Forearm pronation is the action of rotating the forearm inward so that the palm of the hand faces backward, or in the same direction as the elbow. Though it may seem like a wrist action, the hand actually cannot rotate on the wrist independent of the forearm. Instead, forearm pronation is produced when the two parallel bones of the forearm, the ulna and radius, rotate past each other, forming an X. This motion occurs by way of several muscles in the forearm, including the pronator quadratus, pronator teres, and the flexor carpi radialis.
Between the elbow and the wrist are two long bones, the radius and the ulna. These lie parallel to one another but in reverse positions, similar to two people lying in bed but with one person’s head at the foot of the bed and feet on the pillow. The radius lies on the thumb side of the arm and articulates with the carpal bones in the base of the hand to form most of the surface area of the wrist joint and with the humerus bone in the upper arm to form a small portion of the elbow joint. Alongside the radius is the ulna, which is found on the pinky-finger side of the arm and articulates with the humerus to form most of the surface area of the elbow and with the carpus to form a small portion of the wrist.
Just below the elbow joint, the head of the radius bone, which is much narrower than the end of the ulna next to it, fills a cavity in the ulna called the radial notch. Cylindrical in shape, the radial head is held in place against the radial notch by a ring-shaped ligament called the annular ligament and can pivot either direction in this notch. The joint formed by these bones against one another is known as the proximal radioulnar joint.
Similarly, at the place where the head of the ulna meets the end of the radius above the wrist is a nearly identical joint called the distal radioulnar joint. Here, the ulnar head fits into a depression on the radius called the ulnar notch. As with the proximal joint, the ulna pivots within this notch to produce the motion of forearm pronation.
During forearm pronation, the radius bone is pulled across the front side of the ulna bone until the two bones form an elongated X and the palm is facing backward, when the arm is hanging at one’s side, or downward, when the elbow is bent to 90 degrees. The muscles that produce this action are the pronator teres in the upper forearm and the pronator quadratus in the lower forearm, muscles that cross the anterior or front side of the forearm diagonally. When they contract, they pivot the radius medially, or toward the body when the arm is hanging at one’s side. A muscle that assists in forearm pronation is the flexor carpi radialis, a flexor of the wrist joint.