Herman Pontzer, a biomechanics researcher at Washington University in St Louis, said that humans swing their arms simply because it would take extra mental and physical effort to keep them still.

He said that arm swings have the added benefit of keeping our heads from bobbing back and forth as we walk.

While teaching an undergraduate laboratory class, Pontzer asked his students to test a critical prediction of the model: as a person walks, their arms and legs should move in tandem.

Instead, the students found that a person's arms and legs move slightly out of sync. Our torsos act as a dampener, causing arm motions to lag slightly behind the legs, he hypothesised.

A scale model showed the same lag.

"I went to [a store] and we spent half an hour in the toy section looking for a big box of Legos and spent the rest of the night building that thing," New Scientist quoted him, as saying.

Next, Pontzer set out to show that real humans, not just Lego models, swing their arms passively when they walk.

His team analysed the movement and muscles of 10 volunteers as they walked and ran on a laboratory treadmill.

The results strongly confirmed predictions of Pontzer''s original hypothesis.

Adding extra weight to test subjects'' arms caused leg and arm movements to shift even further out of sync.

Similarly, when volunteers folded their arms in, reducing inertia, the lag between arm and leg shortened.

And when walkers and runners crossed their arms, they suffered no loss in efficiency.

Pontzer's team found that those muscle contractions that researchers noticed in the 1960s seem to stabilise the shoulder, not drive motion.

John Bertram, a biomechanics researcher at the University of Calgary in Canada, says understanding how arms swing naturally could aid in the design of prosthetic limbs, making movements more efficient and realistic-looking.