Forearm
Antebrachial fascia: deep fascia enclosing the forearm, continuous with brachial fascia proximally and the flexor/extensor retinacula distally; sends septa to the radius and ulna, dividing the forearm into anterior (flexor) and posterior (extensor) compartments.
- Interosseous membrane: strong fibrous sheet connecting the shafts of radius and ulna; fibres run obliquely, mostly from the radius (proximal) to the ulna (distal), transmitting load from the radius (which bears the wrist) to the ulna (which bears the elbow). Provides additional origin for deep forearm muscles and stabilises the radio-ulnar joints.
- Anterior compartment: flexors and pronators, mostly supplied by the median nerve (with the ulnar nerve supplying FCU and the medial half of FDP).
- Posterior compartment: extensors and the supinator, supplied entirely by the radial nerve (via its deep/posterior interosseous branch, except brachioradialis and ECRL which are supplied by the radial nerve trunk itself).
Superficial Layer
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Pronator teres | Humeral head: medial epicondyle; ulnar head, coronoid process | Lateral surface, mid-shaft of radius | Median nerve (C6, C7) | Pronation; weak elbow flexion |
| Flexor carpi radialis | Medial epicondyle | Base of 2nd (and 3rd) metacarpal | Median nerve (C6, C7) | Wrist flexion, radial deviation |
| Palmaris longus | Medial epicondyle | Palmar aponeurosis | Median nerve (C7, C8) | Wrist flexion; tenses palmar fascia (absent in ~10-15% of people) |
| Flexor carpi ulnaris | Humeral head: medial epicondyle; ulnar head, olecranon and posterior ulna | Pisiform, hook of hamate, base of 5th metacarpal | Ulnar nerve (C8, T1) | Wrist flexion, ulnar deviation |
Intermediate Layer
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Flexor digitorum superficialis | Humero-ulnar head: medial epicondyle, coronoid process; radial head, anterior oblique line of radius | Middle phalanges of digits 2–5 (splits around FDP tendon) | Median nerve (C7, C8, T1) | Flexes PIP joints (and MCP, wrist secondarily) of digits 2–5 |
Deep Layer
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Flexor digitorum profundus | Proximal ¾ of anteromedial ulna and interosseous membrane | Distal phalanges of digits 2–5 | Lateral half: anterior interosseous nerve (median); medial half: ulnar nerve | Flexes DIP joints (and PIP, MCP, wrist secondarily): the only muscle that flexes the DIP joints |
| Flexor pollicis longus | Anterior radius, interosseous membrane | Distal phalanx of thumb | Anterior interosseous nerve (median) (C8, T1) | Flexes thumb IP joint |
| Pronator quadratus | Distal anterior ulna | Distal anterior radius | Anterior interosseous nerve (median) (C8, T1) | Pronation (prime mover, especially in fast/repeated movements); also holds radius and ulna together distally |
Median nerve (in forearm): Enters between the two heads of pronator teres, runs deep to FDS (between FDS and FDP), giving off the anterior interosseous nerve just distal to pronator teres. Supplies all anterior forearm muscles except FCU and the medial (ulnar) half of FDP.
Anterior interosseous nerve: Branch of median nerve; runs on the interosseous membrane with the anterior interosseous artery; supplies FPL, the lateral half of FDP, and pronator quadratus, purely motor (no cutaneous sensory territory), though it does supply the wrist joint.
Ulnar nerve (in forearm): Passes posterior to the medial epicondyle, then between the two heads of FCU; runs on FDP, medial to the ulnar artery in the distal forearm. Supplies FCU and the medial half of FDP only in the forearm; gives off the dorsal cutaneous branch proximal to the wrist.
Radial artery: Arises from brachial artery bifurcation. Runs deep to brachioradialis (a useful surface landmark for the vessel), lateral to the tendons of FCR, giving muscular branches and the radial recurrent artery (contributes to elbow anastomosis). It becomes superficial and palpable at the wrist just medial to the radial styloid (classic pulse site) before winding around into the anatomical snuffbox.
Ulnar artery: Larger of the two terminal branches; passes deep to pronator teres and FDS, gives off the common interosseous artery (which splits into anterior and posterior interosseous arteries supplying the deep forearm compartments). It runs with the ulnar nerve (medial to it) through the distal forearm into Guyon canal.
