Anconeus The structure indicated is the anconeus muscle of the forearm. The anconeus 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 epicondyle of humerus Insertion: olecranon, proximal surface of ulna Innervation: Radial nerve Action: assists elbow extension, stabilises the elbow joint, abducts ulna during pronation Learn about the muscles of the forearm in this tutorial.