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 Australopithecus fossils from 4.2 to 3.9 million years ago, although Sahelanthropus may have walked on two legs as early as seven million years ago."

Source : Wikipedia

The main theories include :

• Savannah-based theory
• Traveling efficiency hypothesis
• Postural feeding hypothesis
• Provisioning model
• Early bipedalism in homininae model
• Warning display (aposematic) model
• Thermoregulatory model
• Carrying models
• Aquatic ape models

All of which can be researched using the Wikipedia link above.

Many of the theories are not mutually exclusive, so the possibility exists that several evolutionary factors could have been working at the same time. Whatever the main factors were, fossil evidence shows that hominins became increasingly bipedal over time - indicating strong evolutionary advantages.

Also see : Human running gaitplugin-autotooltip__plain plugin-autotooltip_bigHuman running gait

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