It’s something so fundamental it can seem arbitrary, but the strides we take when walking are actually unique in the animal kingdom. Many animals, such as cats and dogs, get around on the balls of their feet, while humans have a clear heel to toe stride.
James Webber, a barefoot running enthusiast and doctoral student at the University of Arizona School of Anthropology, has been researching the ways humans run and walk over the past few years. His latest study, published in the Journal of Experimental Biology, explains why human’s walk the way they do
Humans are very much set in the way they walk, something which obscures the fact that our stride is actually pretty curious.
To Webber it is a particularly pertinent issue. Barefoot runners run in a way that sees them land on the middle or balls of their feet instead of their heels, a stride which would feel unnatural when walking.
“Humans are very efficient walkers, and a key component of being an efficient walker in all kinds of mammals is having long legs,” Webber explains in a press release from the University of Arizona. “Cats and dogs are up on the balls of their feet, with their heel elevated up in the air, so they’ve adapted to have a longer leg, but humans have done something different. We’ve dropped our heels down on the ground, which physically makes our legs shorter than they could be if we were up on our toes, and this was a conundrum to us (scientists).”
Webber suggests our stride can be explained by limb length. Key to his explanation is the idea that humans move like an inverted swinging pendulum. The body essentially pivots about the point where the foot meets the ground, with the centre of pressure sliding across the length of the foot from heel to toe as we take a step. This means that the true pivot point occurs in the centre of the foot, but at a ‘virtual point’ several centimetres below the ground.
Our heel to toe stride then, extends the length of our ‘virtual legs’: the points we pivot around which are longer than our actual legs.
“Humans land on their heel and push off on their toes. You land at one point, and then you push off from another point eight to 10 inches away from where you started.” Webber says in the press release. “If you connect those points to make a pivot point, it happens underneath the ground, basically, and you end up with a new kind of limb length that you can understand. Mechanically, it’s like we have a much longer leg than you would expect.”
Webber and his team reached their conclusion after monitoring study participants on a tread mill. They compared those who walked normally with those they’d asked to walk toe first. They quickly observed that those who had to walk toe first were slower, and used 10% extra energy in their strides.
“The extra ‘virtual limb’ length is longer than if we had just had them stand on their toes, so it seems humans have found a novel way of increasing our limb length and becoming more efficient walkers than just standing on our toes,” Webber said. “It still all comes down to limb length, but there’s more to it than how far our hip is from the ground. Our feet play an important role, and that’s often something that’s been overlooked.”
Ancient footprints preserved in volcanic ash in Tanzania show that ancient hominins had a heel to toe stride up to 3.6 millions years ago. Nevertheless, our feet have changed over the years. Early bipeds had longer feet than modern humans – 70 percent of the length of the femur compared to 54 percent in modern humans, which likely helped them become very fast, efficient walkers. Webber suggests that as modern humans evolved the heel first style of walking their feet got proportionally smaller, allowing us to become better runners, possibly as a means to evade predators.
“When you’re running, if you have a really long foot and you need to push off really hard way out at the end of your foot, that adds a lot of torque and bending,” Webber said. “So the idea is that as we shifted into running activities, our feet started to shrink because it maybe it wasn’t as important to be super-fast walkers. Maybe it became important to be really good runners.”