Then as you roll forward, the flat bottom of your foot hits the ground. The muscles of the hindfoot and midfoot, like the achilles tendon and the various plantar muscles on the bottom of the foot, absorb the force from the ground, acting like shocks in a car. Then they have to lift up the heel, levering you forward onto your toes, before the toes push you off into the next step. Walking and running is, basically, falling down and catching yourself over and over.