The shoulder contains several muscles, including the subscapularis, supraspinatus, teres minor and infraspinatus. The muscles and their connected tendons comprise the rotator cuff, which stabilizes the shoulder and supports flexibility of motion.
Attached to the rotator cuff are fluid sacs named bursae, which minimize friction between bones, tendons and muscles as the shoulder moves. The shoulder connects to the arm by a joint at the shoulder blade, which holds the ball of the humerus bone in a socket. This connection is fairly loose, allowing for significant range of motion, but also makes the shoulder more prone to injury and dislocation than other joints.