The shoulder is a very complex joint with muscles, tendons, ligaments, bursae, and fibrous cartilage helping to hold it together and give it smooth motion. Four muscles and their tendons called the rotator cuff surround the shoulder joint like an envelope. These are covered by a fibrous connective tissue called the capsule.

The head of the humerus (upper arm bone) and the shoulder socket are enclosed by the capsule. Normally the capsule is large and loose. When injury or immobility damages the capsule a frozen shoulder can develop. This means the shoulder doesn't move smoothly in the socket. It gets stuck or frozen in place. The medical term for this is adhesive capsulitis.

With a frozen shoulder a typical pattern of movement restriction starts. This is known as the capsular pattern of the shoulder. The greatest loss of motion is in outward or external rotation of the shoulder. Moving the arm away from the body is also limited. This is called shoulder abduction. In a severe capsular pattern inward or internal shoulder motion is also stuck.