When Injured

It happens. You strain your shoulder, sprain your wrist, or even break an arm. Most people think that working out with an injury like that will be a waste of time. But that's plain wrong. In fact, by exercising your strong arm, you can actually maintain - or possibly even improve - the strength in your injured arm.

European researchers put 10 women on a two-month strength-training program for one arm only. At the end of the two months, though the women had done no additional activity with the other arm, they had increased strength in both arms.

Scientists call this the "cross-over" effect. When your brain sends instructions to one arm, the "untrained" arm receives the same instructions. The end result is that you build strength in the untrained arm because of the connection between your brain and your muscles.

So if you injure your arm or shoulder, there's no reason to stop exercising the good arm (provided your doctor clears you for exercise). Just make sure you avoid exercises that would further damage the injured side. You can, for example, continue to press and curl dumbbells with your good arm to keep both arms strong.