For better or worse, the human body adapts to its environment. If you stop applying force to your frame by focusing on low-impact sports, you'll build muscle, but your bod will assume that it can slow down bone maintenance.

This process came to light in 1996, when a study of six Tour de France riders showed bone-density losses of up to 17 percent over the course of the race. Seven years later, the SDSU study demonstrated that two-thirds of the masters bike racers tested had, on average, 10 percent more bone-density loss when compared with the male control group. The results led the study's author, exercise-physiology professor Jeanne Nichols, to warn of the risk in those participating exclusively in low-impact sports—like cyclists and swimmers. The latter, according to Scott Going, associate professor of nutritional science at the University of Arizona, are engaged in the sport ranked dead last for bone maintenance. The best sports for that goal? Weight lifting and gymnastics.

To counteract this problem, just add bone-building workouts to your program, like a weekly ski session this winter. "Logically, the weight-bearing loads that come from mogul skiing or snowboarding should spur bone growth," says Going.

No snow? No problem. Jogging or jumping rope daily will also prompt bone growth. These load-bearing exercises—as well as a weight-lifting program where you pump enough iron to reach muscle failure after eight reps—will stop your bones from weakening and could help multiply bone cells by as much as 2 percent a year. "Just make the impact on your skeleton significant," says Warren A. Scott, a medical director for the Hawaii Ironman Triathlon. Activities that are too mellow, like casual walks, don't cut it.