The pronator quadratus muscle is one of the muscles of the forearm. Anteriorly, it is within the deepest layer it is the most distal. It is quadrangular in shape.
It arises from the distal quarter of the anterior and radial aspect of the ulna. It passes transversely to insert into the distal quarter of the ulnar aspect of the radius.
It is innervated by the median nerve by its anterior interosseous nerve branch (C8, T1).
Pronator quadratus acts to pronate the forearm with the assistance of pronator teres. Also, it helps to maintain the close relationship of the radius and ulna when for example, they are pushed apart by falling onto the outstretched hand.