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People swing their arms when they walk. But did you ever wonder why? Does it help us keep our balance? Is it a relic¹ from our evolutionary² past? Is it because we look like doofuses if we don’t? Now, a report in the Royal Society journal Biological Sciences suggests that we swing our arms while walking, in part, because it takes less energy than it would to stop.

Since people don’t walk on all fours, it’s not clear why we pump our arms when we pick up our feet. And studies have shown that it does require some muscle to do it—those limbs don’t just sway passively like a pendulum³. So there must be some benefit.

To find out, scientists studied walking robots and human volunteers, who walked normally or with their hands secured to their sides. And they found that for people on-the-go, strolling with their arms strapped⁴ down took 12 percent more energy than just letting them fly. Swinging the arms also offsets⁵ the scissoring of the legs, saving even more energy. So next time you go for a walk, don’t forget those arms. ‘Cause it don’t mean a thing if it ain’t got that swing.