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.
