During the first half of the stance phase, muscles vault your body up and over that leg, elevating your center of mass about two inches (five centimeters). That upward lift expends calories but stores potential energy, just as if you were to raise this book. Then during the second half of stance, your body converts that potential energy to kinetic energy by falling downward and forward, as if you were to drop the book. Eventually, your swing leg collides with the ground, halting your body’s fall and starting a new cycle.
