Asked by: Theodore Barkas, Athens
Experts believe that the ancestors of modern humans began walking upright at least 3.6 million years ago. Yet the reason we move our arms out of sync with our legs has only recently been solved.
For years, the answer was thought to be simply balance. Research published in 2010 by a team led by Dr Sjoerd Bruijn at the Free University, Netherlands confirmed this – but with a twist. Swinging our arms doesn’t make us much more stable when walking normally – but it does help restore our balance if we suddenly lose our footing while walking over uneven ground.
Earlier this year, Bruijn and colleagues also found another benefit: swinging our arms while walking is more energy efficient than keeping them still. While it takes energy to move our arms, this is more than compensated for by the reduced energy needed by the rest of the body to propel itself forward.
Subscribe to BBC Science Focus Magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun facts.