They also tested two additional theoretical situations. On the one hand, the researchers programmed the possibility that people's knee cartilage would easily repair itself after repeated minor damage from walking or running – but would not change otherwise. And for the latter scenario, they assumed that cartilage would actively remodel and adapt to the demands of movement, getting thicker and stronger, just as muscles do when we exercise.
The end results of the models were eye-opening. According to the simulations, daily hikers by age 55 had a 36 percent chance of developing arthritis if the model did not include the ability for the knee cartilage to adapt or repair itself. This risk dropped to about 13 percent assuming that cartilage could be repaired or adjusted. This is roughly what studies predict the real arthritis risk for otherwise healthy people.
The numbers for running were more worrying. If the model assumed cartilage could not change, the risk of possible arthritis in runners was a whopping 98 percent and only dropped to 95 percent when the model considered the possibility of cartilage repair. Indeed, according to this scenario, the damage to the cartilage from frequent walking would overwhelm any ability of the tissue to fixate itself.
However, when the model took into account the likelihood that cartilage will actively adapt – getting thicker and more comfortable – as you run, the likelihood of runners developing arthritis dropped to about 13 percent, just like healthy hikers.
These results suggest that cartilage is malleable, says Dr. Horse. It needs to be able to feel the stress and minor damage from running and to rebuild itself as it gets stronger. In this scenario, running strengthens cartilage health.
However, modeled results like this are theoretical and limited. They don't explain how cartilage remodels without a blood supply, or whether genetics, diet, body weight, knee injuries, and other factors affect an individual's risk of arthritis. Such models also do not tell us whether different distances, speeds or running forms would change the results. To find out more, we need direct measurements of the molecular and other changes in living human cartilage after running, says Dr. Miller, but such tests are difficult.
Still, this study can allay the concerns of some runners – and those of their families and friends. "It looks like running probably doesn't cause knee arthritis if the cartilage is worn out," says Dr. Horse.