Supinator has two heads: the superficial, arising from the anterior aspect of the lateral epicondyle, and the deep arising from the supinator crest of the ulna. It inserts into the proximal third of the radius.
It is found in the deep extensor compartment of the posterior forearm and as such cannot be palpated.
Biceps is only a weak supinator when the elbow is extended so in such positions supinator becomes the principal muscle to supinate.
It is supplied by the posterior interosseous branch of the radial nerve.