They also tested two additional 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 actively remodel itself and adapt to the demands of moving, growing thicker and stronger, much as muscle does when we exercise.
The models’ final results were eye-opening. According to the simulations, daily walkers faced about a 36 percent chance of developing arthritis by the age of 55, if the model did not include the possibility of the knee cartilage adapting or repairing itself. That risk dropped to about 13 percent if cartilage were assumed to be able to repair or adapt, which is about what studies predict to be the real-world arthritis risk for otherwise healthy people.
The numbers for running were more worrisome. When the model assumed cartilage cannot change, the runners’ risk of eventual arthritis was a whopping 98 percent, declining only to 95 percent if the model factored in the possibility of cartilage repair. In effect, according to this scenario, the damage to cartilage from frequent running would overwhelm any ability of the tissue to fix itself.
But if the model included the likelihood of the cartilage actively adapting — growing thicker and cushier — when people ran, the odds of runners developing arthritis fell to about 13 percent, the same as for healthy walkers.
What these results suggest is that cartilage is malleable, Dr. Ross says. It must be able to sense the strains and slight damage from running and rebuild itself, becoming stronger. In this scenario, running bolsters cartilage health.
Modeled results like these are theoretical, though, and limited. They do not explain how cartilage remodels itself without a blood supply or if genetics, nutrition, body weight, knee injuries and other factors affect individual arthritis risks. Such models also do not tell us if different distances, speeds or running forms would alter the outcomes. To learn more, we will need direct measures of molecular and other changes in living human cartilage after running, Dr. Miller says, but such tests are difficult.
Still, this study may quiet some runners’ qualms — and those of their families and friends. “It looks like running is unlikely to cause knee arthritis by wearing out cartilage,” Dr. Ross says.