Located in the forward section of the forearm, the supinator is a wide muscle comprised of two separate sets of fibres surrounding the lower branch of the radial nerve. Curving around the radius’ top third, the dual layers lift in tandem from the ulna’s supinator crest, the annular radial ligament, the radial collateral ligament, and finally, the humerus’ lateral epicondyle. The first, lying closest to the skin (the most superficial) is mainly tendon, while the other, much deeper, is fibrous muscle.
Controlled (innervated) by the radial nerve, the supinator muscle, per its name, brings the hand into a supinated (flat) position. With the sole exception of times when the arm is extended at the elbow, the supinator always works in tandem with the biceps.