the next person who tells me not to run gets smacked

Together, the findings lend support to the theory that osteoarthritis, which affects nearly 20 million Americans, is caused mainly by genes and risk factors like obesity (obese men and women are at least four times as likely to become arthritic as their thinner peers), rather than daily exercise or wear and tear of joints. In fact, a "normally functioning joint can withstand and actually flourish under a lot of wear," says Fries. Because cartilage — the soft connective tissue that surrounds the bones in joints — does not have arteries that deliver blood, it relies on the pumping action generated by movement to get its regular dose of oxygen and nutrients. "When you bear weight, [the joint] squishes out fluid, and when you release weight, it sucks in fluid," says Fries, explaining why a daily run or any other workout is useful for maintaining healthy cartilage.

That's not to say that there are no risks in running. It can sometimes cause soft-tissue injuries and stress fractures, also called hairline fractures, which result from the compounding of tiny cracks in the bone over time. It's not uncommon for such tiny cracks to appear in the bones that bear the heaviest loads, like the tibia (shinbone), but they usually heal quickly and go unnoticed. Stress fractures occur when bone damage happens suddenly, without enough time to heal. For instance, high school athletes who stop training all summer and then abruptly start attending practice every day have a much higher risk of stress fractures in their shinbones than their friends who practiced regularly over the break.