From The NY Times:
Many 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 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.
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. “But that’s speculation,” Dr. Miller said.