Speed comes from force into the ground, not from faster legs: top sprinters push down and back with more than 2.5 times their body weight.
The most useful sentence about sprinting is a negative one: faster runners don't swing their legs through the air any quicker. Flight time between steps sits at about 0.12 seconds for fast and slow runners alike, and repositioning time is near identical. The difference lives in force per ground contact — over 2.5 times body weight in the quickest athletes. The second layer is direction: during acceleration, average horizontal ground reaction force runs at roughly 0.35 body weights, about 0.425 across the first ten steps, and elite sprinters generate more of it than sub-elite at any given speed. What doesn't help is the old \"claw the ground\" cue: no correlation showed up between pawing kinematics and horizontal force. What does help is less braking impulse — propulsive impulse alone explains around 57 percent of the differences in sprint speed.