Arising from the lower fourth of the ulna, especially from the sinuous ridge on its anteromedial aspect, the fibres are inserted, superficially into the ridge on the anterolateral border of the radius above the styloid process, more deeply into the anterior surface of the lower end of the radius, and most deeply into the triangular interosseous area just above the ulnar notch. The interosseous membrane clings to the posterior border of this triangular area, the sacciform recess of the distal radioulnar joint lies just in front of it, and the muscle is inserted into the remainder of the area.
Nerve supply of pronator quadratus:
By the anterior interosseous branch of the median nerve (C8).
Action of pornator quadratus:
The superficial fibers pronate the forearm (note the extent of separation of the ridges of attachment in pronation and supination of the dried bones); running transversely, they are well placed for pronating. The deepest fibers possess almost the same length in pronation and supination, and their action would appear to be chiefly concerned with maintaining apposition of the lower ends of radius and ulna. The muscle is too deep to be palpated.