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From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

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And I'm Noah Adams.

Exercise can be hard on the joints, at least that's a common perception - make that misperception. Several recent studies suggest that moderately intense, weight-bearing exercises, including long distance running, do not increase the risk of arthritis. NPR's Allison Aubrey has details.

ALLISON AUBREY: Physician Harvey Simon is 67 now. But back in college, he earned the ironic nickname Gazelle, thanks to his running style.

Dr. HARVEY SIMON (Physician): I'm reasonably lean, but I'm uncoordinated and clumsy. My running stride is something awful to look at.

(Soundbite of laughter)

Dr. SIMON: But I haven't missed a day of running since October 30, 1978.

AUBREY: After clocking a little over 100,000 miles in these years, Simon says his knees and hips are no worse for the wear.

Dr. SIMON: I may be lucky, I may be genetically gifted in that way, but my joints are fine.

AUBREY: So is Harvey Simon an anomaly? Isn't all that pounding bound to do some damage? Consider this, and these are figures Simon quotes. Every time a runner strikes the ground, he or she applies eight times their body weight to the joints. So if you take a 150-pound person, each stride will impose a load of about 1,200 pounds to the body.

Dr. SIMON: And that goes from the foot to the knee to the hip, to the spine, et cetera.

AUBREY: And that's a lot of weight.

Dr. SIMON: Over the course of a mile, that's tons and tons of impact. So it's an understandable thing that people suspect that that might be harmful to joints.

AUBREY: But what the most recent studies are showing is that for many people, it's not harmful. In a new analysis for Harvard Men's Health Watch, Simon cites three different lines of evidence. In one long-term study, which included about 1,200 residents of the town of Framingham, Massachusetts, researchers looked for a correlation between arthritis and exercise. But what they found:

Dr. SIMON: People who were physically active were no more likely to develop symptoms of arthritis or X-ray evidence of arthritis than people who were sedentary.

AUBREY: And then there's a study from Stanford University that tracked a group of long distance runners for 20 years. Researchers compared their health to others of similar age who did less exercise. Stanford's Eliza Chakravarty says they expected to find a lot more problems in the runners' knees, but it turns out they had healthier joints at the end of the study.

Dr. ELIZA CHAKRAVARTY (Assistant Professor of Medicine, Stanford University School of Medicine): It does sort of negate the idea that running is universally bad on the knees.

AUBREY: And the other studies negate the idea that moderately intense exercise, including running, leads to a higher risk of arthritis. Steven Blair is an exercise scientist at the University of South Carolina. He says there are some clear risk factors for osteoarthritis that are related to physical activity.

Dr. STEVEN BLAIR (Professor, University of South Carolina Arnold School of Public Health): The primary determinant or the primary predictor of joint injury is a previous injury. You know, you may have damaged something when you were a kid, let's say, that then causes you trouble later on.

AUBREY: If you suffer an injury in a shoulder or knee, you're two to five times more likely to develop arthritis in that joint. Other risk factors include being overweight, age, or alignment problems - say, if one leg is shorter than the other, this can put unusual stress on the joints. Blair says there are all sorts of individual exceptions, and he says it's important for everyone to be careful and not over-do it. But increasingly, he says the studies are reminding us that the joints were made for moving.

Allison Aubrey, NPR News.

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