Upper Limb Muscles: A Practical Guide to Origins, Insertions, Actions, and Innervation

The upper limb muscles are organized into compartments separated by fascial septa. Each compartment shares a common function and nerve supply — which means learning one nerve tells you the function of an entire group of muscles. Anterior compartment of the arm (biceps, brachialis, coracobrachialis): all flexors of the elbow, all innervated by the musculocutaneous nerve (C5-C7). Posterior compartment of the arm (triceps, anconeus): extensors of the elbow, innervated by the radial nerve (C5-T1). Anterior compartment of the forearm (flexors — 3 layers, 8 muscles): wrist flexors, finger flexors, and pronators. Superficial and intermediate layers innervated by the median nerve. Deep layer: flexor digitorum profundus is split — lateral half (index/middle fingers) by median nerve (anterior interosseous branch), medial half (ring/little fingers) by ulnar nerve. The flexor carpi ulnaris is also ulnar nerve. Posterior compartment of the forearm (extensors — 2 layers, 12 muscles): wrist extensors, finger extensors, and supinators. All innervated by the radial nerve (posterior interosseous branch). The rule of thumb: anterior = flexors = median nerve (with ulnar nerve contribution). Posterior = extensors = radial nerve. This covers about 80% of upper limb innervation. The exceptions are what exams focus on. AnatomyIQ traces each muscle's origin, insertion, action, and innervation when you snap a photo of an anatomy diagram — and highlights which compartment it belongs to so you can learn the pattern, not just the individual muscle. This content is for educational purposes only and does not constitute medical advice.

The rotator cuff is the most tested shoulder topic. Four muscles — SITS: Supraspinatus, Infraspinatus, Teres minor, Subscapularis — that stabilize the glenohumeral joint by holding the humeral head in the shallow glenoid fossa. Supraspinatus: origin = supraspinous fossa of scapula. Insertion = greater tubercle of humerus (superior facet). Action = initiates abduction of the arm (first 15-30 degrees). Nerve = suprascapular nerve (C5-C6). Clinical: the most commonly torn rotator cuff muscle. A positive empty can test (pain or weakness when resisting downward force on the arm held at 90° abduction with thumbs pointing down) suggests supraspinatus tear or impingement. Infraspinatus: origin = infraspinous fossa. Insertion = greater tubercle (middle facet). Action = external rotation of the shoulder. Nerve = suprascapular nerve. Clinical: weakness of external rotation suggests infraspinatus tear or suprascapular nerve injury. Teres minor: origin = lateral border of scapula. Insertion = greater tubercle (inferior facet). Action = external rotation (same as infraspinatus). Nerve = axillary nerve (C5-C6). The axillary nerve also innervates the deltoid — so an axillary nerve injury (from shoulder dislocation or surgical neck fracture) causes both deltoid weakness (can't abduct) and teres minor weakness (weak external rotation). Subscapularis: origin = subscapular fossa (anterior surface of scapula). Insertion = lesser tubercle (the only rotator cuff muscle on the lesser tubercle — the other three attach to the greater tubercle). Action = internal rotation. Nerve = upper and lower subscapular nerves (C5-C6). Clinical: the lift-off test (patient pushes hand away from their lower back against resistance) tests subscapularis strength. Deltoid: not part of the rotator cuff but the most prominent shoulder muscle. Three parts: anterior (flexes and internally rotates), middle (abducts — the primary abductor after supraspinatus initiates), posterior (extends and externally rotates). All innervated by the axillary nerve. The deltoid is tested by resisting shoulder abduction at 90° — if the patient cannot hold the arm up against gravity, the axillary nerve or deltoid is compromised.

