If you're a runner, you've probably heard that biking is a good choice for cross-training on your non-running days. After all, biking offers cardio benefit without the high impact stress that can lead to running injuries.

Likewise, running is a good cross-training choice if your main activity is biking. Running (or jogging or even walking) is a weight-bearing exercise that helps build and maintain bone.

Whatever your favorite cardio activity, remember that strength-training -- of the upper as well as lower body muscles -- should be part of your exercise mix.