Why do our arms move to and fro when we walk?

A: To keep our torso stable and conserve energy, we swing our arms backwards and forwards while walking. When you swing, say, your right leg forward to take a step, you provide a rotational moment about the central vertical axis of your torso. By the principle of conservation of angular momentum, an opposite reactionary moment is felt by your torso.

By swinging your right arm backwards and your left arm forwards, you counterbalance this moment. Just try running without swinging your arms at all.

Or worse still, try running while swinging your arms in the opposite direction to normal: that is, swing your left arm forward when you swing your left leg forward and so on. You will find that your torso rotates from side to side in an uncomfortable and unnatural manner.

Serge Gracovetsky hypothesised in the 1980s that the spine, rather than the legs, is the primary source of power for gait, and this is now accepted by most, if not all, researchers in this field.

The mechanism works because the spine is curved. Any attempt to straighten such a structure will result in a twisting action. The lumbar muscles acting on the lumbar spine cause such a twist and provide the main impetus for placing one foot in front of the other.

Swinging one’s arms while walking assists in this twisting motion, increases efficiency, and reduces the physiological cost of walking.