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* The foot is landing ahead of the hips rather than under the hips. This causes some of the landing force to go up the leg and push back on the runner, acting as a break.
* The leg is straight, so there is little shock absorption from the quads that would occur as the knee bends.
* The heel touches down first, so the calf [[Muscle|muscles ]] cannot absorb the landing forces in the way that happens when the front of the foot lands and moves up.
* Because the foot is so far ahead of the body, there is no chance of paw back, so the foot is travelling forward at the same speed as the body.
These factors cause the foot to make contact with the ground while still moving forward, and that force is not absorbed by the quads or calf. The force slows down the runner and is transmitted through the foot, up the ankle, lower leg bones, through the knee and on up the body putting higher levels of stress than would have been the case with a better running form.