Why do humans step on their heels, while most animals do so on the balls of their feet? A longer leg is better suited to walking and running, so why would we evolve shorter ones? A new study found that stepping heel-to-toes makes for a longer “virtual leg”, allowing us to walk and run more efficiently.

Walking

Image via giphy

For mammals aiming to get better at walking, the best bet is to evolve longer legs. You can see it in how cats’ or dogs’ legs are shaped — the heel is high in the air and the foot touches the ground on the little balls behind the toes. But humans, who are walkers by excellence, have dropped this system altogether and evolved a heel-to-toe stepping style.

For University of Arizona anthropologist James Webber, this question was particularly intriguing — he took up barefoot running 12 years ago and has had plenty of time to ponder on how his feet hit the pavement. Shod runners typically land their steps with an initial heel strike, while barefoot runners tend to land on the middle part or the balls of the foot. This, however, would feel unnatural to them while walking.

So what gives?

In his latest study, Webber describes why humans have evolved this seemingly counter-intuitive stride.

“We’ve dropped our heels down on the ground, which physically makes our legs shorter than they could be if were up on our toes, and this was a conundrum to us (scientists),” Webber said.

So Webber and co-author UA anthropologist David Raichlen set up a treadmill in the University’s Evolutionary Biomechanics Lab and started looking at people walk. They asked some participants to walk normally and others to walk toe-first. The later group moved slower and put in 10% more effort than their counterparts.

The two believe that the answer still comes down to limb length. While animals usually elevate the heel to increase this value, Webber says heel-first walking creates a longer limb by adding some “virtual leg”. He describes a walking human as an inverted pendulum. Our bodies can be seen as pivoting over the point where the soles come in contact with the ground. As we step, our weight is distributed along the length of the sole and the true pivot point “forms” midfoot several centimeters below the ground, allowing for longer strides.

“Humans land on their heel and push off on their toes. You land at one point, and then you push off from another point eight to 10 inches away from where you started. If you connect those points to make a pivot point, it happens underneath the ground, basically, and you end up with a new kind of limb length that you can understand,” Webber explains.

“Mechanically, it’s like we have a much longer leg than you would expect.”

To take it another way, let’s simplify a step taken by an animal into three points forming a downward triangle. The lower point would be where the foot touches the ground, and the two points on top would be where the hips are at the start and the end of the stride. The longer the line connecting the step and hip points (the leg,) the longer the stride becomes. Webber found that while the step point is at ground level for other animals, it’s actually underground for humans. This makes the sides of the triangle longer than our legs actually are because we gain “extra leg” underground, so to speak.

A leg up

The team found who subjects that walked normally had legs that were, in essence, 15 centimeters (5.9 inches) longer. Even better, this virtual limb length means we’re more efficient walkers than if we landed on the balls of our feet.

“The extra ‘virtual limb’ length is longer than if we had just had them stand on their toes, so it seems humans have found a novel way of increasing our limb length and becoming more efficient walkers than just standing on our toes,” Webber said.

“It still all comes down to limb length, but there’s more to it than how far our hip is from the ground. Our feet play an important role, and that’s often something that’s been overlooked.”

When speeding up the treadmill to study the transition from walking to running, the team also found that toe-first participants switched to running at lower speeds — further suggesting that it’s less efficient for humans.

Archaeological evidence (footprints found preserved in volcanic ash in Latoli, Tanzania) shows that ancient hominids have been heel-to-toe walking for at least 3.6 million years now. But they likely had rigid feet, proportionately much longer than our own — about 70% the length of their femur, compared to our 54%. This likely made them better runners than modern humans, but Webber thinks we’ve evolved shorter legs to become better hunters and pursuers.

“When you’re running, if you have a really long foot and you need to push off really hard way out at the end of your foot, that adds a lot of torque and bending.”

“So the idea is that as we shifted into running activities, our feet started to shrink because it maybe it wasn’t as important to be super-fast walkers. Maybe it became important to be really good runners,” Webber concludes.

Well, now at least I know why my girlfriend walks so slow in heels — she’s losing more virtual leg than gaining actual leg. Not that I’m complaining.

I’m all about actual legs.

The full paper “The role of plantigrady and heel-strike in the mechanics and energetics of human walking with implications for the evolution of the human foot” has been published in The Journal of Experimental Biology.

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