Only one major muscle is involved in elbow joint extension - the triceps brachii, which is a large muscle that covers the entire back side of the upper arm. It is divided into three sections, known as the lateral (outer) head, medial (middle) head, and long (inner) head. The lateral head originates on the back of the humerus from the middle of the shaft to almost the very top. The medial head originates on the lower portion of the back of the humerus over a wide space extending nearly two-thirds of the length of the bone. The long head of the triceps originates on the scapula just below the shoulder joint.
All three heads come together into a common tendon which inserts on the olecranon process of the ulna. It should be noted that the olecranon process extends beyond the elbow joint and prevents excessive hyperextension. To work all portions of the triceps muscle maximally, increasingly heavy weights must be used. For example, when light or moderately heavy weights are used, only the medial head of the triceps goes into action. As the resistance increases, the lateral head joins in, and when sufficiently heavy weights are used, the long head goes into contraction. Because the radius also articulates with the humerus it can also be considered part of the elbow. The annular ligament which encircles the head of the radius and attaches to the ulna, allows the radius to rotate around the ulna on a longitudinal axis of the forearm to provide for pronation and supination. The only movements possible at the elbow joint are flexion and extension.