Although most people associate rotator-cuff injuries with baseball pitchers, the vast majority of cases occur in ordinary people over age 55. The rotator cuff is a family of four muscles, but the supraspinatus, which runs along the top of the shoulder blade, causes most of the problems. "Its the only muscle in the body that is sandwiched between two bones. Like a sock that rubs between the heel and shoe, it eventually wears out," says Cleveland Clinic orthopaedic surgeon John Brems, MD. Trouble begins when the muscle is injured. When it swells, it has nowhere to go and rubs on the clavicle bone. This causes a disorder called impingement syndrome. Lifting your arms above your head-to brush your hair, for example-worsens the pain, and reaching behind your back is nearly impossible. Reducing the swelling is the key to resolving impingement syndrome.