Brachioradialis The structure indicated is the brachioradialis muscle of the forearm. The brachioradalis muscle is one of the muscles of the posterior compartment of the forearm. The posterior compartment consists of a superficial layer of muscles comprised of 7 muscles. The deep layer is comprised of 5 muscles. Origin: lateral supracondylar ridge of humerus Insertion: radial styloid process Innervation: radial nerve Action: flexes the forearm at the elbow – strongest when forearm is midway between pronation and supination. Learn more about the muscles of the forearm in this tutorial.