Why crawling makes every step cost more
In ordinary walking, a leg swings forward while the body’s momentum helps carry it over the planted foot. The leg behaves roughly like an inverted pendulum: the body vaults over the stance leg, then the next step catches the fall. Muscles still do work, but the motion recycles some energy rather than lifting the whole body from scratch with every step.
Crawling changes the job. With hands and knees—or hands and feet—on the ground, the arms become weight-bearing limbs. Each move has to support and reposition part of the torso, while the trunk and shoulders work to keep the body from sagging or twisting. Instead of mainly swinging a leg beneath an upright body, the crawler repeatedly braces against the floor and pushes forward.
A different body, a different bargain
That does not make crawling universally wasteful. A small child has a low body to support and short limbs well suited to moving close to the ground. Crawling can also be useful when staying low matters more than saving effort, or when walking is not possible. But for a typical adult traveling across open ground, the extra support work is a poor trade: the arms are doing a job that walking largely assigns to the legs, and the body cannot use the same upright pendulum-like motion.
The contrast is easy to feel. Try moving a few steps on hands and knees, and the shoulders, arms, and core quickly join the effort. A routine trip across a room becomes a sequence of small whole-body pushes—less like a stroll than a workout with forward progress.
