Why Do We Bend Our Arms When We Run?

As we walk, our arms usually hang naturally at our sides and are mostly straight. But when we run, our arms typically swing while bent at the elbow. Why is that? Researchers recently investigated how arm position affects energy efficiency, and they found that walking with bent arms was actually less energy efficient than walking with straight arms.

A bent arm has a shorter arc than a straight arm; bent arms therefore require less energy to swing back and forth and should be more efficient for both running and walking, the researchers initially hypothesized.

Why Do We Bend Our Arms When We Run?

But if bent arms are more energy efficient, why don’t walkers naturally bend their arms? To find out, the authors of the new study examined the movements of eight people — four men and four women — on treadmills. As the subjects walked and ran (performing both activities with straight arms and then with bent arms), the scientists used infrared cameras and motion-capture software to record the subjects’ movements and construct 3D digital models of their bodies.However, the scientists found that when their subjects walked with bent arms, their energy expenditure increased by about 11%, likely because it required more effort to keep their arms bent while moving at a relatively slow speed. Their experiments shed light on why people naturally hold their arms straight when they walk, “but the reason for stereotyped bent arm running remains unclear,” according to the study.

But shorter forearms — and a shorter arm overall — swing less. Shorter arms would therefore benefitted modern humans during long-distance running; selection for this trait could have shaped the evolution of human arm bone length, the scientists wrote.