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Phys Ed Gretchen Reynolds on the science of fitness.

One of the most entrenched beliefs about running, at least among nonrunners, is that it causes arthritis and ruins knees. But a nifty new study finds that this idea is a myth and distance running is unlikely to contribute to the development of arthritis, precisely and paradoxically because it involves so much running.

It’s easy to understand, of course, why running is thought to harm the knee joint, since with every stride, ballistic forces move through a runner’s knee. Common sense would suggest that repeatedly applying such loads to a joint should eventually degrade its protective cartilage, leading to arthritis.

But many of the available, long-term studies of runners show that, as long as knees are healthy to start with, running does not substantially increase the risk of developing arthritis, even if someone jogs into middle age and beyond. An impressively large cross-sectional study of almost 75,000 runners published in July, for instance, found “no evidence that running increases the risk of osteoarthritis, including participation in marathons.” The runners in the study, in fact, had less overall risk of developing arthritis than people who were less active.

But how running can combine high impacts with a low risk for arthritis has been mysterious. So for a new study helpfully entitled, “Why Don’t Most Runners Get Knee Osteoarthritis?” researchers at Queen’s University in Kingston, Ontario, and other institutions looked more closely at what happens, biomechanically, when we run and how those actions compare with walking.

Walking is widely considered a low-impact activity, unlikely to contribute much to the onset or progression of knee arthritis. Many physicians recommend walking for their older patients, in order to mitigate weight gain and stave off creaky knees.

But before the new study, which was published last week in Medicine & Science in Sports & Exercise, scientists had not directly compared the loads applied to people’s knees during running and walking over a given distance.

To do so now, the researchers first recruited 14 healthy adult recreational runners, half of them women, with no history of knee problems. They then taped reflective markers to the volunteers’ arms and legs for motion capture purposes, and asked them to remove their shoes and walk five times at a comfortable pace along a runway approximately 50 feet long. The volunteers likewise ran along the same course five times at about their usual training pace.

The runway was equipped with specialized motion-capture cameras and pads that measured the forces generated when each volunteer struck the ground.

The researchers used the data gathered from the runway to determine how much force the men and women created while walking and running, as well as how often that force occurred and for how long.

It turned out, to no one’s surprise, that running produced pounding. In general, the volunteers hit the ground with about eight times their body weight while running, which was about three times as much force as during walking.

But they struck the ground less often while running, for the simple reason that their strides were longer. As a result, they required fewer steps to cover the same distance when running versus walking.

The runners also experienced any pounding for a shorter period of time than when they walked, because their foot was in contact with the ground more briefly with each stride.

The net result of these differences, the researchers found, was that the amount of force moving through a volunteer’s knees over any given distance was equivalent, whether they ran or walked. A runner generated more pounding with each stride, but took fewer strides than a walker, so over the course of, say, a mile, the overall load on the knees was about the same.

This finding provides a persuasive biomechanical explanation for why so few runners develop knee arthritis, said Ross Miller, now an assistant professor of kinesiology at the University of Maryland, who led the study. Measured over a particular distance, “running and walking are essentially indistinguishable,” in terms of the wear and tear they may inflict on knees.

In fact, Dr. Miller said, the study’s results intimate that running potentially could be beneficial against arthritis.

“There’s some evidence” from earlier studies “that cartilage likes cyclical loading,” he said, meaning activity in which force is applied to the joint, removed and then applied again. In animal studies, such cyclical loading prompts cartilage cells to divide and replenish the tissue, he said, while noncyclical loading, or the continued application of force, with little on-and-off pulsation, can overload the cartilage, and cause more cells to die than are replaced.

“But that’s speculation,” Dr. Miller said. His study was not designed to examine whether running could actually prevent arthritis but only why it does not more frequently cause it.

The results also are not an endorsement of running for knee health, he said. Runners frequently succumb to knee injuries unrelated to arthritis, he said, and his study does not address or explain that situation. One such ailment is patellofemoral pain syndrome, which is often called “runner’s knee.”

But for those of us who are — or hope to be — still hitting the pavement and trails in our twilight years, the results are soothing. “It does seem to be a myth,” Dr. Miller said, that our knees necessarily will wear out if we continue to run.