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2009-07-29 06:18:42
Tue Jul 28, 7:49 pm ET
PARIS (AFP) 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 oxygen consumption 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 energy costs, 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, arm 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.