The rotator cuff is your shoulder and comprises of a group of four muscle-tendon units that wrap around the front, top, and back of the shoulder joint. The muscles, which are connected to bones by tendons, move the shoulder in many directions and hold the humeral head (ball) and the glenoid (socket) together. A bursa, which lies over the tendons, reduces tension between the tendons and bone and protects the tendons from excessive pressure.