Shoulder is the part of a person’s body between the neck and the arm. It includes bones and muscles that attach the arm to the body. The shoulder consists of three bones—a broad, flat scapula (shoulder blade), the slender clavicle (collarbone), and the humerus (bone of the upper arm). The chief muscles of the shoulder are the large deltoid muscle and the rotator cuff muscles. The rotator cuff is a group of muscles and tendons that hold the arm firmly in the joint and help rotate the shoulder.
The shoulder is the most mobile joint in the body. The humerus has a round head that fits into a shallow depression of the scapula to form the shoulder joint. Another joint formed near the front of the shoulder by the clavicle and the scapula permits the scapula to twist. Movement also occurs between the scapula and ribs.
The shoulder muscles also help the arm move. The deltoid muscle raises the upper arm. The rotator cuff muscles also raise the arm, and they rotate the upper arm as well. The combined action of the joints with support from the muscles allows for free movement in all directions.
The shoulder joint becomes dislocated more easily than many other joints. Many athletes dislocate shoulders by falling on their outstretched arm or being pushed hard from behind.
