Why this exercise? Strong hip flexors are crucial for sprinting fast. They’re what lift your leg, so stronger hip flexors = faster knee drive = more speed. Cable knee drives are by far the best hip-flexor exercise you can do, because they build strength both in front of and slightly behind the body—something most hip-flexor exercises don’t do. That’s why you see the fastest men on the planet doing them. They usually go a bit lighter and move fast, but if you don’t have any strength to begin with