It can ward off illness, fill you with energy, improve your sex life, and encourage a good night's sleep. But you can get too much of a good thing. Working out too long and too hard at one activity will lead almost inevitably to injury. If you've been concentrating on just one sport, consider substituting one or two others in your weekly workout schedule. Cross training not only improves overall fitness while reducing the risk of injury, it also can actually help heal damage already done.
Cycling helps develop the quadriceps while putting minimal strain on ankles and knees.
That makes it a good change of pace for runners and people who do aerobics or use stair-climbing machines. Swimming helps add upper body strength and develop flexibility for runners, cyclists, skiers, and people who use stair-climbing machines or do aerobics. It's also an excellent low-impact aerobic activity for weight lifters. Although any weight-training program should be accompanied by aerobic activity, the opposite is also true. Lifting weights can help a swimmer develop upper-body strength, a cyclist balance out leg muscle development to fully protect knees, or a skier develop shoulder strength.