M.D. Bicycle riders have bones that are less dense than people who don't exercise at all, according to a recent study from the American College of Sports Medicine. This led a science writer for one major newspaper to recommend that bicycle riders should lift weights to strengthen their bones, or change sports. Bone density tests do not necessarily measure bone strength.
Birds have unbelievably thin bones that are extremely strong.
Many birds with thin light bones are far more resistant to fractures than many mammals that have much denser bones. There is no evidence that bicycle riders or racers are at increased risk for bone fractures. Racers crash all the time. Lance Armstrong spends as much time on a bike as anyone, and he has had at least two serious high impact crashes this year. If he had weak bones, he would be in a wheelchair, and not be the greatest bicycle racer in the world.