Your body is continuously renovating your skeleton, absorbing old bone tissue and replacing it with calcium that helps form new bone. Physical activity that includes impact and vibration (running, strength training, walking) stresses the tissue and triggers the rebuilding process.
Cycling, however, has no such effect. Thanks to its seated, off-the-ground riding position, it actually minimizes impact on your skeletal system. And there's a chance that the more you ride, the greater your risk of losing bone mass, says Aaron Smathers, MS, lead author of the aforementioned study.
"You can be strong and very fit," says Regina Hammond, a sports nutritionist who studied bone density in cyclists for her master's thesis at the University of Colorado. "But your heart, lungs and muscle don't reflect bone health." In fact, it was thanks to a bone scan done during Hammond's study that Matheny discovered his mountain bike probably was soaking up more impact than he thought.
Complicating the issue is yet another by-product of cycling: sweat. You can lose up to 200 milligrams of bone-building calcium in an hour as you soak your jersey on a hot ride. Put in long training miles each week and ride a century now and then, and you'll deplete your body of so much calcium that it becomes harder and harder to replace, especially as you age. You can counteract this by fueling up on foods with plenty of calcium, but you may need more than just dietary adjustments.
What You Can Do
Keep your bones strong with this exclusive routine from Allison Westfahl, an exercise physiologist in Boulder, Colorado, and coauthor of Tom Danielson's Core Advantage. Plan to do an off-the-bike workout on three nonconsecutive days a week. Each day, pick one move from Group A and one from Group B (Choose a different combination of moves each day). Unless the directions say otherwise, do three sets of 10 to 12 repetitions, resting 30 to 45 seconds between each set. Use the heaviest weight you can handle while still finishing all the reps.