Out on a limb: Arm-swinging riddle is answered

The shadow of a woman is seen on a wall
The shadow of a woman is seen on a wall. Biomedical researchers on Wednesday said they could explain why we swing our arms when we walk, a practice that has long piqued scientific curiosity.

Biomedical researchers on Wednesday said they could explain why we swing our arms when we walk, a practice that has long piqued scientific curiosity.

Swinging one's arms comes at a cost. We need muscles to do it, and we need to provide energy in the form of food for those muscles. So what's the advantage?

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

But a trio of specialists from the United States and the Netherlands have put the question to rigorous tests.

They built a mechanical model to get an idea of 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 and carbon dioxide (CO2) production as the human guinea pigs breathed in and out.

Arm-swinging turned out to be a plus, rather than a negative, the investigators found.

For one thing, it is surprisingly, er, "'armless" in , requiring little torque, or rotational twist, from the shoulder muscles.

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

The arms' pendulum swing also helps dampen the bobbly 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 a whopping 63 percent.

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 Steven Collins at the University of Michigan at Ann Arbor, says we should give the thumb's-up to arm swinging.

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

It appears in Proceedings of the Royal Society B, the biological research journal of the Royal Society, Britain's de-facto academy of sciences.

(c) 2009 AFP

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Jul 28, 2009
So swinging my arms in the opposite direction will burn 25% more calories? Awesome!

Jul 28, 2009
Didn't most of us "discover" this around the same time we learned to walk? Who's scientific curiosity was piqued by this?

Still, I'll bet it was fun playing with the oxygen and carbon dioxide measuring gizmo.

Jul 29, 2009
I knew it was easier to swing my arms, I just knew it!

Jul 29, 2009
I don't doubt the human species has learned to create the most energy saving walk and run. Perhaps those power walkers with the weights have the wrong idea - stop moving your arms!

Jul 29, 2009
How long before walking robots have arms?

Jul 29, 2009
Next we need to study the energy (and monitary) economy of performing test like these...

Aug 03, 2009
@Truth: It obviously is more fun playing with gadgets and [human] guinea pigs, than it is to wade through old dusty research in the library. Besides, the department can then apply for more money for the research, and get more visibility in the press. (And even here.)

Too bad even that money wasn't spent on something that'd create new knowledge.

Another problem is that young people of today inherently believe that anything pre-www-era is worthless knowledge. What a shame.

Aug 12, 2009
Lead author here. You can find the full PDF online at http://www.umich.edu/~shc

As you'll see in the introduction, the literature has been quite conflicting on this issue going back at least two centuries. Modern energy and motion capture techniques now allow the question to be answered clearly.

There actually is useful information in these results. Understanding of our neural circuitry helps us to devise treatments for spinal cord injury and stroke patients. Many researchers believe that connections between the arms and legs might be used to help train people to walk again after such an event.

That said, the investment in time and money was minimal. Our main work is in developing artificial limbs, and this was a useful sub- / side-project.

It's tough to tell these sorts of details from short news snippets. Believe me: we read the literature, and we're not out to waste (our) time or money.

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