BioEd Online

How much energy does it take to move around? You might think it obvious that animals with long legs would use up less energy covering a given distance than would those with short legs. But how much leg length determines the energetic costs of walking or running is hotly debated by scientists.

A mathematical model developed by Herman Pontzer of Washington University in St Louis, Missouri, sheds light on the question, and may even provide an answer to why our ancestors evolved longer legs than their predecessors some two million years ago.

The main cost of running or walking comes from the force needed to support the animal's stance¹. Scientists know that bodyweight plays a big role in determining this force. But what about other physical factors?

Pontzer used basic physics principles to predict how running speed and leg length would affect the forces exerted by an animal. Longer legs, for example, reduce the amount of up-and-down movement in a stride, and so also reduce the force needed to push down with each step. From the total force predicted for a given animal, he could calculate an expected energy use.

To see how well this model works, he put some people, goats and dogs on a treadmill in his lab, and worked out their energy usage by measuring how much oxygen remained in their exhaled air after a stint of exercise. The reality matched up very nicely with his model, he reports in The Journal of Experimental Biology².

Pounding the pavement

Peter Weyand, a researcher of locomotion at Rice University in Houston, Texas, points out that the model doesn't factor in the complexities of muscle physiology, such as how fast-twitch muscle fibres are called into action during intense bursts of activity.

Still, the idea behind the result should open up the way for researchers to study the energetics of locomotion much more easily, which is especially useful for animals that are tricky to get on a treadmill in the lab from tigers to elephants. "All you need are some cameras out in the field to establish hip height and step frequency," says Pontzer.

The model could also offer anthropologists a new way to study energy use in our ancient ancestors. "The fossil record shows that two million years ago, there was a big increase in leg length in early humans," says Pontzer. He suggests that a reason for this increase could have been the energy saved by having longer legs.

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