Bursitis of the shoulder is irritation or inflammation of the bursa, a sac that rests between bone, tendons, muscles and skin in the shoulder joint. The purpose of the bursa is to decrease friction and rubbing, according to WebMD.
The most frequent cause of shoulder bursitis is repetitive impact on an area within the shoulder or a sudden, more traumatic injury. Age is also a contributing factor, as tendons lose their elasticity and stress tolerance over time, explains WebMD. Overuse is another contributing factor, as is incorrect posture. Pain, either swift and severe or building gradually, is the predominant symptom.