Shoulders are miraculous. They move up, down, in, out and all around, but that versatility is also a weakness. The ball and socket joint is relatively unstable and when the rotator cuff is weak you can easily damage your shoulders playing sport or lifting weights. The rotator cuff also degenerates with age, so it's important to start training it now to protect it from injury and so you can keep smashing aces long into the future. Here's how...