While many of the athletes that line up for one of the planet's great marathon races will be sporting the latest high-tech footwear, an increasing number of running enthusiasts are casting aside their Nikes and Asics in favour of a more free style of running. Preferring to run barefoot, these athletes land on the balls of the foot rather than striking the ground heel first. ‘I've been a barefoot runner for about 12 years’, says James Webber from the University of Arizona, who explains that his passion took him to graduate school to learn about the evolution of human running. However, something bothered Webber. ‘In running, people can change foot postures and it does not seem to change energetic costs very much’, he says – which made him wonder: why are we so locked into landing on our heels when we walk?
Walking humans effectively move like an inverted swinging pendulum – pivoting above the foot that is in contact with the ground as we step forward – and Webber explains that when we transfer our weight forward from the heel to the toe, the centre of pressure slides across the sole of the foot. Reasoning that this could mean that the true centre of pendulum rotation is effectively several centimetres beneath the ground, Webber wondered whether our conventional flat-footed style of walking effectively lengthens the leg. If so, this could improve our efficiency and allow us to walk faster than if we walked toe-first like many other animals.
To test this theory, Webber and his thesis advisor David Raichlen set about observing what happened to walkers when they reversed their normal footfall pattern to walk like a ballet dancer – landing on the ball of the foot first before allowing the heal to touch down. Webber recruited 14 volunteers to try out the novel walk. ‘It feels a little like you are walking on the catwalk’, laughs Webber, who admits that it took a while for some of the volunteers to take the alternative gait seriously. But once they had got the hang of sashaying along the walkway, he was able to film them walking over speeds ranging from a gentle 0.78 m s−1 to a brisk 1.47 m s−1 before asking them to revert to their more conventional heel-first footfall pattern.
Calculating the effective leg length of the volunteers as they walked conventionally and toe-first, Webber was impressed to see that the walkers’ legs were effectively 15 cm longer when the heel hit the ground first. And when he and Raichlen compared the cost of the two styles of locomotion, the toe-first walkers were having to work ∼10% harder than when they landed on the heel first. Webber also asked the volunteers to walk on a treadmill as he slowly increased the speed until they had to shift up gear into a run – because it was no longer efficient to continue walking – and found that they switched to a run at lower speeds when touching down with the front of the foot first, suggesting that the toe-first walk was less efficient than conventional walking.
So, we appear to gain the mechanical benefit of longer legs when landing on the heel first while walking because weight is transferred forward along the foot during a stride, which effectively shifts the stride pivot point several centimetres beneath the sole of the foot to extend the virtual leg. And Webber suggests that our unusual running style could be a relic of our evolutionary past. Explaining that the combination of a heel-to-toe gait and a long rigid foot appears to be essential for economical walking in our ancient ancestors, Webber suspects that early humans retained the unusual posture – despite developing shorter toes to generate a stronger push off – when they took up running in pursuit of prey.