Anconeus

This muscle arises from a smooth facet on the lower extremity of the lateral epicondyle. It fans out to its insertion on the lateral side of the olecranon. Its lateral fibers are vertical, in the axis of the forearm, but its upper fibers are horizontal, passing transversely across the forearm.

Nerve supply of anconeus:

By the radial nerve (C7, 8) by a branch that leaves the trunk in the spiral groove and passes through the triceps muscle with the nerve to its medial head.

Action of anconeus:

The vertical fibers may act as a weak extensor of the elbow but the horizontal upper fibers plainly can have no extensor action. Their contraction could abduct and rotate the ulna were such a movement possible at the elbow joint. A slight amount of such movement is in fact possible, and the muscle contracts during pronation of the forearm.