The Supraspinatus muscle runs along the top of the shoulder blade and inserts via the tendon at the top of the arm (humerus bone). This muscle is used to lift the arm up sideways and is also important in throwing sports as it is the muscle that holds the arm in the shoulder when you release what you are throwing. There are massive forces involved in slowing the arm down after you have thrown something but few people bother to train these muscles. A heavy fall onto the shoulder can also result in injuring this muscle.
Over the tendon is a bursa (small sack of fluid used to help lubricate the moving tendon). This bursa can become trapped in the shoulder causing pain and inflammation. The athlete is more prone to this injury if they overuse the shoulder particularly if the arm is at or above shoulder level. Or if the athlete has had a rupture of the supraspinatus tendon.