Bursitis is caused by inflammation of a bursa, a small fluid-filled sac that functions as a gliding surface to reduce friction between muscles, tendons, and joints during movement. There are 160 bursae located throughout the body. The major bursae are adjacent to the tendons near the large joints, such as the shoulders, elbows, hips, knees, and heels.
In the shoulder there are three bursae, and each of these can become inflamed and swell with more fluid. The most common causes of bursitis are repetitive motions or positions that irritate the bursae such as throwing or lifting overhead repeatedly, leaning on your elbows or kneeling for long periods, or prolonged sitting, particularly on hard surfaces. Symptoms of bursitis include joint pain and tenderness when you press around the joint, stiffness and aching when you move the affected joint, or swelling, warmth or redness over the joint.