ] walking [ __ ] fasciotomy if ΔP < [ __ ]
phase IV [ __ ] valves make flow pulse lost [ early / late ]
[ ____-way ] necrosis at [ ___ ] hours
Lab / Self-Exploration
- Find your own reciprocal inhibition. Rest your forearm on a table, palm up. Place your other hand on your triceps and flex your elbow slowly against light resistance. Feel the triceps stay soft. Now deliberately co-contract both. Note how much more effort the second costs, and how much stiffer the joint feels.
- Measure your own mechanical advantage. With a tape measure, find the distance from your elbow crease to the middle of your palm (the load arm). The biceps effort arm is about 4 cm. Compute the mechanical advantage, then compute the muscle force needed to hold a 2 kg object. The number should surprise you.
- Palpate a compartment. Feel the firm muscle just lateral to your tibial shaft — the anterior compartment. Dorsiflex your foot repeatedly for 60 seconds and feel it firm up as the muscle swells. Now imagine that swelling inside fascia that cannot stretch.
- Demonstrate the muscle pump. Stand still for two minutes and look at the veins on the dorsum of your foot. Then rise onto your toes twenty times and look again. The veins should flatten visibly. You have just lowered your own ambulatory venous pressure.
- Find your VMO. Sit with your leg straight and press the back of your knee down into the surface. The bulge that appears on the medial thigh just above the kneecap is the vastus medialis obliquus — the muscle that disappears first after a knee injury.
- Test scapulohumeral rhythm. Have a partner watch your scapula from behind while you slowly abduct your arm to full overhead. Note the point at which the scapula begins to rotate — around 30° — and that it continues throughout. Then try abducting with the scapula deliberately held still, and see how far you get.
Key Terms
agonist (prime mover) · The muscle chiefly responsible for producing a given movement.
antagonist · The muscle opposing the agonist; it lengthens under control and is reflexively inhibited.
aponeurosis · A flat, sheet-like tendon, as in the abdominal wall and the plantar fascia.
arthrogenic muscle inhibition · Reflex suppression of a muscle's motor neuron pool by abnormal afferent input from an injured or swollen joint; the reason quadriceps activation falls after knee injury.
Valsalva manoeuvre · Forced expiration against a closed glottis; produces four hemodynamic phases and large swings in arterial pressure and venous return.
buccinator · Cheek muscle that presses food against the teeth; supplied by CN VII.
compartment syndrome · Raised pressure within a closed fascial compartment sufficient to abolish capillary perfusion; diagnosed by ΔP under 30 mm Hg, not by pulse.
co-contraction · Voluntary simultaneous activation of agonist and antagonist to stiffen a joint at the cost of efficiency.
diaphragm · The dome-shaped principal muscle of inspiration, supplied by the phrenic nerve (C3–C5), responsible for about 70% of tidal volume.
erector spinae · The three vertical columns — iliocostalis, longissimus, spinalis — extending the vertebral column, working largely eccentrically.
fixator · A muscle that stabilizes the origin of an agonist so it has a firm base to pull from.
gait cycle · One complete sequence from initial contact of a foot to the next initial contact of the same foot; 60% stance, 40% swing.
gluteus medius · Hip abductor stabilizing the pelvis in single-leg stance; its weakness produces the Trendelenburg sign.
hernia · Protrusion of viscera through a wall that should contain them; indirect inguinal hernias pass through the deep ring lateral to the inferior epigastric vessels, direct hernias through Hesselbach's triangle medial to them.
insertion · The attachment of a muscle on the more mobile bone.
intra-abdominal pressure · Pressure within the abdominopelvic container, 0–5 mm Hg at rest and over 100 mm Hg during maximal straining; generated chiefly by transversus abdominis with the diaphragm and pelvic floor.
lever, first / second / third class · Fulcrum in the middle / load in the middle (always a force advantage) / effort in the middle (always a speed advantage, and the human default).
levator ani · The main muscle of the pelvic diaphragm; supports the pelvic viscera and, through puborectalis, maintains the anorectal angle.
mechanical advantage · Effort arm divided by load arm; greater than 1 favours force, less than 1 favours speed and range.
myotendinous junction · The extensively folded interface where a muscle fiber attaches to tendon, converting tensile load into shear over a 10–20-fold enlarged area; the commonest site of strain injury.
myotome · The muscle-forming part of a somite and, clinically, the group of muscles supplied by one spinal segment.
origin · The attachment of a muscle on the more stationary bone.
pennation · Attachment of short fibers at an angle to a tendon, increasing physiological cross-sectional area and force at the cost of excursion.
physiological cross-sectional area (PCSA) · Total cross-sectional area of all a muscle's fibers measured perpendicular to their own axis; proportional to maximum force.
reciprocal inhibition · The spinal circuit by which a Ia afferent excites the agonist's motor neurons and, through an inhibitory interneuron, inhibits the antagonist's.
rate–pressure product · Heart rate multiplied by systolic pressure; the clinical index of myocardial oxygen demand.
rotator cuff · Supraspinatus, infraspinatus, teres minor, subscapularis; a dynamic ligament compressing the humeral head into the shallow glenoid.
scapulohumeral rhythm · The roughly 2:1 coordination of glenohumeral to scapulothoracic motion during arm elevation.
skeletal muscle pump · The mechanism by which rhythmic muscle contraction and one-way venous valves propel blood proximally, dropping ankle venous pressure from about 90 to 20–30 mm Hg.
soleus · The principal antigravity muscle, tonically active in standing because the line of gravity falls anterior to the ankle.
sternocleidomastoid · Neck muscle rotating the head to the opposite side and serving as an accessory muscle of inspiration.
synergist · A muscle that assists the agonist by adding force or by cancelling an unwanted secondary action.
transversus abdominis · The deepest abdominal wall muscle, with horizontal fibers; the primary generator of intra-abdominal pressure, activating about 30 ms before limb movement.
Trendelenburg sign · Drop of the unsupported side of the pelvis in single-leg stance, indicating hip abductor weakness on the stance side.
vastus medialis obliquus (VMO) · The distal oblique portion of vastus medialis, active in terminal knee extension and the first part of the quadriceps to atrophy after knee injury.
winged scapula · Lifting of the scapula's medial border from the chest wall, caused by long thoracic nerve injury paralysing serratus anterior.
Next: Chapter 11 · The Nervous System — where the motor neurons that have commanded every muscle in these two chapters are finally examined in their own right, and where the reason Amara's chest pain reached her jaw becomes a question about wiring.