The shoulder jointjointThe junction or articulation of two or more bones that permits varying degrees of motion between the bones. has the largest range of motion out of all the joints in the body. This flexibility allows people to hit a backhand swing in tennis, stretch to reach something on a top shelf or lift heavy items. The shoulder also helps people position their hands for movements such as waving goodbye or using the mouse on their computers.

With all of this flexibility, however, injury can result—along with a variety of painful problems. The shoulder consists of three bones:

• Shoulder blade (scapula)
• Collar boneboneThe hard tissue that provides structural support to the body. It is primarily composed of hydroxyapatite cryst... (clavicleclavicleThe collarbone.)
• Upper arm bone (humerushumerusThe bone of the arm, articulating with the scapula above and the radius and ulna below.)

Two main joints help the shoulder move easily:

• Glenohumeral jointGlenohumeral jointTrue shoulder joint.: Commonly called the shoulder joint, the glenohumeral joint helps people move their shoulders forward and backward. It also enables the arm to rotate in a circular manner or move outward.
  • The glenohumeral joint is composed of the socket on the shoulder blade (glenoid) and the "ball" at the top of the arm bone (the humerus). Uniquely, the ball of the upper arm bone is twice the size of the shoulder blade socket. One way of picturing this joint is to think of a golf ball on a tee.
• Acromioclavicular joint: The acromioclavicular joint is located between the shoulder blade (acromion) and the collar bone (clavicle).

The sternoclavicular joint is another joint in this region that isn‘t part of the shoulder joint, but is a bridge between the upper extremity and the back of the rib cage (thoraxthoraxThe chest or rib cage; also refers to the space containing the lungs and heart. There are 12 vertebral segment...).

Muscles, tendonstendonsFibrous bands, one at each end of a muscle, that connect the muscle to bones. and ligaments also support the shoulder joint. Four tendons make up what is called the rotator cuffrotator cuffThe rotator cuff is made up of four muscles and their tendons. These combine to form a "cuff" over the head of....

This complex arrangement of bones, muscles, tendons, and ligaments can be injured by an accident or overuse. Disease also can affect the shoulder. When that happens, the pain and lack of mobility can be severe enough to affect a person‘s ability to work and enjoy normal activities of daily life.