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They won't have to wait until middleage to find out, because I guarantee they'll feel it once they hit their 30's......I know from first hand experience what bad knees feel like from all the crap I used to do as a kid.....running long distance, jumping things, etc.
its called parcour. A french invention. Its a combination of Running (or "Yohgging") Break dance and Gymnastics. ITs pretty neat to watch. Howver no matter how much they twist, im sure they cant out run a 308