Supinator

Original Editor - Kate Sampson,

Top Contributors - Kate Sampson, Kim Jackson, Nina Myburg, 127.0.0.1 and Joao Costa;

The supinator muscle is located in the posterior compartment of the forearm. As its name suggests, it is the prime supinator of the forearm.

Structure

is a broad muscle in the posterior compartment of the forearm, curved around the upper third of the radius. Supinator consists of two planes of fibers, between which the deep branch of the radial nerve ls. The two planes arise in common — the superficial one by tendinous (the initial portion of the muscle is actually just tendon) and the deeper by muscular fibers —^[2] from the supinator crest of ulna, the lateral epicondyle of humerus, the radial collateral ligament, and the annular radial ligament.^[1]

The superficial fibers (pars superficialis) surround the upper part of the radius, and are inserted into the lateral edge of the radial tuberosity and the oblique line of the radius, as low down as the insertion of the pronator teres. The upper fibers (pars profunda) of the deeper plane form a sling-like fasciculus, which encircles the neck of the radius above the tuberosity and is attached to the back part of its medial surface; the greater part of this portion of the muscle is inserted into the dorsal and lateral surfaces of the body of the radius, midway between the oblique line and the head of the bone.^[2]

The proximal aspect of the superficial head is known as the arcade of Frohse or the supinator arch.

Origin

and has a broad origin from the ulna and humerus and extends down to the posterior surface of the proximal third of the radius.

- Lateral epicondyle of the humerus

- Annular ligament

- Supinator Cest fossa of the ulna

Insertion

- Proximal third of the radius on the anterolateral surface, distal to the radial tuberosity.

Innervation

Supinator is supplied by the posterior interosseous brach of the radial nerve (C5, C6)

It is innervated by the deep branch of the radial nerve. The deep branch then becomes the posterior interosseous nerveupon exiting the supinator muscle

The radial nerve divides into deep and sensory superficial branches just proximal to the supinator muscle — an arrangement that can lead to entrapment and compression of the deep part, potentially resulting in selective paralysis of the muscles served by this nerve (the extensor muscles and the abductor pollicis longus.)^[3] Many possible causes are known for this nerve syndrome, known as supinator entrapment syndrome, including compression by various soft-tissued masses surrounding the nerve, and stress caused by repetitive supination and pronation

Function

Supinator supinates the forearm. It causes the distal end of the radius to move anterolaterally around the ulna, causing them to lie paralell

Supinator works with biceps brachii if powerful supination is required. However biceps brachii is unable to function to supinate when the elbow is extended

Encircling the radius, supinator brings the hand into the supinated position. In contrast to the biceps brachii, it is able to do this in all positions of elbow flexion and extension.^[1]

Supinator always acts together with biceps, except when the elbow joint is extended.^[6] It is the most active muscle in forearm supination during unresisted supination, while biceps becomes increasingly active with heavy loading. Supination strength decreases by 64% if supinator is disabled by, for example, injury.^[7]

Palpation

Place the patients arm in extension at the elbow and the forearm in midposition. Actively resist supination and palpate along the posterior part of the proximal third of the radius.