Arm — anterior compartment (flexors): Biceps brachii is the star — two heads (long head from supraglenoid tubercle, short head from coracoid process), inserts on radial tuberosity. Actions: powerful elbow flexion AND supination (turning the palm up — biceps is actually a stronger supinator than flexor). Test: resist elbow flexion with forearm supinated. Biceps reflex tests C5-C6. Brachialis sits deep to biceps, origin from anterior humerus, inserts on coronoid process of ulna. It is the pure elbow flexor (does not supinate). All innervated by musculocutaneous nerve. Arm — posterior compartment (extensors): Triceps brachii — three heads (long from infraglenoid tubercle, lateral and medial from posterior humerus). Inserts on olecranon of ulna. Action: elbow extension. Innervated by radial nerve. Triceps reflex tests C7. The radial nerve spirals around the posterior humerus in the radial groove — a midshaft humerus fracture can damage it, causing wrist drop (loss of wrist and finger extension). Forearm — the key clinical muscles: Flexor digitorum superficialis (FDS) — the intermediate layer flexor that flexes the PIP joints of digits 2-5. Median nerve. Flexor digitorum profundus (FDP) — the deep flexor that flexes the DIP joints. Split innervation: lateral half (digits 2-3) by anterior interosseous nerve (branch of median), medial half (digits 4-5) by ulnar nerve. This split is high-yield: an anterior interosseous nerve injury causes inability to flex the DIP of the index finger and the IP of the thumb (cannot make an OK sign — the pinch test). Extensor digitorum — the primary finger extensor in the posterior compartment. Extends the MCP joints of digits 2-5. Radial nerve (posterior interosseous branch). Wrist drop from radial nerve palsy involves inability to extend the wrist AND the MCP joints — the fingers hang limp. AnatomyIQ organizes muscle data by compartment and tests you on the clinical correlations — not just isolated origins and insertions.

The hand intrinsic muscles are almost entirely innervated by two nerves: the ulnar nerve (most intrinsics) and the median nerve (thenar muscles and lateral 2 lumbricals). Thenar muscles (the fleshy mound at the base of the thumb): opponens pollicis, abductor pollicis brevis, flexor pollicis brevis — all median nerve (recurrent branch). These muscles allow thumb opposition (touching the thumb tip to each fingertip). Carpal tunnel syndrome compresses the median nerve as it passes under the transverse carpal ligament — causing numbness in the lateral 3.5 digits and weakness of thumb opposition in advanced cases. Thenar atrophy (wasting of the thumb mound) is a late sign of carpal tunnel. Hypothenar muscles (small finger side): opponens digiti minimi, abductor digiti minimi, flexor digiti minimi brevis — all ulnar nerve. These are less commonly tested individually but ulnar nerve injury produces hypothenar atrophy visible on inspection. Interossei (between the metacarpals): dorsal interossei (4 muscles) — ABduct the fingers (spread them apart). Mnemonic: DAB = Dorsal ABduct. Palmar interossei (3-4 muscles) — ADduct the fingers (bring them together). Mnemonic: PAD = Palmar ADduct. All innervated by the ulnar nerve (deep branch). Interosseous weakness from ulnar nerve injury produces the claw hand deformity in digits 4-5: the MCP joints hyperextend (because the intrinsics normally flex them) and the IP joints flex (because the long flexors are unopposed). Lumbricals (4 muscles — one per finger, not thumb): these unique muscles originate from the tendons of FDP and insert into the extensor expansion. Action: flex the MCP joints while extending the IP joints (the position you use to hold a book open). Lumbricals 1-2 (lateral): median nerve. Lumbricals 3-4 (medial): ulnar nerve. This median/ulnar split matches the FDP split — because the lumbricals originate from FDP. The clinical bottom line: median nerve = thumb opposition + lateral 3.5 digit sensation. Ulnar nerve = finger abduction/adduction + claw hand in digits 4-5 + medial 1.5 digit sensation. Radial nerve = wrist and finger extension (no hand intrinsics). These three nerve injury patterns are tested on every anatomy, Step 1, and nursing exam. AnatomyIQ generates hand anatomy practice questions that test both the intrinsic muscle innervation patterns and the clinical nerve injury presentations — building the connections that exams test.