Muscles of the Shoulder: Anchoring the Moving Platform
Almost every joint in your body moves one bone on another fixed bone. The shoulder does something far bolder: it moves a bone on a bone that is itself moving. The scapula floats on the back of the chest wall, held there by nothing but muscle — no locking socket, no bony strut except one slender clavicle. That is the price the shoulder pays for the widest range of motion in the human body, and the reason a whole committee of muscles must first steady the platform before the arm can safely be lifted. Understand the shoulder as platform plus lever, and everything from a tennis serve to a winged scapula suddenly makes sense.
Watch a swimmer at the moment her hand enters the water. Her arm is fully overhead — and to get it there, seventeen muscles have already negotiated with one another in a fraction of a second. The trapezius and serratus anterior have spun the scapula upward so that its socket faces the sky. The supraspinatus has cracked the arm off her side. The deltoid has hauled it up through the arc. The rotator cuff has been quietly pulling the head of the humerus down and inward the whole time, so that it does not ram into the bony roof above it. Then the latissimus dorsi fires like a whip and drags her body past her fixed hand. Not one of these muscles could do the job alone. The shoulder is not a joint so much as a committee — and every classic shoulder injury is a story about one member of that committee failing to show up.
Two families of muscle: those that move the platform, those that move the arm
Every muscle here belongs to one of two groups, and the group tells you what it does. The axioappendicular muscles run from the axial skeleton — the skull, vertebrae and ribs — to the pectoral girdle, and their job is to position the scapula and clavicle. The scapulohumeral muscles run from the girdle itself to the humerus, and their job is to move the arm on that positioned platform. It is a two-stage system, and it must run in that order: aim the platform, then fire the lever. This is why the bones described in the clavicle and scapula are shaped as they are — the scapula is essentially a broad muscular billboard with three bony handles (the spine, the acromion and the coracoid process) for tendons to grab. The clavicle, meanwhile, is the single strut that holds the whole apparatus out from the chest so the arm can swing free.
The superficial back: trapezius and latissimus dorsi
The trapezius is the great diamond of the upper back, arising from the external occipital protuberance, the nuchal ligament and the spinous processes of C7–T12, and inserting along the lateral clavicle, the acromion and the spine of the scapula. Its three sets of fibres pull in three directions: the upper fibres elevate the scapula (the shrug), the middle fibres retract it (pulling the shoulder blades together), and the lower fibres depress it — while upper and lower together rotate the glenoid cavity upward. It is supplied by the spinal accessory nerve (CN XI), a cranial nerve that has travelled all the way down the neck to reach it, plus proprioceptive fibres from C3–C4. The latissimus dorsi is the muscle of climbing and swimming: a vast sheet from the spinous processes of T7–T12, the thoracolumbar fascia, the iliac crest and the lower three or four ribs, converging into a narrow tendon that twists to insert into the floor of the intertubercular sulcus of the humerus. Innervated by the thoracodorsal nerve (C6–C8), it extends, adducts and medially rotates the arm — and when the hand is fixed above you, it does the opposite: it pulls your whole trunk up toward your hand.
The deep back: levator scapulae and the rhomboids
Under the trapezius sit three retractors, all fed by one small nerve. Levator scapulae arises from the transverse processes of C1–C4 and inserts on the medial border of the scapula above the spine; it elevates the scapula and tilts the glenoid cavity downward. Rhomboid minor runs from the spinous processes of C7–T1 to the medial border at the level of the spine of the scapula; rhomboid major runs from T2–T5 to the medial border below it. Both retract the scapula, rotate the glenoid downward, and — critically — press the medial border firmly against the thoracic wall. All three are supplied by the dorsal scapular nerve (C5), which arises directly from the root of C5 before the trunks form. When that nerve is injured, the scapula on that side drifts laterally away from the midline and the medial border lifts slightly — a subtler cousin of true winging, and a reminder that the muscles of the girdle are as much stabilizers as movers.
The front wall: pectoralis major, pectoralis minor and subclavius
Pectoralis major is the fan of the chest, and it has two heads with different jobs. The clavicular head arises from the medial half of the clavicle and flexes the arm; the sternocostal head arises from the sternum and upper six costal cartilages and extends the flexed arm back down. Both converge onto the lateral lip of the intertubercular sulcus of the humerus, and together they are the great adductor and medial rotator of the arm — the muscle of a bear hug, a bench press, a push-up. It is supplied by the lateral pectoral nerve (clavicular head, C5–C7) and the medial pectoral nerve (sternocostal head, C8–T1). Beneath it, pectoralis minor runs from ribs 3–5 to the coracoid process, depressing and protracting the scapula and stabilizing it against the thorax; it is a key surgical landmark, dividing the axillary artery into its three parts. Subclavius is a small muscle from the first rib to the inferior surface of the clavicle, supplied by the nerve to subclavius (C5–C6): it anchors and depresses the clavicle and cushions the underlying vessels — a small crumple zone protecting the subclavian vessels when the clavicle fractures.
