Muscle · Forearm

Pronator quadratus

Musculus pronator quadratus

A flat square of muscle lying across the front of the two forearm bones just above the wrist, deep to every flexor tendon in the region, with its fibres running straight across from the ulna to the radius. It is the prime mover of turning the palm downward, and pronator teres higher up joins in only when speed or force is needed. Its deep fibres do a second job that is easy to overlook: they hold the lower ends of the radius and ulna together, so the joint the two bones make at the wrist stays congruent while the forearm rotates through its full arc.