The shoulder joint has three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone). The acromioclavicular (AC) joint is located between the acromion (part of the scapula that forms the 'shoulder cap') and the clavicle. The glenohumeral joint, commonly called the shoulder joint, is a ball-and- socket type joint that allows a wide range of motion. It is surrounded by the capsule, a soft but strong tissue that helps stabilize the joint. The labrum is a rim of cartilage that helps deepen the shallow socket and helps with shoulder stability.
There are many muscles used during motion of the shoulder joint. One of the most important, the rotator cuff, is a group of muscles and tendons that provide strength and stability to the ball and socket.