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Humans tend to run with arms bent at an acute angle. The reasons are unclear. A 2019 study published in the Journal of Experimental Biology found that there could be an energy advantage for walking with elbows slightly bent - but was unable to account for the bent-elbows-whilst-running phenomenon
Although we have focused on energetic cost as a mechanism in this study, stereotypical bent arm running may result from a non-energetic benefit. For instance, the biceps muscle is thought to link the inertia of the forearm and the head during running for the purpose of head stabilization (Lieberman, 2011); bent arms increases the flexion moment at the elbow and therefore may provide a head stabilization benefit. We must also consider that the stereotyped behavior during running may not provide a benefit at all.
See: Straight arm walking, bent arm running: gait-specific elbow angles Journal of Experimental Biology, 2019 222: jeb197228
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