Straight arms or bent? For walking, it's clear. For running, less so

It is a question that perhaps only a scientist would ask and try to answer: why do we walk with straight arms but run with them bent?

Months after the conundrum struck Andrew Yegian as he strolled across campus at Harvard, he has part of the answer.

He asked volunteers to walk on a treadmill while wearing an oxygen mask and found they used up 11% more energy while bending their arms than while keeping them straight.

Bending the arms raised the amount of oxygen used from an average of 643ml per minute to 712ml per minute, revealing a clear advantage for straight-armed walking.

The study, published in the Journal of Experimental Biology, involved eight

students who ranged from avid marathon runners to people who ran only a couple of times a week.

The participants were fitted with reflective markers on their shoulders, elbows and wrists to track their movements. Each was asked to run and walk on the treadmill, once with arms straight and again with them bent.

“Bending the arm reduces the energy you need to spend at the shoulder but increases the energy you need to spend at the elbow,” Yegian said.

Yegian and his team suspected that bending the arms would be more efficient when running but the assumption turned out to be wrong. “The most surprising thing was that we found equal costs for running with a straight or bent arm,” he said.

The researchers concede they do not know why runners tend to bend their arms, but Yegian is convinced there is some benefit. It may be that bent arms help to stabilise the head when running, he said. Previous research has suggested it may help runners maintain balance.

Yegian’s research looked at only one running speed, so the energy spent may be different when running faster.

“We have a pretty good idea now that energy is why we keep our arms straight when we’re walking and there’s probably a specific reason for bending your arms during running,” Yegian said. He plans to conduct more research on that subject.

Christopher Arellano, who studies biomechanics in sport at the University of Houston and was not involved in the study, said humans were “pretty clever” at saving energy, so Yegian’s original assumption that bending the arms helped runners made sense.

The finding that it apparently made little difference left a big part of the mystery unanswered. “The question remains: what advantage does bending the arms have during human running?” Arellano said.