As a result of wear and tear, overuse or possibly an accident, some of the muscles deep inyour shoulder (called the rotator cuff) have worn. This means it is much harder to raise your arm. Unfortunately it is very difficult and often impossible to repair these muscles. There is, however, another powerful muscle on the outside of your arm (the deltoid muscle) that may be re-trained to do some of the work of the torn rotator cuff muscle.