Pronator quadratus is a square shaped muscle on the distal forearm

Forearm

-See also:*Forearm flexors*Forearm muscles...

 that acts to pronate (turn so the palm faces downwards) the hand

Hand

A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

.

As it is on the anterior side of the arm, it is innervated by a branch of the median nerve

Median nerve

The median nerve is a nerve in humans and other animals. It is in the upper limb. It is one of the five main nerves originating from the brachial plexus....

, the anterior interosseous nerve (roots C8 and T1 with T1 being primary). Arterial blood comes via the interosseous artery.

Origin and insertion

Its fibers run perpendicular to the direction of the arm, running from the most distal quarter of the anterior ulna

Ulna

The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...

 to the distal quarter of the anterior radius

Radius (bone)

The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...

. It is the only muscle that attaches only to the ulna at one end and the radius at the other end.

Action

When pronator quadratus contracts, it pulls the lateral side of the radius towards the ulna, thus pronating the hand. Its deep fibers serve to keep the two bones in the forearm bound together. It also aids in wrist extension.