For more than a century, evolutionary biologists have admired the exquisite design of the human foot and how its features make it possible for us to effortlessly walk upright. Our short toes, for example, enable us to run long distances.
Now, a paper published Wednesday in Nature makes the case that another part of our anatomy—an arch running over the top of the foot—plays a bigger role than previously thought in mobility. The finding increases our understanding of the evolution of foot biomechanics, experts say, and could lead to more accurate robotic and prosthetic feet, help orthopedic doctors treat foot disorders and even inspire better shoe designs.
Called the transverse tarsal arch (that’s the horizontal curve across the top of your foot), this previously underappreciated attribute accounts for more than 40 percent of the stiffness of the modern human foot, according to the team of researchers from the United States, Japan and the United Kingdom. This upper arch tag teams with the better known example along the bottom side of the foot called the medial longitudinal arch. Together, they account for our uniquely human feet’s stiffness, which allows us to push off without falling over and distinguishes us from other primates that need a more flexible foot to grasp tree branches.
“We were surprised by what an effect it had,” says Madhusudhan Venkadesan, the study’s lead author and an assistant professor in mechanical engineering and materials science at Yale University. “There have been major debates on how the shape of the foot relates to stiffness, but they’ve concentrated on the medial longitudinal arch [the long one from the ball to the heel on the inside of the foot].”
It’s easy to comprehend the relationship between an arch’s curve and foot stiffness if you grab a dollar bill. Lay the money flat and slightly curl its long edges so the middle bends up—as if forming a tube or highway tunnel. This creates an arch, running lengthwise down the bill. Push a finger on the middle of the bill’s arch, and you’ll notice some resistance or stiffness. Venkadesan’s team wanted proof that a similar principle—that also explains why folding pizza makes it less floppy—was at work in our feet.
“We had to come up with a way to test this idea in real feet,” he says.
So they designed a series of experiments in which they conducted bending tests on the feet of two human cadavers. In living humans, it’s too difficult to isolate the role of the transverse arch because it works in sync with other foot parts. But in the cadaver feet, the researchers were able to remove the elastic tissue in between the long bones—called metatarsals—in order to directly measure the arch’s impact on foot stiffness.
The next step was to understand the role of the transverse arch in the context of human evolution. So Venkadesan’s team developed a mathematical model to reconstruct the history of the human foot by comparing our current arch with fossils from extinct hominin species.
Just as they suspected, the appearance of the transverse arch—which appeared in other hominins more than 3 million years before modern humans walked the earth —was an important element of bipedalism. The medial longitudinal arch followed—arriving 1.8 million years ago, to be precise. And the combo created the necessary stiffness that enabled us to eventually run marathons and take jumping pictures for social media.
Experts say the new study is valuable because it’s the first to quantify the stiffness of the transverse arch.
“We’ve known about the presence of the transverse arch for a long time, but we’ve never had a way to measure it, and we didn’t know how it affected the overall function of the foot,” says Nicholas Holowka, an assistant anthropology professor at the University of Buffalo in New York who studies the evolution of the human foot. “This profoundly adds to our understanding of how the unique shape of the human foot enables our unique bipedal locomotion.”
So what does this research mean for flat-footed folk? The transverse arch is their supportive unsung hero.
The flat-footed lack of a medial longitudinal arch can cause stress to other areas of the body and lead to foot pain. At one point, it was grounds for automatic rejection from the military.
But Venkadesan’s research sheds light on why the majority of flat-footed people don’t suffer from chronic pain or injuries, Holowka says.
“You can imagine you can have flat feet with a low longitudinal arch, but because you have a relatively high transverse arch, you can still have a stiff foot.” Holowka says, adding that future research should examine any links between people’s degrees of flat-footedness and their transverse arches. He is also calling for ways to quantify this transverse arch curvature in living people to better understand foot pain, which might be the key to building corrective orthotics.
Other future research should look at the range of transverse arch anatomy among humans to probe the correlation between high curvature and high levels of stiffness, adds Glen Lichtwark, an associate professor in biomechanics at the University of Queensland in St Lucia, Australia.
“You might have a high curve, but you might have a tradeoff somewhere else. Or you might use your muscles differently. We don’t know these things yet,” Lichtwark says
According to Lichtwark, who co-authored an accompanying article in Nature, this research has practical applications for foot health, including designing robotics and prosthetics and explaining the mystery why orthopedic surgeries provide pain relief for some patients and not others. Also in the future, shoe store employees might be able to scan your foot and provide personalized recommendations based on the total structure of your foot.
“This research gives us another dimension of the complex structure of the foot,” Lichtwark says. It highlights that the foot is three-dimensional, and we need to start to start thinking about it like that.”