When Herman Pontzer was a teaching assistant for an undergraduate class designed to show that shoulder muscles drive arm swinging as we walk, he was in for a surprise. The class's results showed that arms and legs weren't swinging completely out of sync, as he had expected. Also, the torso muscles were firing simultaneously, as if they were stabilising the body's rotation rather than driving it. Discussing his results with colleagues Daniel Lieberman and David Raichlen, Pontzer speculated that arm swinging could be entirely passive and driven by the body's rotation, caused by the swinging legs. Pontzer remembers that Lieberman and Raichlen weren't so sure, believing that there must be some shoulder muscle activity driving the movement. But when Pontzer turned up in the lab the next day with a model man built from lego bricks and showed that the model's arms swung naturally when he set the legs swinging, Lieberman and Raichlen became more convinced. But they still needed evidence that something as complex as a human body with nerves, muscles and tendons could drive arm swinging simply as a consequence of dissipating the body's rotational energy (p. 523).

Setting up his own lab at Washington University, Pontzer had access to a treadmill and team of enthusiastic students to test his theory. Recruiting active joggers, Pontzer had the students run and walk on the treadmill while he filmed their movements. Then he repeated the experiments, either attaching a 1.2 kg weight to the athlete's arms, or asking the students to fold their arms across their chests to see how changing the limbs' weight distribution affected their movements. Pontzer also measured muscle activity in the students' shoulders to see if any of the muscles could be driving the swinging arms.

Working with John Holloway to track 15 points on each student's body,Pontzer saw that changing the arm's moment of inertia (mass distribution) by folding them across the chest had a significant effect on the body's rotation when the students ran. The runners' shoulders twisted significantly more than when their arms were free to swing. And when Pontzer increased the arms'moment of inertia by adding weights, the runners' shoulders twisted less. The swinging arms were dissipating the upper bodiy's rotation to prevent the torso from twisting too much, as he expected.

Pontzer also realised that the muscle activation pattern in the joggers'shoulders was completely wrong for driving the swing. If the shoulder muscles swung the arm, then the front shoulder muscle would be active as the arm swung forward, and the back muscle active as the arm swung back. Instead, he found that both muscles activated simultaneously, as if they were stabilising the arm. So the shoulder muscles do not drive arm swinging.

But where does the rotation that swings the arms coming from? Pontzer explains that the rotation is transmitted from the swinging legs, through the hips and torso to the shoulders and arms. He adds that the arms don't swing perfectly out of sync with the legs because of damping of the leg's rotation as it is transmitted through the body.

Pontzer suspects that we have evolved to take advantage of our body's intrinsic mechanics because it is metabolically and neurologically cheaper than micromanaging our every move. And by swinging our arms, we stop our heads from twisting too, making it easier to keep an eye on what's in front.

Pontzer, H., Holloway, J. H., 3rd, Raichlen, D. A. and Lieberman, D. E. (
). Control and function of arm swing in human walking and running.
J. Exp. Biol.