Pronator quadratus

Origin: Medial, anterior surface of the ulna

Insertion: Lateral, anterior surface of the radius

Artery: Anterior interosseous artery

Nerve: Median nerve (anterior interosseous nerve)

Action: Pronates the forearm

Antagonist: Supinator muscle

Description:
The Pronator quadratus is a small, flat, quadrilateral muscle, extending across the front of the lower parts of the radius and ulna. It arises from the pronator ridge on the lower part of the volar surface of the body of the ulna; from the medial part of the volar surface of the lower fourth of the ulna; and from a strong aponeurosis which covers the medial third of the muscle. The fibers pass lateralward and slightly downward, to be inserted into the lower fourth of the lateral border and the volar surface of the body of the radius. The deeper fibers of the muscle are inserted into the triangular area above the ulnar notch of the radius—an attachment comparable with the origin of the Supinator from the triangular area below the radial notch of the ulna.
Variations.—Rarely absent; split into two or three layers; increased attachment upward or downward.