Not sure I really buy that. Surely the everyday jumping activities of kids have higher impact stresses, like jumping out of trees and off of swings! Not to mention school long and triple jump and then there is basketball and dunking. Most American kids seem to spend all their time doing lay-ups and trying to touch the rim. I think all of these activities have higher impact loads than sprinting a slight gradient on grass… even runnning on a hard track in sprint spikes.