There are four muscles that make up the rotator cuff. The tendons from each muscle encircle the top of the shoulder, much like the cuff of a sleeve goes around the entire wrist. The rotator cuff holds the humerus in the socket of the shoulder joint while still allowing it to turn, or rotate (hence the word "rotator").

Usually only one or two tendons of the rotator cuff are torn. The tear commonly occurs close to where the tendon attaches to the bone. It is rare for all four tendons to be injured at the same time.