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About this time every year, with the fall marathon season at its zenith, racers in training begin to hear the refrain, ‘‘You are going to ruin your knees.’’ The idea that distance running inexorably leads to arthritis is deeply entrenched, despite the publication of a number of recent studies (detailed in a Phys Ed column last year) that have found otherwise. In one representative experiment, the knees of experienced marathoners, with multiple races behind them, were scanned with magnetic resonance imaging technology, and then scanned again 10 years later. The runners’ knees were and remained robust throughout that time, with few significant cartilage abnormalities. The only truly unhealthy knee in the study belonged to a former marathoner, who had quit the sport. In the years since he stopped running, his joint had deteriorated badly.

But then came the latest study on the issue, this one from researchers at the University of California at San Francisco, using a more sensitive type of M.R.I. technology than had been available in the past. For this study, the researchers recruited a group of beginner marathon runners. The runners were 40 and younger and had completed fewer than three marathons in their lifetimes. Some were training for their first. At the time they enrolled in the study, none of the runners reported knee problems. ‘‘They had virgin knees,’’ said Anthony Luke, M.D., director of primary care sports medicine at U.C.S.F. and the study’s lead investigator. In the days before the runners’ marathons (either the San Francisco or Nike women’s marathon), they scanned the racers’ knees, employing a type of M.R.I. technology that evaluates the metabolic activity and health of the cartilage at a cellular level. They repeated the scans within 48 hours after the event and again about three months later.

The results, published earlier this year in The American Journal of Sports Medicine, are eye-opening. On these more-sensitive M.R.I. scans, the researchers found evidence of significant biochemical changes in the runners’ knee cartilage, particularly in the days immediately after the race. According to their postrace scans, the racers had elevated values for two technical measures of the health of their cartilage matrix. Elevations in these measures, known as T2 and T1rho values, have been linked to cartilage degeneration and incipient arthritis in other studies of the knee.

Three months after the race, the runners’ T2 values had returned to normal, but their T1rho values remained elevated, although they were declining. Whether the remaining increase was permanent and whether it indicated that, at some deep, molecular level the marathon had changed the runners’ knee cartilage, ‘‘is simply unclear at this point,’’ Dr. Luke said.

In other words, the issue of whether distance running does or does not harm your knees would appear still to be open (to the considerable satisfaction of some of my nonrunning friends).

Yet another new study, however, offers some additional and consequential evidence. For that study, published in July in The Journal of the American College of Sports Medicine, researchers at Monash University in Melbourne, Australia, compiled and reviewed several decades’ worth of studies about activity and knee health. The reviewed studies involved a variety of sports, not just distance running. But those other sports, notably basketball and soccer, demand a considerable amount of running.

What the researchers found was that, at first blush, strenuous physical activity does seem to damage knees. Activity, especially lots of it, was ‘‘associated with an increase in radiographic osteophytes,’’ or bone spurs, the authors wrote, a condition that long has been accepted as an early indication of knee arthritis. Some of the studies under review had, in fact, concluded that activity must eventually end in arthritis, since the examined knees appeared to be imperiled.

But as the Australian researchers pointed out, some of those same studies, as well as others, did not find other characteristic changes in the knee that indicate damage. There was, for instance, almost no joint-space narrowing in active people. Joint-space narrowing is a necessary if unwelcome step on the way to full, bone-on-bone knee arthritis. The shock-absorbing cartilage in the joint wears away, the bones move closer together, and the space between tapers. Active people did not display this narrowing. In fact, according to a number of the studies reviewed, active people had greater cartilage volume than sedentary people. They weren’t losing the tissue; they were vigorously maintaining it.

Why, then, were their knees so often sprouting bone spurs, supposedly a marker of damage? The answer may be that in an active person’s otherwise uninjured knees, spurs are healthy, said Flavia Cicuttini, Ph.D., a professor in the School of Public Health and Preventive Medicine at Monash University. The spurs, she said, ‘‘may simply be a way that the bones adapt to forces pulling on the joint.’’

Similarly, adaptive transformations may underlie the cartilage changes visible in the U.C.S.F. marathon study, Dr. Luke said. ‘‘Running a marathon involves a lot of repetitive forces moving through the knee,’’ he said. ‘‘That kind of force is bound to have consequences’’ within the knee joint. ‘‘But that doesn’t mean,’’ he continued, that the molecular changes necessarily are destructive. ‘‘It’s my personal opinion,’’ he said, that the same signals on an M.R.I. that would suggest incipient arthritis in a sedentary person’s knee ‘‘may indicate some kind of necessary adaptation’’ in the knees of a marathoner.

There remains ‘‘a lot of research to be done,’’ though, he cautioned, before that theory can be proved. ‘‘The main thing we’re learning at the moment is that cartilage’’ and knees are ‘‘far more complex than we once thought.’’ Still, the bottom line, based on the current science, is cheering. ‘‘There’s no strong evidence,’’ he said, that, if your knees are healthy to start with, ‘‘running a marathon will hurt them.’’