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). At the top of the humerus, these muscles and their tendons blend together, creating a “cuff” around the shoulder’s ball-and-socket joint. 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.