[wyclif]

I suspect that the cause of that is running on pavement. Doing that for decades is hard on the knees and joints. I wonder if there would be more people in the over-50 age group still running if they had spent their younger years running on grass or sand. I don't have any evidence that they would, but it's an interesting question.

[maxmcorp]

But nature is hardly better for running than pavement. Stones, grass, roots, mud etc. There are a lot more ways to get injuries in nature than on a gravel path.

[Tade0]

I guess people just don't take into account the fact that our ancestors usually didn't live to their 50s, so they couldn't experience most of the problems that nowadays middle-aged runners do.