Tennis elbow is an injury to the muscles and tendons on the outside (lateral aspect) of the elbow resulting from overuse or repetitive stress. The narrowing of the muscle bellies in the forearm, as they merge into the tendons, creates highly focused stress in the areas where they insert into the bone of the elbow. Injury to the lateral aspect of the elbow is the most common upper extremity tennis infliction. Tennis elbow is generally caused by overuse of the extensor tendons in the forearm, particularly the extensor carpi radialis brevis.