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

Also see : Human bipedalism - originsplugin-autotooltip__plain plugin-autotooltip_bigHuman bipedalism - origins

"There are at least twelve distinct hypotheses as to how and why bipedalism evolved in humans, and also some debate as to when. Bipedalism evolved well before the large human brain or the development of stone tools. Bipedal specializations are found in