Exercise is good, but improper form--specifically, lifting weights while moving in a way that is unnatural for your body--can be dangerous, especially if you're prone to a rotator cuff injury. People who bodybuild, swim or play sports that require repetitive overhead arm movements are more likely to develop this injury, which can take anywhere from two to six months to heal. That's a long time to be out of the game.Here's what you need to know about your rotator cuff--and how you can exercise to avoid this painful injury.Your Rotator Cuff: Why it MattersTo lift, move and push objects, we depend on the movements of our arms. The rotator cuff, a series of muscles and tendons gathered around the humerus, helps us do that by enabling our arms to move freely. Without it, we wouldn't be able to