Muscles supinating the forearm

Supinator

Biceps brachii

Brachioradialis

Supinator

Supinator lies deep in the upper part of the forearm concealed by the superficial muscles as it surrounds the upper end of the radius. The two heads of the supinator arise in a continuous manner from the inferior aspect of the lateral epicondyle of the humerus, the radial collateral ligament, the annular ligament, the supinator crest and fossa of the ulna. It is often convenient, however, to think of the supinator as arising by two heads, the humeral and ulnar, between which passes the posterior interosseous nerve to gain access to the extensor compartment of the forearm. From this extensive origin, the muscle fibers pass downwards and laterally to wrap around the upper third of the radius. They insert into the posterior, lateral and anterior aspects of the radius, as far forwards as the anterior margin between the neck and the attachment of pronator teres.

Nerve supply

Supinator is supplied by the posterior interosseus branch of the radial nerve, root value C5 and 6. However, the skin overlying the supinator is supplied by roots C5, 6 and T1.

Action

Supinator, as its name suggests, produces supination of the forearm, in which there is an anterolateral swing of the lower end of the radius around the ulna causing the two bones to lie parallel to each other. Unless a particularly powerful supinatory action is required, supinator is probably the prime mover. However, if a powerful movement is required, biceps brachii is recruited to produce the movement. It must be remembered, however, that biceps brachii cannot function as a supinator with the elbow fully extended, and consequently powerful supinatory movements are performed with the elbow flexed to about 120°.

Palpation

With the arm fully extended at the elbow, and the forearm in a mid-pronated position, supinator can be felt contracting over the posterior part of the upper third of the radius when the arm is supinated against resistance.