How heels help people walk

By David Daversa
Institute of Zoology, University of Cambridge, UK

This summary was highly commended by the judges for Access to Understanding 2013


Most people may not think very much about reasons explaining the shape of our feet. For evolutionary biologists and designers of prosthetic legs however, this topic is of major interest.

A recent study, led by Dr James Usherwood from the Royal Veterinary College in England, provides new evidence that our feet are specifically designed for walking.

To understand why however, requires a brief overview of human walking 101. Just as people dance with certain styles, we also walk with a certain style. This movement is no hokey-pokey, however. Rather, scientists call our walk the inverted pendulum. Inverted pendulums are characterized by objects that move in an arching, rainbow-like fashion overtop of a fixed pivot. A catapult exemplifies an inverted pendulum. Likewise, while walking our bodies act as a weighted object that propels forward over our feet, which serves as a fixed pivot. Imagine the way a wiper blade moves across a car windshield. Such is the motion made by both walking people and inverted pendulums.

Yet, this confusing concept may be best grasped while actually walking. So, let’s do the inverted pendulum (imagine beat of “the hokey pokey”):

We put one foot out, and place the heel down.
We push our bodies forward with the foot on flat on the ground.
Arriving overtop we stand with bodies and legs erect.
We pivot further forward, lifting our heel and using the toes to push off into the next step.

That is what the inverted pendulum is all about.

What has specifically confused scientists is why humans are flat-footed, with heels that touch the ground while walking and standing. As ostriches illustrate, such a foot is not essential for doing the inverted pendulum. These large, flightless birds known for running at impressively high speeds use the inverted pendulum style to walk as well, but their heels are always elevated off the ground. Furthermore, women in high heels still do the inverted pendulum as well. Thus, the ground-touching heel is inessential. Moreover, ostrich feet may actually be preferred, as the need to spend energy lifting the heel off the ground is eliminated.

What these past studies fail to consider however, is the burden that walking places on the leg muscles. Therefore, James Usherwood, from the Royal Veterinary College in the United Kingdom, and colleagues examined muscle use throughout the inverted pendulum walk, to see whether any insights into the function of the heel could be afforded. They did this by first breaking down the inverted pendulum into 3 steps. In Step 1 we place our heel down on the ground. In Step 2 we stand overtop of our feet with bodies and legs erect. Finally, in Step 3 people push off the ground, lifting our heel and using the toes, into the next step. Then they determined when and how lower leg muscles were put to work during each step. What they found was that the shin muscles were used in Step 1 to absorb the initial impact of hitting the ground with our heel. In Step 3 the calf muscles were triggered for the push off. Interestingly, during Step 2 these muscles were relaxed. Furthermore, the heel, then positioned on the ground, absorbed the pressure placed on the foot by standing on top of it. Therefore, Usherwood and colleagues concluded that the grounded heel functions to provide a brief respite for our shin and calf muscles during the inverted pendulum walk.

Coincidentally, this finding explains why people can stand for long periods of time without getting sore shin and calf muscles.

These results provide new insight for human evolution. Recently, some scientists have argued that humans are specifically adapted for running. These findings however, offer new evidence that human bodies are more likely to be designed for walking.

Such findings also have important implications for the design of prosthetic legs for amputees. In light of the results, prosthetic legs would best be designed with so that the heel touches the ground behind the leg, correct? Actually, the opposite is the case. Prosthetic legs do not incorporate muscle-like components as our bodies do. Thus, the value of the ground-touching heel (to reduce the amount of work placed on the leg muscles) is eliminated. Rather, its planted position represents a cost, since lifting the heel off the ground requires unneeded effort.

For this reason, Usherwood and colleagues propose that designs for prosthetic legs mimic ostrich feet, keeping the heel permanently lifted off the ground. Such designs have already been put to the test and are proving effective. The prosthetic leg used by accomplished Paralympic athlete Oscar Pistorius, known as the blade runner, mimics a raised heel form. This South African sprinter holds the world record time for several track and field events. Now that is no hokey-pokey.

This article describes the research published in:

The human foot and heel–sole–toe walking strategy: a mechanism enabling an inverted pendular gait with low isometric muscle force? (2012) J. R. Usherwood, A. J. Channon, J. P. Myatt, J. W. Rankin, T. Y. Hubel J. R. Soc. Interface 9(75), 2396–2402

This article was selected for inclusion in the competition by the Wellcome Trust.