Why do we land on our heels when other animals walk on their toes?
A study on the mechanics of walking could provide glimpses into how our earliest ancestors got around too.
Next time you see a person out walking their dog, watch how their feet land with each stride. The dog trots along on its toes, but its owner strikes the ground with his or her heel, then rolls the weight towards their toes before pushing off for the next step.
It turns out our stumpy toes and penchant for strolling mean this way of walking is the most energy efficient way of getting around – even though most other animals, including our nearest primate relatives, tend to walk on their toes.
James Webber and David Raichlen from the University of Arizona in the US filmed the gait of 14 people walking normally – with their heels striking the ground first – and abnormally, with the balls of their feet making initial contact.
Not surprisingly, they found the more awkward, non-heel-strike gait meant slower walking speeds and more difficult transitions to running. But they suggest that the shape of the human foot, with relatively short toes, is ideally suited to a heel-strike stride style.
The work, which may also help provide insights into our hominin ancestors, was published in the Journal of Experimental Biology.
Our heel-strike style of locomotion, evolutionary biologists believe, evolved to reduce energy expenditure while walking. Generally, a longer stride means less energy is used, but when we land on our heel instead of our toes, we are in effect shortening our limb length.
Basically, our feet are ill-equipped for non-heel-strike walking. Bonobos (Pan paniscus) and chimpanzees (Pan troglodytes), with their comparatively long toes, are able to change up their gait between heel-strike and other stride types.
So to better uncover the advantages and disadvantages of different gaits, Webber and Raichlen stuck motion capture reflectors on seven men and seven women and filmed them walking barefoot on a track and on a treadmill using a heel-strike gait followed by a non-heel-strike gait, where they were instructed to let the balls of their feet make first contact with the walking surface.
When hitting the ground with the balls of their feet, the participants walked slower. And the transition from walking to running, when the treadmill was cranked up, was trickier.
During a normal stride, a heel strike does shorten leg length – but only momentarily, as a person's centre of pressure shifts towards their toes. A non-heel-strike gait means centre of pressure shifts back towards the heel then to the toes.
Given our foot shape and toe length, heel strike is the most energy efficient way of walking, the pair conclude.
It appears this style has been around for a few million years. Australopithecus africanus, a hominin that walked on two legs but still spent significant time in the trees and preceded Homo, had relatively long feet. And fossilised A. africanus footprints found in volcanic ash in Laetoli, Ethiopia, suggest that by 3.6 million years ago, hominins walked with a heel-strike technique.