Do the fastest sprinters move their legs faster, or hit the ground harder?

The fastest sprinters are not faster because they move their legs faster. They are faster because they hit the ground harder. When researchers measured athletes at top speed, the time it took to swing a leg through the air for the next step was the same for elite sprinters as for other runners. What differed was the force they delivered at each footstrike, relative to their body weight. Every step is a push against the ground, and by Newton's third law the ground pushes back just as hard: that push, the ground reaction force, is what keeps a sprinter flying forward.

Researchers at Southern Methodist University in Texas then asked how the best sprinters do it. They compared Olympic-level 100 and 200 metre runners with soccer, lacrosse and football players on a treadmill that measures forces. Most runners use the leg like the spring of a pogo stick, catching the body and bouncing it back up. Elite sprinters don't. They raise the knee high, drive the foot down with a stiff ankle, and stop the lower leg abruptly, in just over two-hundredths of a second after touching down. It is less a bounce than a punch.

Direction matters as much as size. At the start, sprinters lean forward to aim their push more horizontally, and studies find that how force is applied, not just how much, best predicts speed. Even Usain Bolt is a puzzle here: scoliosis gives him an uneven stride, and researchers still can't say whether it helps or hurts.