Efficient race walkers do not let their feet lift passively off the ground; instead, just before the rear foot breaks contact with the ground, they actively push the big toe against the ground. If you add this simple flick, you feel a slight float to your stride. Done properly, overall race walking technique becomes less mechanical and movements feel as if they are flowing together in a forward motion. Done improperly, an overly hard push off causes over-striding and flagrant loss of contact with the ground.
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