Deep extensors of the forearm

Supinator muscle

The supinator muscle lies in the proximal aspect of the forearm surrounding the upper third of the radius. It originates by two muscle layers (heads) from the lateral epicondyle of humerus, radial collateral and annular ligaments and supinator crest of ulna. The muscle fibers extend obliquely distally and laterally to encompass the proximal third of the radius by inserting onto its lateral, posterior, and anterior surfaces.

The supinator muscle is innervated by the posterior interosseous nerve (C7, C8). The superficial layer of the supinator muscle receives its blood supply via the radial recurrent branch of the radial artery, while its deep part is supplied via the anterior and posterior interosseous recurrent branches of the ulnar artery.

The prime action of the supinator muscle is to rotate the radius at the proximal radioulnar joint, bringing the radius in a parallel position to the ulna. This results in the supination of the forearm, which is essentially an external rotation of the forearm in which the palm of the hand is facing upwards.

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Abductor pollicis longus muscle

The abductor pollicis longus is a long muscle that originates from the posterior aspect of the proximal half of the radius and ulna, and the adjacent interosseous membrane. The muscle then runs distally and ends in a tendon, which inserts onto the base of metacarpal bone 1 and the trapezium bone. Before inserting, the tendon of abductor pollicis longus forms the lateral border of the anatomical snuffbox.

The abductor pollicis longus receives its innervation from the posterior interosseous nerve (C7, C8) and its blood supply via the anterior and posterior interosseous arteries. 

The name of the muscle points to its prime function of abducting the thumb at the first metacarpophalangeal joint. Additionally, the abductor pollicis longus is an important contributor to the extension of the thumb at the metacarpophalangeal joint, and abduction of the hand (radial deviation) at the radiocarpal joint.

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Extensor pollicis longus muscle

The extensor pollicis longus is a slender muscle that extends across the distal half of the forearm. It arises from the posterior surface of the middle third of the ulna and the interosseous membrane, and runs distally and medially across the distal radioulnar joint. Just proximal to the wrist, the extensor pollicis longus extends into a tendon that inserts onto the base of the distal phalanx of the thumb.

Like other muscles from this group, the extensor pollicis longus receives its nervous supply via the posterior interosseous nerve (C7,C8). Its blood supply comes from the posterior and anterior interosseous arteries. 

The prime function of extensor pollicis longus is to extend the thumb in the metacarpophalangeal and interphalangeal joints. This action occurs in synergy with the action of extensor pollicis brevis muscle. Additionally, extensor pollicis longus contributes to the extension and abduction of the hand at the wrist joint. 

Extensor pollicis brevis muscle

The extensor pollicis brevis is a short, posterior forearm muscle that originates from the distal third of the radius and the adjacent interosseous membrane, and inserts onto the base of the proximal phalanx of the thumb. 

The extensor pollicis brevis muscle is innervated by the posterior interosseous nerve (C7, C8). Its blood supply is provided via the posterior and anterior interosseous arteries. 

This muscle primarily acts as an extensor of the thumb at the carpometacarpal and metacarpophalangeal joints. This action occurs together with the action of its long counterpart, the extensor pollicis longus muscle. Like other muscles from this group, the extensor pollicis brevis contributes to the movements in the wrist joint, primarily to the extension of the hand.

Extensor indicis muscle

The extensor indicis is a narrow fusiform muscle found in the distal aspect of the forearm. It originates from the distal third of the ulna and the adjoining interosseous membrane. The muscle fibers run distally towards the hand converging onto a narrow tendon which inserts on the extensor expansion of the index finger. 

The extensor indicis muscle is innervated by the posterior interosseous nerve (C7, C8) and receives its blood supply via the posterior and anterior interosseous arteries. 

Like its name suggests, the prime action of extensor indicis is the extension of the index finger at the metacarpophalangeal and interphalangeal joints. As it crosses the wrist joint, the extensor indicis also contributes to the extension of the hand.

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