For something so routine, walking is shockingly complicated. Biomechanists break a single step into several phases: First there’s touchdown, when your heel strikes the floor. Next comes the single support phase, when you’re balancing on that leg. After that, you roll onto your toes for takeoff and your leg goes into a forward swing.
All of this contains a mystery. Researchers have long observed that when we walk, our planted leg bounces twice before swinging into the next step. That is, the knee bends and extends once when the foot first touches down, then again just before takeoff. That first bounce helps our foot absorb the impact of our weight as we
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