The rotator cuff is a set of muscles that extend from the scapula (shoulder blade) and attach to the top of the humerus (upper arm bone), creating a “cuff” around the shoulder’s ball-and-socket joint at the top of the humerus. When working properly, the four muscles of the rotator cuff help the shoulder move smoothly throughout its range of motion and stabilize the shoulder’s ball-and-socket joint. A rotator cuff tear is a common injury in sports that require repetitive overhead arm movement.