Serratus anterior: the muscle that holds the scapula to the chest
If one muscle deserves a chapter of its own, it is this one. Serratus anterior arises by fleshy digitations from the outer surfaces of ribs 1–8 (or 1–9), sweeps around the chest wall deep to the scapula, and inserts along the whole length of its medial border — with the heaviest slips gripping the inferior angle. It is supplied by the long thoracic nerve (C5, C6, C7 — "C5, 6, 7 keeps the wing from heaven"), which runs superficially down the outer chest wall, exposed and vulnerable. Serratus anterior does two things nothing else can do as well. First, it protracts the scapula — the punching and reaching motion — and it powerfully rotates the glenoid cavity upward, working as a force couple with the trapezius. Second, and more fundamentally, it holds the scapula flat against the thoracic wall at all times. Lose it, and the medial border and inferior angle lift off the back like a folded wing: the winged scapula, the single most recognizable sign in upper-limb neurology.
Think of the scapula as a camera tripod head and the arm as the camera. The axioappendicular muscles are the three legs and the locking knobs: trapezius pulling up and in, serratus anterior pulling forward and around, rhomboids pulling back, pectoralis minor pulling down. Each pulls in a different direction, and it is precisely because they pull against one another that the head stays exactly where you set it. The deltoid and rotator cuff are then free to aim the camera. Cut one leg of the tripod — paralyse serratus anterior — and it no longer matters how strong the camera operator is: the whole mount tilts and shakes the instant any load is applied.
Scapulohumeral muscles: deltoid, teres major and the rotator cuff
The deltoid is the shoulder's contour — a thick triangle arising from the lateral third of the clavicle, the acromion and the spine of the scapula, and converging onto the deltoid tuberosity of the humerus. Like the trapezius it works in thirds: anterior fibres flex and medially rotate the arm, middle (acromial) fibres are the powerful abductors beyond the first 15°, and posterior fibres extend and laterally rotate it. It is supplied by the axillary nerve (C5–C6), which curls around the surgical neck of the humerus — described with its companions in the radial, axillary and musculocutaneous nerves. Teres major arises from the dorsal surface of the inferior angle of the scapula and inserts on the medial lip of the intertubercular sulcus; supplied by the lower subscapular nerve (C5–C6), it adducts and medially rotates the arm, acting as latissimus dorsi's short-armed partner. Then there is the rotator cuff — supraspinatus, infraspinatus, teres minor and subscapularis — whose tendons fuse with the fibrous capsule of the joint itself and grip the head of the humerus into the shallow glenoid, the arrangement examined in the shoulder joint.
Four cuff muscles, four precise jobs — and one of them starts every abduction you make. Supraspinatus arises in the supraspinous fossa, passes beneath the acromion, and inserts on the superior facet of the greater tubercle; supplied by the suprascapular nerve (C5–C6), it initiates abduction through roughly the first 15° and then helps the deltoid through the rest. Infraspinatus fills the infraspinous fossa and inserts on the middle facet of the greater tubercle; also suprascapular (C5–C6), it is a lateral rotator. Teres minor runs from the upper lateral border of the scapula to the inferior facet of the greater tubercle; supplied by the axillary nerve (C5–C6), it too laterally rotates the arm. Subscapularis is the only one in front: it fills the subscapular fossa on the costal surface of the scapula and inserts on the lesser tubercle, supplied by the upper and lower subscapular nerves (C5–C6), and it is the great medial rotator. Every one of these nerves is a branch of the network mapped out in the brachial plexus — which is why a single plexus lesion can silence half the shoulder at once.
The most beautiful piece of teamwork in the upper limb is the abduction sequence. From 0–15°, supraspinatus alone cracks the arm away from the side — the deltoid's line of pull at that angle is almost straight up the shaft of the humerus, so it can only shrug the bone upward, not lift it. From 15–90°, the deltoid takes over as the prime mover, while the other three cuff muscles pull the humeral head downward and inward to stop it grinding into the acromion. Beyond 90°, the arm itself has run out of joint: further elevation comes from the scapula rotating upward, driven by the force couple of the upper and lower trapezius with serratus anterior, in a roughly 2:1 ratio of glenohumeral to scapulothoracic movement. Three muscle groups, three phases, one continuous arc — and if any phase fails, the arm simply stops at that angle.
