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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.
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.
If you hold your arms while walking, this movement, called vertical ground reaction moment, rises by a whopping 63 percent.
The downside, though, is that opposite-to-normal swing forces up the metabolic rate by a quarter.
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