The rotator cuff muscle sits between two bones: the shoulder blade and upper arm. It is made up of four muscles. Injury or overuse, especially in sports, can inflame it. Then, when you lift your arm, the shoulder blade pushes, or impinges, on the muscles, tendons, and bursa around it. This often causes pain and tenderness, especially in the front of your shoulder. Rest and icing are important. Anti-inflammatory meds and physical therapy can help, too.