Right arm or folded? To walk, it's clear. To run, less well | Science

This is a question that only a scientist might ask and try to answer: why do we walk with outstretched arms but bend over?

Months after the riddle struck Andrew Yegian while he was walking on the Harvard campus, he had some of the answer.

He asked volunteers to walk on a treadmill while wearing an oxygen mask and discovered that they were using 11% more energy to bend their arms than to keep them straight.

Flexing of the arms increased the amount of oxygen used, from an average of 643 ml per minute to 712 ml per minute, which reveals a clear advantage for walking with the right arm.

The study, published in the Journal of Experimental Biology, looked at eight students, ranging from passionate marathon runners to people who ran only two or three times a week.

Participants were equipped with reflective markers on the shoulders, elbows and wrists to track their movements. Everyone was invited to run and walk on the treadmill, once with arms extended and arms folded again.

"Bending the arm reduces the energy you have to spend in the shoulder, but increases the energy you have to spend on your elbow," Yegian said.

Yegian and his team suspected that bending the arms would be more effective during the race, but the assumption was proven wrong. "The most surprising thing was that we found equal costs to run with a straight arm or bent," he said.

The researchers admit that they do not know why riders tend to bend their arms, but Yegian is convinced that there is a benefit. It may be that the bent arms help stabilize the head during the race, he said. Previous research has suggested that this could help runners maintain balance.

Yegian's research has focused on a single speed of operation, so the energy expended can be different when running faster.

"We have a pretty good idea now that energy is the reason we keep our arms stretched when we walk and that there is probably a specific reason for you to look down during the race," he said. Yegian. He plans to conduct more research on this topic.

Christopher Arellano, who studies biomechanics in sport at the University of Houston and did not participate in the study, said humans were "smart enough" to save energy. Yegian's initial idea that bending arms made sense to runners.

The discovery that he apparently made little difference left much of the mystery unanswered. "The question remains: what advantage has flexing of the arms during the human race?" Said Arellano.