The Supinator has superficial and deep layers between which the
posterior interosseus nerve passes.
The muscle has the form of a parallelogram. Its fibres curl in a
distal direction around the proxial third of the radial shaft.
The two parts arise together - the superficial by tendinous and the deep by muscular fibres- from the lateral epicondyle of the humerus,from the radial collateral ligament of the elbow joint and the annular ligament of the superior radio-ulnar joint, from the supinator crest of the ulna and the posterior part of the triangular depression in front of it.
The distal attachment is to the lateral surface of the proximal third of the radius down to the insertion of the pronator teres.
The muscle is covered proximally by the anconeus(1) and the extensor carpi ulnaris (2) and distally by the muscle belly of the extensores carpi radialis longus and brevis (4) and the brachoradialis (3).