The rotator cuff is a group of four muscles and two tendons that connect the humerus, or upper arm, to the scapula, or shoulder blade. The muscles and tendons include the teres minor, infraspinatus, supraspinatus, subscapularis, and supraspinatus and subscapularis tendons.
The muscles and tendons that comprise the rotator cuff allow for a wide range of motion while still providing stability to the joint. The head of the humerus articulates with a shallow groove of the shoulder blade called the acromium process. This joint is unstable and needs the rotator cuff, along with the capsular ligament surrounding the joint, to remain in articulation. This delicate balance between stability and range of motion is the reason why the rotator cuff is easily injured.