Appearance
6.5 — Muscles of the Upper Limb
The shoulder is stable only because four muscles hold the humeral head against a socket a third of its size (Chapter 5.4). The hand is powerful only because most of the muscles that move the fingers are not in the hand at all — they are in the forearm, pulling on long tendons, which is why your fingers can be strong without your hand being bulky.
Both arrangements are compromises, and both produce characteristic injuries.
Moving the scapula
Before the arm can move usefully, the scapula must be positioned and held (Chapter 5.4).
Trapezius — the upper fibres elevate the shoulder, the middle fibres retract it, and the lower fibres depress and help rotate it upward.
Serratus anterior — runs from the ribs at the side of the chest to the medial border of the scapula, holding it flat against the ribcage and rotating it upward. It is essential for raising the arm above shoulder height.
When the nerve to serratus anterior is damaged, the medial border of the scapula lifts away from the chest wall — winged scapula. It is most visible when the person pushes against a wall. The nerve runs a long superficial course down the side of the chest and is vulnerable to pressure — carrying a heavy pack, or surgery in the armpit.
Rhomboids — retract the scapula, pulling the shoulder blades together.
Levator scapulae — elevates it, and is a common site of neck and shoulder tension.
The rotator cuff
Four muscles, all running from the scapula to the top of the humerus, whose tendons blend into the joint capsule. They are the shoulder's stability, and they generate very little of its power.
Supraspinatus — from above the scapular spine, over the top of the joint. Initiates abduction for the first 15 degrees, then deltoid takes over. Its tendon passes through a narrow space between the humeral head below and the acromion above.
Infraspinatus — below the scapular spine. Laterally rotates the arm.
Teres minor — laterally rotates.
Subscapularis — on the front surface of the scapula. Medially rotates, and it is the only cuff muscle on the front, which is part of why dislocations go forward.
A useful way to see what they do: the three at the back and top pull the humeral head down and inward against the socket; the deltoid pulls it upward. Without the cuff, contracting the deltoid would simply drive the humeral head up into the acromion instead of raising the arm. The cuff provides the anchor.
Cuff problems, which are extremely common
The supraspinatus tendon is the one that fails, and the reason is where it runs. It passes through the subacromial space — a gap of only about 6 to 14 millimetres between the humeral head and the acromion, also containing a bursa. Anything that narrows that space compresses the tendon.
Subacromial impingement — pain on raising the arm, characteristically in a "painful arc" between about 60 and 120 degrees, because that is where the tendon is squeezed most, and less painful above and below it.
Rotator cuff tendinopathy — degeneration of the tendon, which is a wear-and-failed-repair process rather than true inflammation, hence the shift in terminology from "tendinitis".
Rotator cuff tear — partial or full thickness. Prevalence rises steeply with age, and a substantial proportion of tears in people over 60 cause no symptoms at all, which is an important fact: finding a tear on a scan does not establish that it is the cause of the pain.
Frozen shoulder (adhesive capsulitis) is a different condition often confused with the above. The capsule itself thickens and contracts. The distinguishing feature is loss of passive range — someone else moving your arm cannot move it either, whereas in a cuff problem passive movement is preserved. It is strongly associated with diabetes, and it follows a three-phase course over one to three years: painful, stiff ("frozen"), then thawing. Most eventually recover, and telling someone it will take two years is more honest and more useful than implying a quick fix.
Treatment for cuff problems is overwhelmingly non-surgical. Structured exercise therapy performs as well as surgery in several trials for impingement, and placebo-controlled trials of subacromial decompression surgery found no benefit over sham surgery — a result that substantially changed practice.
Muscles moving the arm
Deltoid — caps the shoulder. Three parts: the front fibres flex the arm, the middle fibres abduct it (from 15 degrees upward), the back fibres extend it. It is the standard site for intramuscular injections, including most vaccines, because it is superficial, easy to locate and has no major structures immediately beneath at the injection point.
Pectoralis major — the large fan across the chest, from the sternum, clavicle and ribs to the humerus. Adducts and medially rotates the arm, and flexes it from an extended position. This is the muscle of a bench press, a push-up, and of climbing.
Latissimus dorsi — a broad sheet from the lower spine and pelvis to the humerus. Adducts, extends and medially rotates, and it is the muscle that pulls the body upward in a pull-up. Its old teaching description as the "handcuff muscle" describes its action of pulling the arm down and behind the back.
Because latissimus dorsi is broad, flat and its blood supply enters at one point, it can be detached and rotated to reconstruct elsewhere, most commonly in breast reconstruction after mastectomy — the muscle is swung round on its vessels with the skin over it.
The upper arm
Biceps brachii — two heads, crossing both the shoulder and the elbow. It flexes the elbow and — often forgotten — it is the most powerful supinator of the forearm.
That is why a right-handed screw tightens clockwise. Driving a screw in with the right hand is a supination movement, and supination is stronger than pronation because biceps assists it. The convention favours the majority.
