The Joy of Movement: How Exercise Helps Us Find Happiness, Hope, Connection, and Courage

by Kelly McGonigal

you can observe many features in your own physique that help you run. Large gluteal muscles and longer Achilles tendons propel us forward. Compared to other primates, humans have more slow-twitch muscle fibers, which resist fatigue, and more mitochondria in running muscles, allowing them to consume more oxygen as fuel. We are also the only primate to have a nuchal ligament, the strip of connective tissue that fixes the base of the skull to the spine. This ligament—shared by other running species, such as wolves and horses—keeps your head from bobbing when you run. All of these adaptations suggest that we evolved as endurance athletes. Because the survival of early humans depended on traveling far and fast, you were born with bones, muscles, and joints that help you go the distance. David Raichlen, an anthropologist at the University of Arizona, was familiar with the idea that natural selection favored traits that allowed humans to run.