The ulna is one of two long bones in the lower arm or forearm. The other bone is the radius, which is a little bit smaller and shorter than the ulna. Generally, muscles are composed of tissues that can contract and return in order to cause corresponding body parts to move. The pronator quadratus is a muscle that is near the lower part of the radius.  Its function is to rotate the forearm and keep the proper distance and rotation between the ulna and radius. It is considered a deep muscle and is a quadrangle shape. The pronator quadratus muscle is unique in that it is the only muscle attached only to the radius at one end and the ulna at the other. It is also used to turn the wrist and palm of the hand. An example of a common daily action the pronator quadratus muscle would assist with is turning a screwdriver. If you were to fall down and land on your hand with fingers outstretched, the pronator quadratus muscle would help to maintain the spatial relationship between the radius and the ulna and thereby prevent injury.