Rupture of the long head of biceps at the shoulder produces a visible bulge in the lower arm — the "Popeye sign" — as the muscle belly retracts. It is often surprisingly well tolerated, because the short head and brachialis still flex the elbow, and in older patients it is frequently left alone.
Brachialis — lies underneath biceps and is actually the main elbow flexor, working regardless of forearm position.
Brachioradialis — in the forearm, but it flexes the elbow, and is strongest with the thumb pointing up.
Triceps brachii — three heads on the back of the arm. The only significant elbow extensor. Used in pushing, in the last part of a bench press, and in the essential function of pushing yourself up out of a chair — which is why triceps weakness matters more than it sounds in older people.
The forearm and hand
The forearm contains around 20 muscles in two compartments.
Anterior (flexor) compartment — flexes the wrist and fingers, and pronates. Mostly supplied by the median nerve, with two muscles from the ulnar nerve.
Posterior (extensor) compartment — extends the wrist and fingers, and supinates. All supplied by the radial nerve.
Most muscles moving the fingers are in the forearm, with long tendons crossing the wrist. The advantage is that the fingers stay slim and can be strong; the cost is that the tendons have to be held down and lubricated over a long course, and that is where several problems arise.
The intrinsic hand muscles — around 20 small muscles within the hand itself — do the fine work: spreading and closing the fingers, and the complex movements of the thumb. Almost all are supplied by the ulnar nerve, which is why ulnar palsy produces a claw hand and loss of dexterity out of proportion to the sensory loss.
The tendon problems
Lateral epicondylitis — "tennis elbow". Pain at the outer elbow where the wrist extensors attach. Around 90 to 95 percent of cases occur in people who do not play tennis, and it is caused by repetitive gripping and wrist extension — using a screwdriver, painting, keyboard and mouse work, carrying. It is a degenerative tendon change rather than inflammation, which is why anti-inflammatories give limited benefit and eccentric loading exercises work better (Chapter 6.1). Most cases resolve within 12 to 18 months.
Medial epicondylitis — "golfer's elbow" — the same at the inner elbow, affecting the flexors.
De Quervain's tenosynovitis — inflammation of the sheath around the two thumb tendons at the wrist. Pain on gripping and on ulnar deviation of the wrist. Common in new parents, from repeatedly lifting an infant with the thumbs extended, and in people who use a phone one-handed for long periods.
Trigger finger — a nodule on a flexor tendon catches at the entrance to its sheath, so the finger snaps rather than moving smoothly, and in advanced cases locks in flexion and must be straightened with the other hand. Commoner in diabetes. Treated with a steroid injection or a small surgical release of the sheath entrance.
Dupuytren's contracture — thickening of the fascia in the palm, drawing the ring and little fingers into permanent flexion. Not a tendon problem at all, though it is often assumed to be. Strongly hereditary, commoner in men, in Northern European ancestry, in diabetes and with alcohol excess.
Carpal tunnel syndrome
Nine flexor tendons and the median nerve pass through a tunnel roofed by a rigid ligament (Chapter 5.5). The tunnel cannot expand, so anything that increases its contents or reduces its size compresses the nerve — and the nerve is the softest structure there, so the nerve loses.
Symptoms: numbness and tingling in the thumb, index, middle and radial half of the ring finger. Characteristically worse at night, and people describe waking and shaking the hand to relieve it — the reason is that the wrist tends to be held flexed during sleep, which raises tunnel pressure. Later, weakness and wasting of the thumb muscles.
Note what is spared: the little finger. It is supplied by the ulnar nerve, which does not pass through the tunnel. Numbness including the little finger is not carpal tunnel syndrome, and that single observation redirects the diagnosis.
Causes: frequently none identifiable. Pregnancy (from fluid retention, and usually resolving after delivery), hypothyroidism, rheumatoid arthritis, diabetes, and repetitive forceful work. The evidence that ordinary computer keyboard use causes it is weak, despite the popular association.
Treatment: a night splint holding the wrist neutral, steroid injection, and surgical release of the ligament, which is a small operation with a high success rate when the diagnosis is correct.
Compartment anatomy, and why it matters
The limb muscles are enclosed in compartments bounded by inelastic fascia, and this is the anatomy behind compartment syndrome (Chapter 5.9). The forearm has two main compartments; a supracondylar fracture in a child bleeding into the anterior compartment is the classic cause, and the result if missed is Volkmann's contracture, in which the flexor muscles die and are replaced by scar that permanently claws the hand.
The signs are pain out of proportion, pain on passive stretch of the fingers, and a tense compartment — and the pulse is present. The treatment is fasciotomy within hours.
What the next page fixes
The lower limb muscles do something quite different: they do not manipulate, they carry, propel and — most of the time — resist gravity rather than generate movement. Chapter 6.6 covers them, including why hamstring tears are the commonest injury in sprinting sports, why the gluteal muscles matter far more than their reputation as sitting muscles suggests, and what the calf muscle pump does for your circulation.