The physics of human walking and considering the wider picture

Gorilla walking like a man. CC
A gorilla walking like a man. Photo by is licensed under CC BY 2.0

Impulsive ankle push-off powers leg swing in human walking
Susanne W. Lipfert, Michael Günther, Daniel Renjewski, and Andre Seyfarth
J Exp Biol 2014 217:1218-1228.

I love papers like this. The extreme level of detail people go to in the quest to discover is fascinating. The question that always comes to my mind is “What made them decide to research this in the first place?” Was it through trying to understand more about where humans come from or was it purely a mathematical and physical curiosity about how we walk? The beauty of science is that it is allowed to be curiosity driven – just like those papers and pieces of research which made it into the IgNobel Awards and have ended up being pivotal in their respective fields.

“Impulsive ankle push-off powers leg swing in human walking” is a rather complicated paper, full of mathematical terminology and notation, but if you can get through the equations you’ll discover a story that shows how human walking has been fine-tuned, linking this physical paper to fundamental questions of human origins.

Lipfert and colleagues describe the walking cycle, and focus on the transition between planting the trailing foot, to lifting off the ground and finally to propelling the hind leg forward. I’d always thought that it was the arches of my feet (especially since I am very proud of my ballet-style high arches!) that provides the propulsion, but Lipfert and colleagues suggest that the more important contribution is made by the combined system of the muscles and tendons in the ankle. Only once your leading leg is planted, taking the weight of your body, is the energy stored in the tendons and muscles of your ankle joint realised, spurring your body forward. This is part of the refined and efficient motion produced by evolution.

Figure 1 from Lipfert and colleagues' paper
Figure 1 from Lipfert and colleagues’ paper

The authors don’t really discuss the evolutionary implications behind their study and previous studies, but that is exactly where this paper interests me. (Bipedal) Walking is one of the attributes that distinguishes us from other apes. There are many obvious attributes that make Homo sapiens, from hair reduction to the size of our brains, making us appear more distantly related to other apes than we are, but it’s when I see gorillas or chimps walking or wading on two feet that I am once again reminded of our shared origins.

Evolutionary anthropologists could argue for years about the ‘most important’ attribute leading to the radiation of humanity but, without walking, who is to say that Homo sapiens would have achieved the global distribution that it has? Would Homo sapiens have been able to embark on those voyages of self and external discovery had they not been able to wander through the savannahs from Ethiopia? Without walking, would Homo sapiens have been able to track and hunt the large game that fed our growing brains and at the same time contributed to the loss of Earth’s Pleistocene megafauna?

Lipfert et al. 2014
Lipfert et al. 2014

It’s grand questions like this that drive scientists to work day by day on the minute details. One of my favourite science quotes is simply “Consider the embryo” – it’s a piece of advice all developmental biologists should heed as they do their thousandth in situ on the same region of their embryo. I took that advice during my own PhD; when I hit a rut and got lost in the repetitive details I was only refreshed once I took a step back and thought about the big picture. Though this paper focuses on a very specific aspect of walking, detailing the precise biomechanics of the ankle movement, consider it in the bigger picture – it contributes to the story of humanity. And that’s what made Lipfert and colleagues decide to do this piece of research.

“During alleviation, support of the body mass is discontinued by the opposing motions of the knee and ankle joints of the trailing leg (contrasting the in-phase motion of both joints observed in human running). With that, launching is enabled, where the smaller mass of the trailing leg exhibits a powerful acceleration into swing by efficiently utilizing elastic energy storage.”

Walking is fascinating.

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Lipfert et al., 2014, Impulsive ankle push-off powers leg swing in human walking, J Exp Biol – pdf freely available from Oregon State University here


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