system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do you use more energy walking slower than your normal pace?
gpt
Your arms and legs are pendulums, so they have a natural swing rate, which depends on their length, mass distribution and also how your body (the attachment point) is moving.

Walking at the speed at which your legs naturally swing is most efficient. Walking slower than that means you're having to use muscles to control the speed rather than just letting it all hang out.