Have you ever wondered why we swing our arms while walking?

It's a curious phenomenon, since arms don't play an obvious role in locomotion. You can still walk normally when your arms are full of groceries or with your hands in your pockets, so why do we do it? Isn't it a waste of energy?

Two independent studies, both published in 2009, may provide some answers to this mystery.

The Answer...

Using a combination of mechanical models, computer simulations and tests with human walkers and runners, the studies concluded that arm swinging just sort of...happens. In other words, you don't need muscles for your arms to start swinging, although you can use muscles to swing your arms higher or more forcefully.

If you're walking and your arms are hanging relaxed by your sides, they will normally swing in sync with the opposite leg. That is, your right arm swings forward when your left leg steps forward and vice versa.

Arm Swinging And Stride

But although it happens passively, arm swinging plays an important role in making your stride more efficient. It does this by counterbalancing your torso and hips and keeping them from twisting and bobbing too much.

Holding your arms still against your sides while walking requires twelve percent more energy than if your arms are allowed to swing naturally. Forcing your arms to swing in sync with the leg on the same side of your body uses a whopping twenty-six percent more energy than normal walking.

Even when walkers had their arms strapped to their sides, so no muscle control was required to keep them still, they used seven percent more energy than when their arms could swing freely. This suggests that arm swinging is not just a passive byproduct of walking, but actually makes walking easier.

Read More: Dynamic arm swinging in human walking (PDF) and Control and function of arm swing in human walking and running (PDF)