A practice that has long irked scientific curiosity may have been solved - why we swing our arms when we walk.
Scientists have discovered that swinging is more energy-efficient than holding them still.
Experts have long been baffled about why humans move the arms when strolling, since they play no obvious role in helping propel the body forward.
However, an experiment carried out by researchers in the US and Netherlands has found that the movements actually provide considerable benefits, which are hidden.
The study was based on the movements of 10 volunteers. They were asked to perform a series of unnatural walks under experimental conditions has shown that swinging the arms in opposition to the legs significantly increases the efficiency of walking.
Steven Collins, a biomechanical engineer at Delft University of Technology in The Netherlands, said normal arm swinging while walking requires little effort and makes it easier than keeping the arms still.
"This puts to rest the theory that arm swinging is a vestigial relic from our quadrupedal ancestors," he said.
To reach the conclusion, Collins and his colleagues established an experiment where people were analysed as they walked in a variety of poses - normally with the arms swinging in opposition to each leg, with their arms at their sides either tied or held there voluntarily, or with each arm unnaturally forced to move in synchrony with each leg, reports The Independent.
From analyses, boffins discovered that arm swinging required very little effort from the shoulder muscles as the movements tended to arise naturally from the twisting movement of the body as it walked.
"Further, our results showed that normal arm swinging made walking much easier. Holding the arms at one's sides increased the effort of walking - measured by metabolic rate - by 12 per cent, which is quite a lot of walking, about the same as walking 20 per cent faster or carrying a 10 kg backpack," Collins said.
The study has been published in the journal Proceedings of the Royal Society B.