high jumper seems to translate the horizontal velocity of the run-up into vertical motion over the bar, but what actually happens is more related to springs, Dapena says. “The fast run-up makes the muscles of the takeoff leg stretch very quickly after the takeoff foot is planted on the ground, and this stimulates those muscles, which can then make larger forces.”

To get the fastest vertical acceleration, your foot must push against the ground for as long as possible. And that requires the runner to, as Dapena says, run with “the butt scraping the ground.” Still, there’s a tradeoff — if you run too low, your overly flexed knees will create a puny push-off.