Pump Up Those Bones With Exercise

Furthermore, lifting weights or cycling causes the muscles to contract around the bone, which further contributes to bone density by stressing the bone.

But the cyclists and the weight lifters differed in the distribution of bone density.

Because cyclists primarily use their lower bodies, the bones in their legs and hips were denser than the bones in their upper bodies. The weight lifters, meanwhile, had the highest whole-body bone density.

"It can be very site-specific," Hinton said.

Though the runners in the study didn't have as much muscle mass as the cyclists and weight lifters, they had higher bone densities because their bones absorbed more impact, particularly in the legs, hips and spine, Hinton said.

The runners' relative lack of muscle mass might have further contributed to their bone density because muscles absorb impact, lessening the impact on the bone.

The study participants' high bone densities can be attributed to their dedication to exercise. To qualify for the study, participants had to work out a minimum of six hours a week for the past two years.

"The importance of exercise or mechanical loading of the skeleton becomes very obvious when you look at the effects of weightlessness," Hinton said.

Astronauts in space or patients on bed rest tend to lose bone density in a short period of time because, like muscles, bones start to atrophy with disuse, Hinton said.