The anconeus is a tiny muscle located on the posterior, or back, side of the forearm, just alongside the elbow joint. It is in fact so small that some experts consider it an extension of the triceps brachii muscle on the back of the arm. Also debated is whether it should be grouped with the muscles in the posterior compartment of the forearm or with the muscles in the posterior compartment of the arm — the triceps being the only other muscle found here. Both the upper and lower arm receive their blood flow from the radial artery. The debate is over whether the anconeus should be counted among the muscles in the forearm or as a continuation of the triceps.
Situated just laterally to the olecranon process, which is the large bony protrusion of the ulna bone at the elbow joint, the anconeus can be felt by placing the fingers on the elbow and sliding them just off the bone toward the thumb side of the arm. It originates on the back of the lateral epicondyle of the humerus bone, which is one of the two rounded prongs on the bottom of the humerus. From there it borders the olecranon process and inserts on the underside of the process and along the posterior side of the ulna.
Only a few inches in length with fibers that run diagonally, the anconeus plays a minor role in extension of the arm at the elbow joint. As the three heads of the triceps muscle are the primary extensors of the elbow, this smaller muscle assists the triceps during extension. It also protects the capsule containing the elbow joint during arm extension. When the arm straightens, the bony olecranon process of the ulna pushes into a cavity on the humerus called the olecranon fossa; the anconeus prevents the joint capsule from getting crushed in this space.
Another function of the anconeus muscle is to stabilize the elbow joint. Though it is not the only muscle that crosses the joint on the back side of the arm, it is the only one that begins and ends just to either side of the elbow. With its short length, it can be thought of as a small, tight rubber band stretching across the elbow and controlling movement forces on the joint.
This location means that the anconeus is relatively protected from severe injuries, except when hyperextension of the elbow occurs, but it is susceptible to strains and tendinitis. Tennis elbow is a condition affecting not only the anconeus but the forearm supinator muscles, which rotate the forearm palm up. Overuse of these muscles and their attaching tendons, as in repeatedly swinging a tennis racket, can lead to severe inflammation — tendinitis — or even small tears in the tendon.