The shoulder joint connects the upper arm bone (humerus), the shoulder blade (scapula), and the collarbone (clavicle). Shoulder movement occurs between the head of the upper arm (humerus) which moves in a shallow cup of the shoulder blade (glenohumeral joint), and the shoulder blade moving on the rib cage. In combination, these joint movements make it possible for the arm to move in all directions through a wide range of motions. This complex joint must maintain a balance between stability and mobility.
Image 1: Shoulder Anatomy