The way humans plant their heel with every step, something so automatic most people never think about, may have played a more important role in human evolution than previously recognized.
A study published Tuesday in the Proceedings of the National Academy of Sciences found that heel-first walking is more distinctly human than researchers had assumed. The researchers argue that the energy advantage of this gait may have helped early hominins travel longer distances than would have been possible with a more variable, ape-like foot-strike pattern.
The research was led by Nicholas Holowka, an assistant professor of anthropology at Penn State, with collaborators from Stony Brook University, the University at Buffalo, New York Institute of Technology, and several other institutions.
A Closer Look at How We Actually Land
For years, the working assumption in anthropology was that humans share their heel-first gait with other great apes, including chimpanzees, based largely on observational data. The new study set out to test that assumption directly using detailed biomechanical measurements.
The team filmed three male chimpanzees walking on all fours and on two legs along a roughly 36-foot walkway at Stony Brook University, lined with force plates that measured impact on the ground. They supplemented this with walking data from three additional chimpanzees collected by another research group, along with data from two further studies covering 15 more chimpanzees.
On the human side, 12 participants were recruited for parallel testing at the University at Buffalo. Nine walked barefoot while wearing reflective tracking markers along a 25-foot runway equipped with force-sensing plates and high-speed cameras.
Each person walked the course multiple times: once using their natural stride, and once trying to mimic the way chimpanzees often move, landing near the outside of the forefoot before lowering the heel. The researchers described this as a midfoot-strike pattern.
A subset of the participants also walked on a treadmill while researchers measured oxygen consumption and carbon dioxide production to estimate metabolic energy use.
What the Data Showed
The results pointed to a clear difference between the two species.
Chimpanzees switched between landing styles depending on the situation, more often striking with the middle or side of the foot and using heel-first landings less frequently when walking upright than when moving on all fours.
Chimpanzees also showed 2.4 to 8.6 times greater ranges of foot-strike angles than the human participants, highlighting how much more variable their gait was.
Humans showed far less variation. Every recorded step during normal human walking used a heel strike, with the pattern remaining consistent across participants and strides.
That consistency appears to come with an energy advantage. According to the study, heel-first walking reduced estimated metabolic energy expenditure by about 26% to 41% compared with the experimentally altered midfoot-strike pattern.
Holowka explained that heel-first walking allows humans to roll smoothly from the back of the foot toward the front, helping make each step more economical.
But the trade-off is greater impact.
Among chimpanzees walking on two legs, impact-loading rates were up to 138% higher during heel strikes than during midfoot strikes. In humans, the increase reached as much as 162%.
Nathan Thompson, an associate professor at New York Institute of Technology’s College of Osteopathic Medicine and a study co-author, compared the softer forefoot-style landing to the way someone might cross a squeaky floor quietly, since it reduces the rate at which force is applied.
Why the Trade-Off May Have Been Worth It
Holowka’s team suggests that early human ancestors may have used a more variable foot-strike pattern similar to that seen in chimpanzees.
A shift toward consistent heel-first walking may have made long-distance travel more economical, an advantage that could have supported hunting and gathering lifestyles that required regular movement across large areas.
The higher impact forces, however, would have placed greater stress on the skeleton.
The researchers suggest that anatomical changes such as thicker heel bones and larger knee and ankle joints may have helped hominins cope with those forces as human walking became increasingly specialized.
Relevance for Modern Walkers
Holowka also pointed to an important difference between ancient and modern environments.
Early humans generally walked on natural ground, which is often softer than the pavement, concrete and hard flooring common today. The research team is interested in examining how surface hardness may affect the forces created by heel-first walking, although the current study does not draw firm conclusions about that question.
Holowka also emphasized that walking remains a healthy form of physical activity and is something the human body evolved to perform efficiently.
The study’s authors also include Penn State doctoral students Zacchariah Apolito and Kevin Palmisano, along with researchers from Midwestern University, the Renaissance School of Medicine at Stony Brook University, the Icahn School of Medicine at Mount Sinai, and Harvard University.
Funding came from the National Science Foundation, the Leakey Foundation, and the Wenner-Gren Foundation.
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