The anconeus muscle is a small triangular elbow muscle connecting the medial side of the ulna to the lateral side of the humerus. It functions mainly to allow a person to extend his elbow and rotate the forearm. This action is used to carry something, as a food server would carry a tray. Because of its location and function, many anatomists believe it is more accurately categorized as an extension of the triceps brachii muscle. The anconeus muscle is not considered to be a critical piece of the arm's musculature. However, its absence would make it more difficult to extend the arm. It also functions to prevent the ulna and humerus from pinching the elbow joint capsule when the arm is flexed and released. An injury or strain to the anconeus muscle may result in tennis elbow. This is a painful condition that makes it difficult to bend the elbow or carry any weight with a bent elbow. Notably, the anconeus muscle is only one of several muscles in the same group. It is not necessarily involved when such an injury occurs.