• Most people tend to walk with their elbows slightly bent.
• Most people tend to run with their elbows acutely bent.
• No-one knows why.
There is however, a(n) hypothesis. It’s the ‘Mechanical Tradeoff Hypothesis.’ which was descibed by Andrew K. Yegian, Yanish Tucker, Stephen Gillinov and Daniel E. Lieberman in their 2019 paper for the Journal of Experimental Biology, 222: jeb197228, entitled Straight arm walking, bent arm running: gait-specific elbow angles
Their hypothesis provides possible answers for the ‘walking’ scenario – but the bent-arms-whist-running phenomenon is still a mystery.
“Optimal energetics appears to be the reason why straight arms are stereotyped during walking, but the reason for stereotyped bent arm running remains unclear.”
The authors suggest, however, that although bent-arm running might not have an energy-saving benefit, it could be related to balance instead – or :
“We must also consider that the stereotyped behavior during running may not provide a benefit at all.”