In a recent Moving Crew online chat, I referred to cycling as a weight-bearing exercise -- the kind that can boost bone density. No way, one chatster charged: Cycling isn't weight-bearing.

It turns out we're both right. More on that below.

More important, many people overstate the importance of weight-bearing exercise as protection from osteoporosis, said Robert Recker, professor of medicine at Creighton University in Omaha and a scientific adviser to the National Osteoporosis Foundation.

Recker, who also heads the foundation's grant program, verifies that weight-bearing activity benefits bone density. But popular beliefs notwithstanding, he adds, he is not aware of studies showing a strong bone density benefit of weight-bearing exercise over the non-weight-bearing sort. "You would need a head-to-head study to make that conclusion," he said, and no such study exists.

"If I had a patient who was swimming [a non-weight-bearing exercise] every day, I would not say, 'Stop that and switch to walking,' " he said. "Anything you do is good for the skeleton: It builds muscle, which helps your righting reflexes and fall reflexes, and that is good for someone with osteoporosis."

The term "weight-bearing" means your bones are working against gravity to support your body weight or, in the case of weight lifting, other weight. So walking, running, push-ups, using an elliptical machine (and anything else done on your feet) and weight lifting are weight-bearing exercise. Swimming, in which your body is floating in the water, is not weight-bearing exercise.

And cycling?

If you stay seated, cycling is non-weight-bearing: The bike is supporting your weight. But when you stand in the pedals (as I often do when huffing up a hill or through a sprint), the exercise becomes weight-bearing: Your feet, braced on the pedals, are supporting part of your weight, said Recker.

Similarly, a workout on an elliptical machine provides no-impact, weight-bearing exercise: While your bones are supporting your weight, your feet are not striking the ground, as they do when you walk or run. So are you missing bone benefits by giving up the impact?

If so, not much, Recker said. The bone density benefit from an elliptical machine workout is comparable to that from a run of equal time.

"You might get a little more benefit from the run," he said. "A study of gymnasts showed they have better bone density than non-gymnasts, but there is not a lot of research on that."

Bones respond to pressure by adding osteoblasts, or bone-forming cells, which boosts density, said exercise physiologist Colin Wilborn, coordinator of weight training studies at Baylor University in Houston.

"It's like the effect of weight lifting on your muscles," Wilborn said. "Stress the muscle enough and it will adapt to handle that load. Your bones do the same thing" by adding osteoblasts.

For adults, the gains from "adding" bone density usually serve, at best, to keep pace with the loss of bone density that comes with age. (Most women, for example, lose up to 20 percent of their bone mass in the first five to seven years after menopause, according to the National Osteoporosis Foundation.) "It is very hard for adults to add new bone," Recker said.

No chat this week; we'll be back online next Thursday, Aug. 5, at 11 a.m. Meantime, send any thoughts, questions or suggestions -- and of course your disagreements -- to move@washpost.com.

-- John Briley