Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

Almost all the features I just reviewed that help humans run are what biologists term convergent, which means they evolved independently in humans and other animals adapted for running. However, because we are unsteady bipeds that evolved from tree-climbing apes, running humans are uniquely prone to falling. A slight shove or an unfortunately placed banana peel is more likely to topple a running human than any quadruped. Because a sprained ankle or broken wrist could be a death sentence in the Stone Age, it makes sense that humans evolved a suite of unique and crucial features for stabilization. My favorite of these adaptations is the enlarged gluteus maximus, the largest and plausibly most shapely muscle in the human body. If you walk a few steps with your hands on your buttocks, notice that this muscle is mostly dormant, but when you start to run, feel how it clenches up forcefully with every step. Experiments in my lab showed that this impressive muscle primarily acts to prevent the trunk from falling forward following each landing.