News in Science

Arm-swinging riddle answered

Biomedical researchers believe they know why we swing our arms when we walk, a practice that has long piqued scientific curiosity.

Swinging one's arms appears to come at a cost. We need muscles to do it, and we need to provide energy for those muscles. So what's the advantage?

Little or none, say some experts, contending that arm-swinging, like our appendix, is an evolutionary relic from when we used to move about on all fours.

But a trio of specialists from the United States and the Netherlands, reporting in the journal Proceedings of the Royal Society B, have put the question to the test.

They built a mechanical model to examine the dynamics of arm-swinging and then recruited 10 volunteers, who were asked to walk with a normal swing, an opposite-to-normal swing, with their arms folded or held by their sides.

The metabolic cost of this activity was derived from oxygen consumption and carbon dioxide (CO2) production as the human guinea pigs breathed in and out.

Surprising result

The investigators found that arm-swinging turned out to be a plus, rather than a negative.

For one thing, it is surprisingly small in energy costs, requiring little torque, or rotational twist, from the shoulder muscles.

Holding one's arms as one walks requires 12% more metabolic energy, compared with swinging them.

The arms' pendulum swing also helps dampen the bobbing up-and-down motion of walking, which is itself an energy drain for the muscles of the lower legs.

If you hold your arms while walking, this movement, called vertical ground reaction moment, rises by 63%.

Should you prefer to walk with an opposite-to-normal swing, meaning that your right arm moves in sync with your right leg and your left arm is matched to the motion of your left leg, the energy cost of using your shoulder muscles will fall.

The downside, though, is that opposite-to-normal swing forces up the metabolic rate by a quarter.

The study, headed by Dr Steven Collins at the University of Michigan, says we should give the thumb's-up to arm swinging.

"Rather than a facultative relic of the locomotion needs of our quadrupedal ancestors, arm swinging is an integral part of the energy economy of human gait," say the team.