As a former HS track and field coach, the vast majority of skinny female knee injuries was due to overuse. Those athletes who didn't overexert themselves and took the recommended "rest" days rarely had over-exertion based injuries. The not-so-skinny female athletes (usually the shot and discus athletes) often had chronic knee pain. Based on my memory and non-scientific analysis this chronic knee pain was more common in the heavier girls. Those girls in the normal wight range (they all believed they were overweight) had the least knee issues (less chronic pain, and less over-exertion injuries). I would love to hear from a medical person to either corroborate my anecdotal observations or contradict them for my own education.