BONE OF CONTENTION

In one study,researchers found no significant differences in the spine or neck-bone densities of young women who did resistance-style exercise training (not heavy weight-lifting) and a similar group who did not.
Researchers admit that they don’t fully understand why some exercise is good for bones and some just isn’t. As the articles in this month’s Medicine & Science in Sports & Exercise make clear,scientists actually seem to be becoming less certain about how exercise affects bone.

Until recently,many thought that the pounding or impact that you get from running,for instance,deformed the bone slightly. It bowed in response to the forces moving up the leg from the ground,stretching the various bone cells and forcing them to adapt,usually by adding cells,which made the bone denser. This,by the way,is how muscle adapts to exercise. But many scientists now think that that process doesn’t apply to bones. “If you stretch bone cells” in a Petri dish,says Alexander G. Robling,an assistant professor in the department of anatomy and cell biology at Indiana University School of Medicine,“you have to stretch them so far to get a response that the bone would break.”

So,many researchers now maintain that bone receives the message to strengthen itself in response to exercise by a different means. He says that during certain types of exercise,the bone bends,but this doesn’t stretch cells; it squeezes fluids from one part of the bone matrix to another. The extra fluid inspires the cells bathed with it to respond by adding denser bone.

Why should it matter what kind of message bones are receiving? Because,Professor Robling and others say,only certain types of exercise adequately bend bones and move the fluid to the necessary bone cells. An emerging scientific consensus seems to be,he says,that “large forces released in a relatively big burst” are probably crucial. The bone,he says,“needs a loud signal,coming fast.” For most of us,weight-lifting isn’t explosive enough to stimulate such bone bending. Neither is swimming. Running can be,although for unknown reasons,it doesn’t seem to stimulate bone-building in some people. Surprisingly,brisk walking has been found to be effective at increasing bone density in older women,Dr Barry says. But it must be truly brisk. “The faster the pace,” he says—and presumably the greater the bending within the bones—the lower the risk that a person will fracture a bone.

There seems to be a plateau,however,that has also surprised some researchers. Too much endurance exercise,it appears,may reduce bone density. In one small study completed by Dr Barry and his colleagues,competitive cyclists lost bone density over the course of a long training season. Dr Barry says that it’s possible,but not yet proved,that exercise that is too prolonged or intense may lead to excessive calcium loss through sweat. The body’s endocrine system may interpret this loss of calcium as serious enough to warrant leaching the mineral from bone.

The current state-of-the-science message about exercise and bone building may be that,silly as it sounds,the best exercise is to simply jump up and down,for as long as the downstairs neighbour will tolerate. “Jumping is great,if your bones are strong enough to begin with,” Dr Barry says. “You probably don’t need to do a lot either.” (If you have any history of fractures or a family history of osteoporosis,check with a physician before jumping.) In studies in Japan ,having mice jump up and land 40 times during a week increased their bone density significantly after 24 weeks,a gain they maintained by hopping up and down only about 20 or 30 times each week after that.
If hopping seems an undignified exercise regimen,bear in mind that it has one additional benefit: It tends to aid in balance,which may be as important as bone strength in keeping fractures at bay. Most of the time,Dr Barry says,“fragile bones don’t matter,from a clinical standpoint,if you don’t fall down.”
_Gretchen Reynolds,NYT