Awkward, but no less energy efficientDescription:Everst / Alamy By Sam WongHave you ever tried running with your arms straight? Most people find it quite a challenge, so it comes as a surprise to learn that it doesn’t require more energy than running with bent arms. Running with bent arms and walking with straight arms are almost universal habits, but until now, there has been no research that explains why. Andrew Yegian and colleagues at Harvard University recruited eight students to walk and run on a treadmill with their arms straight and bent. Six of the subjects also had their oxygen consumption measured. Advertisement As you might expect, walking with bent arms proved to be more energetically demanding than with straight arms, increasing oxygen consumption by 11 per cent. But unexpectedly, running with straight arms does not appear to be more efficient than running with straight arms. “Pretty much every subject in the study said that straight-arm running was the most challenging condition,” says Yegian. “That’s why it was very surprising when we couldn’t find any difference in the energetics.” Energy trade-off The way we hold our arms is influenced by a trade-off between energy spent at the shoulder and the elbow, Yegian explains. Bending the arms uses more energy at the elbow to resist gravity, but should save energy at the shoulder since it effectively makes the arm shorter, reducing the force needed to swing the arms. The results for walking suggest that with bent arms, we spend more energy at the elbow than we save at the shoulder. As there was no difference in oxygen consumption while running, this suggests that the trade-off between shoulder and elbow energy is balanced. But it leaves the reason why we bend our arms when we run unexplained. The study only tested running at a relatively low speed, so perhaps the benefit of bending our arms is only apparent at a higher speed. Maybe bending the arms spreads some energy demand from the shoulder to the elbow, stopping the shoulder muscles from tiring out. “These are speculative hypotheses to test in the future,” says Yegian. Our running style might have been an important factor in human evolution, he adds. Between 1.5 and 2 million years ago, our ancestors became accomplished long-distance runners, and evolved shorter forearms around the same time. “This might suggest the mechanics of the elbow during running might have something to do with that evolutionary change,” he says. Journal reference: Journal of Experimental Biology , DOI: 10.1242/jeb.197228 More on these topics: human evolution exercise
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