The rotator cuff is a group of four muscles that surround the shoulder joint like a “cuff.” The rotator cuff overlaps the shoulder joint capsule that, together with tendons, keep the ball of the humerus (bone of the forearm) attached in its socket. The structure of the rotator cuff affords the arm a great amount of mobility—much more than that of the ball and socket of the hip joint. Because the boney structure underneath the rotator cuff relies on this soft tissue so much, the rotator cuff is especially prone to overuse and associated injuries.