Why do humans swing arms while walking?
In: Biology
Balance and inertia. Our fully upright posture raises our center of gravity far higher than other primates (although other primates also often swing their arms somewhat when they choose to walk fully upright). By swinging our arms in concert with the swinging of our legs we keep our actual mechanical center more stabilized than if we didn’t – and by being relaxed our hands become something of a pendulum that swing even further than is technically necessary which is why walks with exaggerated arm swinging like “saunters” or “struts” are perceived as being done by people who are especially relaxed/confident.
When you walk/run your legs are moving in opposite directions, each offset from your body’s centerline.
This movement of mass results in an unbalanced rotational force on your body, that changes direction with each step.
To counteract this you swing your arms, with the motion opposite to the direction your legs are moving on each side, this generates a rotational force in the opposite direction. Your arms are lighter than your legs, but also further from your body’s centerline so apply more rotational force for their mass than your legs do. The goal is to mostly cancel out the rotations from the legs so your legs don’t have to impart a twist themselves and your body is more stable so able to more effectively deliver power.
At walking speed the motion is about equal to your arms natural frequency, so they almost resonate with the forces from the legs, and then help dampen the energy from the offset forces so some other part of your body doesn’t have to absorb it. A similar premise is used in earthquake engineering with things called [tuned mass dampers](https://www.atlasobscura.com/places/tuned-mass-damper-of-taipei-101)
You can kinda try this out. Stand on your left foot and then swing your right leg from back to front without moving any limbs or trying to stabilize the twist, you’ll rotate clockwise. Then stand on your right foot and move your left leg from front to back, you’ll rotate clockwise. Then stand relaxed and quickly move your arms – right arm forwards, left arm back – opposite to your legs. You’ll rotate counter-clockwise. These movements when added together (eg walking) cancel out.
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