To run fast, an athlete needs to run on the balls of their feet. But most young athletes sprint on their heels. Using a sled weighted heavy helps to reinforce good sprint mechanics we want them to have.
By marching with the heavy sled, the athlete presses the ball of his foot forcefully into the ground. Bill Parisi, from Parisi Speed School, says: “An athlete’s ability to accelerate is summed up by their ability to “push”. Objects do not speed up, slow down, or change direction unless they are acted upon.”
Newton’s Second Law states this idea: “The acceleration of an object results from the application of a force”.
So we give the body something to react to: a heavy sled. It takes “force” to get that thing going! And we teach our athletes to “press hard into the ground” with ball of their feet.