Could running actually be good for your knees?

That idea is at the heart of a new study of the differing effects of running and walking on the knee joint. Using motion capture and sophisticated computer modeling, the study confirms that running pummels knees more than walking does. But in the process, the authors conclude, running likely also fortifies and bulks up the cartilage, the rubbery tissue that cushions the ends of bones.

The findings raise the beguiling possibility that, instead of harming knees, running might fortify them and help to stave off knee arthritis.

The notion that running wrecks knees is widespread and entrenched. Almost anyone who runs is familiar with warnings from well-meaning, nonrunning family members, friends and strangers that their knees are doomed.

This concern is not unwarranted. Running involves substantial joint bending and pounding, which can fray the cushioning cartilage inside the knee. Cartilage generally is thought to have little ability to repair itself when damaged. So, the theory goes, repeated running wears away fragile cartilage and almost inevitably leads to crippling knee arthritis.

But in real life, it does not. While some runners develop knee arthritis, many others never have a problem.

The question of why running spares so many runners' knees has long intrigued Ross Miller, an associate professor of kinesiology at the University of Maryland in College Park. He knew that recent studies with animals intimated that cartilage might be more resilient than researchers previously had believed.

In those studies, animals that ran tended to have thicker, healthier knee cartilage than comparable tissues from sedentary animals, suggesting that the active animals' cartilage had changed in response to their running. Perhaps, Miller speculated, cartilage in human runners' knees likewise might alter and adapt.

To find out, he asked a group of healthy young men and women to walk and run along a track containing force plates measuring how hard their feet were hitting the ground. Then he and his colleagues modeled what the future might hold for the volunteers' knees.

They used the force-plate numbers, plus extensive additional data from past studies of biopsied cartilage pulled and pummeled in the lab until it fell apart and other sources to create computer simulations. They wanted to see what, theoretically, would happen to healthy knee cartilage if an adult walked for 6 kilometers (about 3.7 miles) every day for years, compared with if they walked for 3 kilometers and ran for another 3 kilometers each of those days.

They also tested various theoretical situations. For one, the researchers programmed in the possibility that people's knee cartilage would slightly repair itself after repeated small damage from walking or running — but not otherwise change. And for the last scenario, they presumed that the cartilage would remodel itself and adapt to the demands of moving, growing thicker and stronger, much as muscle does when we exercise.

In the final results, for the model that did not include the possibility of the cartilage adapting or repairing itself, daily walkers faced about a 36% chance of developing arthritis by the age of 55. But that risk dropped to about 13% if cartilage were assumed to be able to repair or adapt, which is about the real-world arthritis risk for otherwise healthy people.

The eye-opening part of the data concerned runners. In the model that accounted for no cartilage change, the runners had a whopping  98% chance of developing arthritis. But in the version that included the likelihood of the cartilage actively adapting, the number fell to 13%, the same as for walkers.

What these results suggest is that cartilage is malleable, Ross said. It rebuilds itself, becoming stronger. In this scenario, running bolsters cartilage health.

Modeled results like these are theoretical, and limited. They do not take into account genetics, nutrition, body weight, knee injuries and other factors that affect individual arthritis risks. They also do not tell if different distances, speeds or running forms would alter the outcomes.

Still, the results might ease some runners' qualms. "It looks like running is unlikely to cause knee arthritis by wearing out cartilage," Miller said.