What causes shoulder tendonitis? The rotator cuff consists of four separate muscles, each of which has a tendon that attaches to different parts of the shoulder blade. These tendons form a cuff around the head of the humerus, and the muscles of this group work together to control and stabilize the shoulder. As a ball-and-socket joint, the rotator cuff helps secure the ball portion of the shoulder as the arm moves and rotates within the socket. Shoulder tendonitis occurs when these tendons are injured and become inflamed as a result. The most common cause of shoulder tendonitis is overuse of the shoulder. This means the tendons in the rotator cuff are overworked beyond what they can handle, and over time they eventually become damaged.