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Impingement is one of the most common causes of pain in the adult shoulder. It results from pressure on the rotator cuff from part of the shoulder blade (scapula) as the arm is lifted. The rotator cuff is a tendon linking four muscles: the supraspinatus, the infraspinatus, the subscapularis, and the teres minor. These muscles cover the "ball" of the shoulder (head of the humerus). The muscles work together to lift and rotate the shoulder. The acromion is the front edge of the shoulder blade. It sits over and in front of the humeral head. As the arm is lifted, the acromion rubs, or "impinges" on, the surface of the rotator cuff. This causes pain and limits movement. The pain may be due to a "bursitis", or inflammation, of the bursa overlying the rotator cuff or a "tendonitis" of the cuff itself.

In some circumstances, a painful tear of the rotator cuff may cause impingement pain. Impingement is common in both young adults and middle-aged people. Young athletes who use their arms overhead for swimming, baseball, and tennis are particulary vulnerable. Those who do repetitive lifting or overhead activities using the arm, such as paper hanging, construction, or painting are also susceptible. Pain may develop as the result of minor trauma or spontaneously with no apparent cause.