KnowHow team explains: To keep our torso stable, we swing our arms backwards and forwards while walking.

When we swing, say our right leg forward to take a step ahead, the to and fro motion of our arms provide a rotational thrust about the central vertical axis of the torso. This phenomenon occurs in accordance with the principle of conservation of angular momentum. While we move our hands forward and backward we counterbalance this momentum.

Simply try running without moving your arms at all. Or worse, try running while swinging your hands in the opposite direction to the normal: that is, swing your left arm forward while you move your left leg forward and so on. You will observe that your torso will start rotating in an awkward and unnatural manner.

Swinging one’s arm while walking assists in the twisting motion. This increases efficiencyand reduces the physiological cost of walking. Indeed everything that we do while moving is done in such a manner that minimum amount of energy is expended to deserved the desired result.

Other two-legged animals balance themselves by synchronising the movement of the backbone to the side of the leg that stays in contact with the ground. This keeps their centre of gravity close to the leg.

The question was sent by
Anisur Hossain, Howrah.