Most running mammals totter along on their toes. In fact, toe running is far more efficient than landing heel first like humans. Yet when it comes to long distance endurance running, humans are some of the best-adapted animals for clocking up the miles, all be it inefficiently. So, why have we stuck with our inefficient heel first footfall pattern when the rest of our bodies are honed for marathon running? This paradox puzzled Nadja Schilling and Christoph Anders from the Jena University, Germany, and Christopher Cunningham and David Carrier from the University of Utah, USA, until they began to wonder whether our distinctive heel first gait, inherited from our ape forefathers, might be an advantage when we walk. The team put young healthy volunteers through their paces to find out why we walk and run heel first and publish their results on 12 February 2010 in The Journal of Experimental Biology at http://jeb.biologists.org.
Measuring the amount of oxygen consumed as their human subjects walked, the team asked the volunteers to walk in one of three different ways: normally, with the heel contacting the ground first; toes first, with the heel slightly raised so that it didn't contact the ground; and up on tip-toes. Then the scientists asked the athletes to repeat the experiments while running heel first and with their heels slightly raised. Calculating the amount of energy required to run and walk, the team found that walking with the heel slightly raised costs 53% more energy than walking heel first, and walking on tip-toe was even less economical. However, there was no difference between the runners' efficiencies when they ran with flat feet and up on their toes.
Our 'heel first' gait makes us incredibly efficient walkers, while both postures are equally efficient for runners. Human walkers burn roughly 70% less energy than human runners when covering the same distance. However, this efficiency would be completely wiped out if we switched to walking on our toes. 'Our ability to walk economically may largely be the result of our plantigrade [heel first] posture,' says Carrier.
But why is heel walking so much more efficient than walking on our toes? To find out, Carrier and his colleagues asked volunteers to run and walk at various speeds in the three postures while recording electrical activity in their muscles to see if the heel first walkers were saving energy by using their muscles differently from toe first walkers. The team also measured the volunteers' metabolic cost of standing on their toes, to find out if increasing stability saved energy, and the forces exerted by the ground on the volunteers' bodies, in case they were reduced in any way that could result in an energy saving.
Analysing the results, the team realised that we lose less energy as our heels collide with the ground than we do when we walk toes first. Landing heel first also allows us to transfer more energy from one step to the next to improve our efficiency, while placing the foot flat on the ground reduces the forces around the ankle (generated by the ground pushing against us), which our muscles have to counteract, resulting in another energy saving.
So we still use our ancestor's heel first gait because it makes us better walkers and Carrier adds, 'Given the great distances hunter-gatherers travel, it is not surprising that humans are economical walkers'.
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REFERENCE: Cunningham, C. B., Schilling, N., Anders, C. and Carrier, D. R. (2010). The influence of foot posture on the cost of transport in humans. J. Exp. Biol. 213, 790-797.
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THIS ARTICLE APPEARS IN THE JOURNAL OF EXPERIMENTAL BIOLOGY ON: 12 February 2010. EMBARGOED UNTIL FRIDAY, 12 February 2010, 00.15 HRS EST (05:15 HRS GMT)
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Journal of Evolutionary Biology