The skip in the human footstep – the flexing of the arch with each step – does little to help carry the body forward, a new study has found. Instead, it serves to keep the ankle upright, so that we walk in the characteristic way of human beings and not like other apes.

The finding overturns conventional wisdom and may help in the treatment of people whose arches have become rigid due to illness or injury.

“We thought originally that the spring-like arch helped to lift the body into the next step,” says Lauren Welte, a researcher at UW-Madison and the first author of the study, in a press release. But the project found that “the spring-like arch recoils to help the ankle lift the body,” she says.

Why Humans Walk Like Humans

The subtle flexing acts as a linchpin for humans to remain upright, the team from Canada, the United States, Sweden and Australia found. In particular, the study points to one of the most underappreciated joints in the human body, the union between the navicular and medial cuneiform, which is crucial to the recoil. Without it, humans would walk more like chimpanzees, our close genetic relatives.

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In such a scenario, our ankles would angle forward as we walked and not remain relatively upright, as they do now. We’d lose the efficiency of the latter, especially when running, which might be how we evolved arch recoil in the first place. The usefulness of it while hoofing away from predators (or toward food) may have driven natural selection, the researchers say.

“The mobility of our feet seems to allow us to walk and run upright instead of either crouching forward or pushing off into the next step too soon,” says Michael Rainbow, an assistant professor at Queen’s University, in a press release.

How the Study Investigated Arch Recoil

The study scanned the feet of seven young men and women, with high-speed X-ray photography, as they walked and ran, to watch the complex interplay of bones and connective tissue. The team saw that the recoil allowed the foot to remain in contact with the floor longer and better propel the body forward.

In another phase of the project, the researchers used a computer model to simulate a foot with no flex and found that its style of walking would have consumed more energy.

Rainbow says the team’s surprising take on arch recoil needs additional research and testing, given the initial study’s small sample size.

“That said, our work sets the stage for an exciting new avenue of investigation,” he says.

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