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The “Double Bounce”

We take it for granted. We do it every day. It’s a basic function of us bipeds.


There are 7.8 billion of us on the planet now and word has it (source: Google) that we walk an average of about 5,000 steps a day. That’s a lot of steps.


Walking is simple, right?


Wrong.


Walking, in fact, is “shockingly complicated,” according to this article in Wired magazine.


Have you forgotten how to walk? YouTube is here to help. There are lots of instructional videos like this one (with some pretty good tips).


Compared to other mammals, us humans walk “weird.”


In fact, our gait has “confounded” both engineers and biomechanists.


A single step can be broken down into several individual motions.



Heel strikes floor. Balance on one leg. Roll onto your toes. Your free leg goes into a forward swing. Next heel strikes floor. And so on.


But here’s the mystery: the planted leg bounces twice before swinging into the next step.


You can see the bounce in the knee as it bends and extends. First, when the foot first touches down. Second, just before takeoff.


The first bounce makes sense. It helps the foot absorb the impact of our weight as we hit the ground.


The second bounce? It’s kind of a mystery.


Photo by Brian Mann on Unsplash


But (drum roll, please) scientists at the University of Munich think they may have found an answer.


It might be an “energy saving technique for a species that has long prioritized endurance over speed.”


Blame the design of the human foot! Humans have a 90-degree angle between the foot and the leg. And, compared to other mammals, that’s strange. Most animals walk on their toes or the balls of their feet while we walk heel to toe. To add to the complicating factors, our legs are relatively heavy as a proportion of our entire body.


As a result, forward motion is a “mechanical challenge.”


Scientists have determined that the foot stays planted for up to 70% of the step cycle. This helps us stay balanced when we’re walking. (When we run, there is no double bounce.)


Researcher Daniel Renjewski said it’s as if nature came up with a “clever trick” to circumvent the limits of human body design.


The foot stays planted to keep us balanced while the ankle takes advantage of that downtime and builds up energy for the eventual release.


Renjewski says that walking this way would have given early humans an edge in persistence, hunting, pursuing animals until they surrendered from fatigue.


“Our flat feet and heavy legs aren't optimized to let us move as fast as four legged sprinters, so it's possible that our gait pattern evolved to grant us an advantage for distance, not speed, because the second bounce catapults the leg from the ankle rather than powering its swing from the hip. The motion uses a lot less energy, allowing our ancestors to stalk prey for hours or days without needing to recover,” says the article in Wired.


Nature usually takes the simplest route, says Renjewski. Humans wouldn't have evolved this complexity unless it conferred an advantage, he says. It obviously gave our ancestors some extra benefit that was worth the effort.


Heading out to stalk prey? Or just taking the dog for a walk?


Now you know.

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