Flexi Says: The shoulder is a ball-and-socket joint which allows the bone of the upper arm (humerus) to rotate in a circle. Multiple muscles connect the humerus to the shoulder blade (scapula) and to the collarbone (clavicle). These muscles contract and relax in different combinations to move the upper arm back, upward and forward. Some of the muscles also help stabilize the joint as the arm moves around.
