Cycling has long been considered one of the easiest and safest ways to get exercise - it's a great aerobic workout, it's gentle on the joints and pretty much everyone knows how to ride a bike.
But recent research suggests that serious cyclists may need to worry about more than just avoiding cars on the roads. People who devote too much time to cycling and don't get enough of other forms of exercise may have a higher risk of developing osteoporosis because their bones don't get enough day-to-day pounding.
It's not something most recreational bike riders need to worry about, but avid cyclists who spend long hours on the bike without doing other forms of exercise might.
"We've had 65-year-old men whose bones look like 65-year-old women. They've suffered fragility fractures as a result," said Dr Srinivas Ganesh, a sports-medicine specialist with Kaiser Permanente in Redwood City, California, who is a cyclist himself. Women generally have a lower bone mass than men.
"We really try to tell them it's great what they're doing, but you need to realise that the bones underneath those muscles are not as strong as they could be," he said. "I tell my cyclists that they've got to get out there and do something else, too."
Several recent studies, including one article published in the International Journal of Sports Medicine, have found that professional cyclists have lower bone-mineral density - a way of measuring the risk of developing osteoporosis - than people of the same age who are moderately active but don't bike.
The theory is that bones are designed to get regular impact, even if it is just from walking around, and without that impact the bones lose strength and density.
Cycling is often considered a healthy choice for exercise because it's no-impact - people don't pound their joints the way they might with running or jumping. Doctors say they often recommend biking for people who are trying to lose weight. It's not as efficient as running - the average person will burn more calories in 30 minutes of running than an hour of cycling - but it's not as hard on the body.
In fact, for people with injuries, cycling can actually be part of their rehabilitation. Easy pedaling on a stationary bike can get blood moving into an injured knee, for example.
But serious cyclists may spend so much time on their bikes that they end up with almost no weight-bearing activity. Even a casual walk down the street puts more impact on the bones than biking, and cyclists who spend all their time biking would probably benefit from regular walks - or even better, a short, intense run a couple of times a week, said Dr Thor Besier, director of research at Stanford University's Human Performance Lab.
"Bones and cartilage are designed to withstand pretty large impact loads, and they like that kind of loading," Besier said. "If all you did was cycle, and you compared yourself to someone who all they did was walk, well, walking would provide much more stimulus for bone and cartilage health."
Still, cyclists are often a stubborn bunch when it comes to getting off their bike. Besier himself said he is a competitive cyclist who doesn't spend nearly enough time cross-training. "I should just get out and do a 10-minute run," he said. "But I usually don't."
Recreational cyclists probably don't need to worry about their bone health, sports doctors said, but they still should consider adding in some sort of moderate to high-impact workout every week.