UNIVERSITY PARK, Pa. — The average person takes thousands of steps each day, instinctively walking heel-to-toe. Although they hardly register them, those consistent heel-first strides are more unique to humans than previously thought and allowed early hominins to walk farther more efficiently, but at the cost of higher impact forces on bones and joints, according to a new study led by a researcher at Penn State.
The team said the findings, published today (Sept. 8) in the Proceedings of the National Academy of Sciences , shed light on an evolutionary adaptation that made early human walking more efficient, ultimately helping our ancestors to adopt hunting and gathering practices and spread across the world.
Previous research relying on descriptive data has suggested that humans share the heel-first, or heel-strike, walking style with other great apes like the closely related chimpanzee. The authors of the new study used high-speed, high-resolution video equipment and a flat runway lined with metal plates that measure impact forces to examine how chimpanzees and humans walk.
They found that chimpanzees vary their walking style, landing on the side or ball of their foot a majority of the time and landing heel-first less often when walking on two legs compared to moving on all fours. Humans, on the other hand, always land with a heel-strike. The researchers found that this walking style saves modern humans an estimated 26% to 41% of the metabolic energy it would take to walk on the ball of the foot but results in higher impact forces, which required bigger, stronger bones and joints — later adaptations that would allow early humans to cover more ground in a safe, consistent way.
"When people think about what makes human walking unique, they talk about upright walking — our backs are straight and our legs are extended," said study author Nicholas Holowka , assistant professor of anthropology and co-hire with the Huck Institute of Life Sciences at Penn State. "What we're saying in this article is that landing on our heel is a unique aspect of human walking that's maybe been underappreciated. This study presents the best measurable comparison of humans to our closest living relatives, chimpanzees, which shows that what we're doing when we land is distinctive in key ways."
The earliest human ancestors likely walked the way chimpanzees do — with less consistent, more variable landings, sometimes on the balls of their feet or flat-footed, other times on their heels, said Holowka. He described the walking style as less efficient than that of modern humans and explained that it probably confined how far they could travel. The heel strike, he continued, is much more efficient, allowing humans to roll from the back of the foot to the front for a smooth ride. But it comes with the tradeoff of higher impact forces, that cause more rapid loading on the leg.
"Imagine you are trying to sneak across a creaky wooden floor," said study co-author Nathan Thompson, an associate professor at New York Institute of Technology, College of Osteopathic Medicine. "You tend to walk on the balls of your feet, because this reduces the rate of loading on the floor and creates less creaking. It's a softer way to walk."
These more impactful forces could have potentially been damaging to the body and ultimately required later adaptations to allow early humans to walk in a safe, consistent way, according to the researchers.
"It seems like humans needed to evolve anatomical adaptations — thicker heel bones and bigger knee and ankle joints — to cope with these high forces," Holowka said. "We think that the heel-strike played a role in enabling key human behaviors, such as using hunting and gathering strategies that require people to walk really long distances every day to get food, but for a big payoff in food resources."
The researchers used high-speed, high-resolution cameras and an approximately 36-foot-long walkway lined with metal force plates at Stony Brook University to examine how three male chimpanzees walked on all fours and on two legs. They augmented their data collection by analyzing walking data from three more chimpanzees collected by another research team in a separate study and reviewed additional data from two other studies from a combined 15 chimpanzees.
The team also recruited 12 human participants for similar laboratory experiments at the University at Buffalo. Nine participants wore reflective markers and walked barefoot on an approximately 25-foot-long runway lined with force plates and high-speed, high-resolution cameras. These participants walked the runway multiple times, using their natural gait as well as an altered gait that mimicked the way the chimpanzees preferred to walk, landing on the ball of the foot just behind the little toe before bringing down the heel. The researchers referred to the latter style as midfoot-strikes. The human participants also walked on a treadmill while wearing a portable respirometry system that measured oxygen consumption and carbon dioxide production. They used these measurements to calculate how much energy the participants expended using the different walking styles.
The researchers found that the chimpanzees varied their walking style, using a mix of heel-strikes and midfoot-strikes. However, they used fewer heel-strikes when walking on two legs than when walking on all fours.
"It seems that when walking on two legs, chimpanzees prefer the 'softer' way to land on their feet," Thompson said.
All normal gait human steps, however, were heel-strikes, with the angle of foot contact relative to the ground very consistent within and between individuals. Chimpanzees, on the other hand, used 2.4 to 8.6 times greater ranges of foot-strike angles than the human participants, the researchers reported.
The team also found that the heel-strike generated much higher impact forces than midfoot-strikes in both chimpanzees and humans. In chimpanzees walking on two legs, the rate of impact force loading ranged up to 138% higher for heel-strikes than midfoot-strikes. In humans, these forces ranged up to 162% higher than midfoot-strikes. However, walking heel-first saved the human participants an estimated 26% to 41% in expended metabolic energy compared to midfoot-strikes.
"We humans have evolved to use this high-impact walking style in a way that is safe and natural for us," Holowka said. "The novel environments that we currently live in with their hard, paved surfaces might upset the evolutionary balance in some ways. It's something that we'd be interested in looking at in future research. Still, walking a lot is a very healthy exercise and, although high impact, our bodies evolved to cope with it, so we shouldn't avoid it."
Penn State doctoral students Zacchariah Apolito and Kevin Palmisano contributed to this work. Other study co-authors include Matthew O'Neill, Midwestern University; Vincent Bhandal, Otto Lam and Brigitte Demes, Renaissance School of Medicine at Stony Brook University; Caleb Massimi, Icahn School of Medicine at Mount Sinai, New York; and Steven Worthington, Harvard University.
The research was funded by the U.S. National Science Foundation under grant BCS-0935321 , the Leakey Foundation and the Wenner-Gren Foundation. This content is the responsibility of the authors and does not necessarily reflect the views of the funders.
