The pronator quadratus is a deep muscle of the anterior forearm, located on the palm-side of the arm above the wrist. Responsible for pronation, or rotating the forearm palm-down, it acts on the radioulnar joint, the articulation between the radial and ulna bones just above the wrist in the lower arm. For this action this muscle is significant to players of racket sports such as tennis or ping-pong as well as to swimmers, who use it to pronate the forearm during the freestyle stroke. As such these populations may be at an increased risk for pronator quadratus overuse injuries like tendinitis.
Originating on the distal quarter, or lowest 25 percent, of the anterior side of the ulna, the pronator quadratus is distinguishable by it horizontal fibers. Running perpendicular to the length of the arm rather than parallel like those of most muscles, its fibers cross the forearm laterally to insert along the radius, the bone nearest the thumb side of the arm. Of the forearm muscles, it is the only one with one end attaching to the ulna only and the other end attaching to the radius only.
Along with the pronator teres, which is located nearer to the proximal or upper end of the anterior forearm, the pronator quadratus is one of only two muscles responsible for pronation. In anatomical terms, this is the position in which he arms are at the sides with the palms facing forward. Pronation is the act of rotating the palms backward without an accompanying rotation in the shoulder joint. From anatomical position, the hands would simply turn to face the other direction.
This specific forearm movement initiated by the pronator quadratus and pronator teres involves changing the position of the radius and ulna relative to one another. In anatomical position, the ulna is medial, or closer to the midline of the body, and the radius is lateral, or farther from the midline, and the two are parallel. When pronation occurs, contractions of the pronator quadratus muscle pull the distal or bottom end of the radius anteriorly and medially to the ulna. In other words, as the forearm rotates palm-down, the radius bone crosses in front of the ulna bone toward the midline of the body so that the two form an "X." As the muscle lengthens back to its neutral position, so the bones return to their neutral, parallel position.