The motion of the forearm is controlled by over 15 muscles. In this case we see the posterior side of the forearm, where the extensor muscles are found. Most of these muscles originate near the elbow joint, and insert at the tendons that move the wrist and hand. As their name would imply, these muscles primarily function to extend the both the wrist and hand. Deep to the extensor muscles are the muscles used for forearm supination - the outward rotation of the forearm