Digital Images/Getty Images Several weeks ago, The Journal of the American Medical Association published a study that should give pause to anyone who plans to live a long and independent life. The study looked at the incidence of hip fractures among older Americans and the mortality rates associated with them. Although the number of hip fractures has declined in recent decades, the study found that the 12-month mortality rate associated with the injury still hovers at more than 20 percent, meaning that, in the year after fracturing a hip, about one in five people over age 65 will die.
Meanwhile, another group of articles, published this month as a special section of Medicine & Science in Sports & Exercise, the journal of the American College of Sports Medicine, underscore why that statistic should be relevant even to active people who are years, or decades, away from eligibility for Medicare. The articles detailed a continuing controversy within the field of sports science about exactly how exercise works on bone and why sometimes, apparently, it doesn’t.
“There was a time, not so long ago,” when most researchers assumed “that any and all activity would be beneficial for bone health,” says Dr. Daniel W. Barry, an assistant professor of medicine at the University of Colorado, at Denver, and a researcher who has studied the bones of the elderly and of athletes. Then came a raft of unexpected findings, some showing that competitive swimmers had lower-than-anticipated bone density, others that, as an earlier Phys Ed column pointed out, competitive cyclists sometimes had fragile bones and, finally, some studies suggesting, to the surprise of many researchers, that weight lifting did not necessarily strengthen bones much. In one representative study from a few years ago, 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 readily 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 fairly 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 and the author of an article in Medicine & Science in Sports & Exercise, “you have to stretch them so far to get a response that the bone would break.”
So he and many other 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 and confounded 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. Dr. Barry is in the middle of a long-term study to determine whether supplementing with calcium-fortified chews before and after exercise reduces the bone-thinning response in competitive cyclists. He expects results in a year or so.
In the meantime, 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 neighbor 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.”
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