The elbow is an example of a hinge joint or a joint moving in one direction permitting only flexion and extension. The elbow joint is formed by three bones - the humerus of the upper arm, and the bones of the forearm, the radius laterally and the ulna medially. The joint is actually formed by the trochlea of the humerus articulating with the ulna and the capitulum of the humerus articulating with the head of the radius. Although there are two sets of articulations, there is only one joint capsule and a large bursa to lubricate the joint. An extensive network of ligaments helps the elbow joint maintain its stability. The ligaments of the elbow joint include the ulnar collateral, and the radial collateral ligaments and the annular ligaments. Because so many muscles originate or insert near the elbow, it is a common site for injury. One common injury is lateral epicondylitis or "tennis elbow", which means inflammation surrounding the lateral epicondyle of the humerus. Six muscles that control backward movement (extension) of the hand and fingers originate on the lateral epicondyle. Repeated strenuous striking while the muscles are contracted and against force such as that occurring with the backhand stroke in tennis causes strain on the tendinous muscle attachments and can produce pain around the epicondyle. Rest for these muscles will usually bring about recovery.