Illumination | Spring 2008

The reason why involves what scientists call "fluid shear stress." Bone cells are organized in a fluid-filled, lace-like network. Rapid movement of the fluid across the surface of the bone cells during movement -- fluid shear stress -- signals the production of new bone-forming cells. High-impact activities, such as running and jumping, produce a large amount of fluid shear stress, thus stimulating bone growth. Low-impact exercises have the opposite effect.

Each time the runner comes in contact with the ground, Hinton says, an "impact force" or "ground reaction force" is exerted on the bones of the hips, legs and feet. This stress peaks and fades with each stride, creating a rhythmic series of compression and relaxation. It is this rhythm that causes fluid flow across bone cells, thus stimulating them to adapt to the stress by adding new bone.

Barring disaster, a cyclist never comes in direct contact with the ground: The only forces exerted on the skeleton during cycling are those produced by contraction of the leg muscles during pedaling. Because the magnitude of these forces is much less than those of high-impact ground reactions, this no-impact form of exercise is much easier on joints. But over time the lack of strain and the resulting deficiency of fluid shear stress could contribute to bones' deterioration.

Still, not many of the athletes showed concern about their bones prior to the study, Hinton recalls. After all, they were all young and fit. "A few that had had fractures were a little more accepting of my hypothesis," Hinton recalls. "The others were like, 'Yeah, whatever, my bones are fine.'"

For these dismissive athletes in particular, Hinton's findings came as a shock. More than one-half of the cyclists, 63 percent, had osteopenia of the spine or hip as compared to 19 percent of the runners. Put another way, the cyclists were seven times more likely to have osteopenia of the spine than runners, despite similar height, weight and body composition between the two groups.

"To be honest, I didn't expect to find that many (cyclists) with osteopenia," Hinton said. "I would expect this in professional cyclists who are very thin and spend 30 hours a week on the bike, but I was surprised to see it in this level of athlete."

After completing the DXA scans, Hinton shared the results in a letter to participants. "The message was, this is not something that you should ignore," Hinton says. "You're at increased risk for low bone density now, and time is just going to magnify that effect, so it's probably a good idea for you to do something about it.

Published by the Office of Research.