Superficial Muscles
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Brachioradialis | Lateral supracondylar ridge of humerus | Base of radial styloid process | Radial nerve (C5, C6) | Elbow flexion (strongest in mid-pronation); brings forearm to neutral from full pronation or supination |
| Extensor carpi radialis longus | Lateral supracondylar ridge of humerus | Base of 2nd metacarpal | Radial nerve (C6, C7) | Wrist extension, radial deviation |
| Extensor carpi radialis brevis | Lateral epicondyle (common extensor origin) | Base of 3rd metacarpal | Deep branch of radial nerve (C7, C8) | Wrist extension |
| Extensor digitorum | Lateral epicondyle | Extensor expansions of digits 2–5 | Posterior interosseous nerve (C7, C8) | Extends MCP joints (and IP joints via extensor expansion) of digits 2–5 |
| Extensor digiti minimi | Lateral epicondyle | Extensor expansion of 5th digit | Posterior interosseous nerve (C7, C8) | Extends little finger |
| Extensor carpi ulnaris | Humeral head: lateral epicondyle; ulnar head, posterior ulna | Base of 5th metacarpal | Posterior interosseous nerve (C7, C8) | Wrist extension, ulnar deviation |
Deep Muscles
| Muscle | Origin | Insertion | Innervation | Action |
|---|---|---|---|---|
| Supinator | Lateral epicondyle, radial collateral and annular ligaments, supinator crest of ulna | Proximal lateral shaft of radius | Deep branch of radial nerve (C5, C6) | Supination (prime mover with forearm extended; biceps dominates when flexed) |
| Abductor pollicis longus | Posterior radius, ulna, interosseous membrane | Base of 1st metacarpal | Posterior interosseous nerve (C7, C8) | Abducts and extends the thumb CMC joint |
| Extensor pollicis brevis | Posterior radius, interosseous membrane | Base of proximal phalanx of thumb | Posterior interosseous nerve (C7, C8) | Extends thumb MCP joint |
| Extensor pollicis longus | Posterior ulna, interosseous membrane | Base of distal phalanx of thumb | Posterior interosseous nerve (C7, C8) | Extends thumb IP joint; adducts thumb |
| Extensor indicis | Posterior ulna, interosseous membrane | Extensor expansion of index finger | Posterior interosseous nerve (C7, C8) | Extends index finger independently |
Deep branch of radial nerve: Arises in the cubital fossa, pierces supinator (passing through the arcade of Frohse, a potential entrapment site, posterior interosseous nerve syndrome), emerges on the posterior forearm as the posterior interosseous nerve.
Posterior interosseous nerve: Continuation of the deep branch of radial nerve after passing through supinator. Runs on the interosseous membrane with the posterior interosseous vessels. Supplies all posterior forearm muscles except brachioradialis and ECRL (both supplied by the radial nerve trunk proper, before the deep branch forms). Purely motor distally (no cutaneous territory of its own), though it supplies the wrist joint.
Posterior interosseous vessels: Posterior interosseous artery (from the common interosseous branch of the ulnar artery) accompanies the nerve, supplying the posterior compartment; anastomoses with the anterior interosseous artery near the wrist.
- Superior (proximal) radio-ulnar joint: Pivot synovial joint; radial head rotates within the ring formed by the annular ligament and the radial notch of the ulna; shares the elbow joint's synovial cavity and capsule.
- Interosseous membrane: Connects the shafts of radius and ulna; fibres run obliquely, distally from the radius; binds the two bones together, transmits force from radius to ulna, and provides an axis for rotation; also gives origin to deep forearm muscles.
- Inferior (distal) radio-ulnar joint: Pivot synovial joint between the head of the ulna and the ulnar notch of the radius. Stabilised by the triangular fibrocartilage complex (TFCC), which also separates the joint from the wrist joint proper and cushions the ulnar side of the carpus.
- Pronation: Rotates the radius medially around a relatively fixed ulna, carrying the hand from palm-up to palm-down; the radial head spins in place at the proximal joint while the distal radius swings around the ulnar head.
- Supination: Reverse movement, radius rotates laterally back to the anatomical position (palm-up); considered the more powerful movement (biceps assists strongly).
- Muscles responsible:
- Pronation: Pronator teres (prime mover with resistance/speed), pronator quadratus (prime mover at rest/slow movements).
- Supination: Supinator (prime mover, forearm extended), biceps brachii (prime mover, forearm flexed, most powerful supinator overall).
- Axis of movement: An oblique axis running from the centre of the radial head (proximally) to the centre of the ulnar head (distally), the radius rotates around the relatively stationary ulna along this axis.
