Your shoulder joint has the largest range of motion out of all the joints in your body. This flexibility allows you to hit a backhand swing in tennis, stretch to reach something on a top shelf, or lift heavy items. The shoulder also helps you position your hand for movements such as waving goodbye or using the mouse on your computer.

With this flexibility, however, injury can result — along with a variety of painful problems. The shoulder consists of 3 bones:

• shoulder blade (scapula)
• collar bone (clavicle)
• upper arm bone (humerus)

Two main joints help your shoulder move easily:

• Glenohumeral joint

  • Commonly called the shoulder joint, the glenohumeral joint (def.)Glenohumeral Joint The glenohumeral joint is composed of the glenoid (socket), the humeral head (ball) and the capsule or ligaments that surround it. It allows the arm to move in many directions. helps you move your shoulder forward and backward. It also enables your arm to rotate in a circular manner or move outward.

    The glenohumeral joint is composed of the socket on your shoulder blade (glenoid) and the "ball" at the top of your 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 (def.)AC Joint (Acromioclavicular Joint) The acromioclavicular joint is located at the tip of the shoulder where the acromion (shoulder blade) and collarbone (clavicle) come together. is located between your shoulder blade (acromion) and your collar bone (clavicle).

The sternoclavicular joint (def.)Sternoclavicular Joint The sternoclavicular joint is at the junction between the collarbone (clavicle) and breastbone (sternum). It anchors the shoulder to the wall of the chest. 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 your rib cage (thorax).

Muscles (def.)Muscles Muscles are tissues composed of fibers that contract, allowing body parts to move. One of the most important shoulder muscles is the supraspinatus muscle — one of four rotator cuff muscles., tendons (def.)Tendons Tendons are cord-like structures made of collagen that connect muscles to bones. and ligaments (def.)Ligaments Ligaments are thick fibrous bands of tissue that link two or more bones together and frequently stabilize joints. also support the shoulder joint. Four tendons make up what is called the rotator cuff (def.)Rotator Cuff The rotator cuff is a muscle tendon unit composed of four muscles that attach the humeral head ("ball" of the upper arm bone) to the scapula (shoulder blade)..

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 your ability to work and enjoy normal activities of daily life.

Learn about common shoulder problems.