Not all cardio was created equal. Weight-bearing activities such as walking, running, basketball and dancing are the smartest, most effective ways to build bone density. But if you start to experience pain in your shins or feet, you may need to lighten up -- heavy pounding can result in stress fractures, tiny cracks in the bones of your feet and lower legs. Biking and swimming aren’t any better. Non-impact activities don’t build bone strength at all. In fact, a recent Journal of Sports Medicine and Physical Fitness study found that competitive cyclists had lower bone density than non-athletes, thanks to all that time in the saddle. Pete McCall, an exercise physiologist with the American Council on Exercise, suggests changing up your cardio routine, incorporating weight-bearing activities that require you to move in multiple directions with a full range of motion. Jog outside one day, take a Zumba class the next, and join a softball league for the weekends. Don’t forget to lift weights, a stellar move for strong bones.