Mine is from football, one of our biggest rivals QB threw a pass right into the back of the centre and it bounced straight up, I collected it and headed for the end zone when he cheap shot me in the knee. Jumping out of planes and impacting the ground at 30-60 km/h didn't help either, lol. I used to bike a lot more than I do now, just no time for it now, that will change come spring.I have to agree with @ReeferRob - I played too much basketball in my younger days which buggered up my knees. So, bike riding is the best option for me now.