Running causes knee arthritis.
Runners of a certain age are often encouraged to enjoy every mile while they're still healthy because their knees are doomed. And it's true that with every sneaker-clad step, you hit the ground with enormous force—it's estimated to be about eight times your body weight. But all that pounding does not substantially increase the risk of knee arthritis in healthy people, several studies have concluded—including one of almost 75,000 runners. While runners hit the ground harder, they also take longer strides and fewer steps, and therefore hit the ground less often. So, over time, running produces no more wear and tear on the knees than walking. (Note: Runners' knees could still be susceptible to injuries unrelated to arthritis, like patellofemoral pain syndrome, which can happen at any age.)