A young man is asked to do a push-up against a wall, and as he presses, the medial border of his right scapula lifts clean off his back — a winged scapula from a long thoracic nerve injury, perhaps after carrying a heavy rucksack strap or an axillary lymph node dissection. A footballer dislocates her shoulder anteriorly; weeks later the smooth curve of her deltoid has flattened into a squared-off shoulder with a palpable bony point, and she cannot feel the skin over the outer arm — the axillary nerve was stretched around the surgical neck of the humerus. A decorator complains that his arm is fine to about 60°, agonizing between 60° and 120°, then comfortable again above that — the painful arc of supraspinatus tendon impingement under the acromion, often eased with the anti-inflammatories discussed in how NSAIDs work. And a patient after neck surgery has one shoulder visibly drooping and cannot shrug it: the spinal accessory nerve was damaged in the posterior triangle, and the trapezius no longer holds the girdle up against gravity.
- Axioappendicular muscles (trunk → girdle) position the scapula; scapulohumeral muscles (girdle → humerus) move the arm on it.
- Trapezius (CN XI, spinal accessory): upper fibres elevate, middle retract, lower depress; upper + lower rotate the glenoid upward.
- Latissimus dorsi (thoracodorsal n., C6–C8): extension, adduction, medial rotation — the climbing and swimming muscle.
- Levator scapulae + rhomboid major and minor (dorsal scapular n., C5): elevation, retraction, downward rotation of the glenoid.
- Pectoralis major (lateral + medial pectoral nn.): clavicular head flexes, sternocostal head extends; together adduct and medially rotate.
- Serratus anterior (long thoracic n., C5–C7): protraction, upward rotation, and holding the scapula against the chest wall.
- Deltoid (axillary n., C5–C6): anterior fibres flex, middle abduct beyond ~15°, posterior extend and laterally rotate.
- Teres major (lower subscapular n., C5–C6): adduction and medial rotation — latissimus dorsi's short-armed partner.
- Rotator cuff: supraspinatus (initiates abduction), infraspinatus and teres minor (lateral rotation), subscapularis (medial rotation).
- Cuff nerves: suprascapular (supra- and infraspinatus), axillary (teres minor), upper/lower subscapular (subscapularis).
- Abduction sequence: supraspinatus 0–15° → deltoid 15–90° → scapular upward rotation (trapezius + serratus anterior) beyond 90°.
- Classic lesions: long thoracic → winged scapula; axillary → deltoid wasting + regimental badge numbness; CN XI → drooping shoulder.
- Saying "the deltoid abducts the arm" and stopping there. The deltoid cannot start abduction from 0° — supraspinatus must break the arm away from the side first, and the scapula must rotate for anything beyond 90°.
- Confusing teres major with teres minor. Teres MINOR is a rotator cuff muscle (axillary n., lateral rotation); teres MAJOR is not part of the cuff at all (lower subscapular n., medial rotation and adduction).
- Blaming any protruding shoulder blade on serratus anterior. True medial winging on pushing is long thoracic; a scapula that drifts laterally with a drooping shoulder points instead to the dorsal scapular or spinal accessory nerve.
A patient cannot begin to abduct the arm from the anatomical position, but once the arm is passively lifted to 20° he can continue the movement unaided. Which muscle is most likely affected?
- The scapula is a moving platform held to the chest only by muscle; axioappendicular muscles aim the platform, scapulohumeral muscles move the arm on it.
- Girdle movers: trapezius (CN XI), latissimus dorsi (thoracodorsal), levator scapulae and rhomboids (dorsal scapular), pectoralis major/minor and subclavius in front, and serratus anterior (long thoracic) holding it all flat.
- Arm movers: deltoid (axillary) in three fibre groups, teres major (lower subscapular), and the four rotator cuff muscles that centre the humeral head in the shallow glenoid.
- Abduction is a relay — supraspinatus, then deltoid, then scapular upward rotation — and its classic failures (winged scapula, deltoid wasting, painful arc, drooping shoulder) each name a single muscle or nerve.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Upper limb: shoulder region and scapular muscles.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — Pectoral, scapular and deltoid regions.
- Netter FH. Atlas of Human Anatomy — Plates of the superficial and deep muscles of the back, pectoral region and rotator cuff.
- Sinnatamby CS. Last's Anatomy: Regional and Applied — The shoulder girdle and its musculature.
- Snell RS. Clinical Anatomy by Regions — The upper limb: muscles connecting the upper limb to the trunk.
- TeachMeAnatomy — Muscles of the Shoulder: intrinsic and extrinsic groups.
