Which do you think is a bigger concern, especially for females??? A torn ACL or Osteoporosis. A torn ACL, which is extrememly unlikely from running, b/c its only purpose is to stabilize the knee from moving left and right. Osteoporosis is a major concern and one way to help prevent is through running, the pounding you speak of, actually stimulates osteoblast to lay down new bone. Don't get me wrong, I am not recommending anyone to be a marathon runner, but then again, most of them lead extremely healthy lives and some even compete after the age of 70. An age when most of the power walkers have already died off. Nautica