This document discusses the three energy systems - ATP/PC, lactic acid, and aerobic - that provide ATP to allow human movement. The ATP/PC system provides ATP for 2-10 seconds of high-intensity exercise like sprinting using stored ATP and phosphocreatine. The lactic acid system takes over for 10 seconds to 2 minutes of high-intensity exercise, producing ATP without oxygen and leading to lactic acid buildup. The aerobic system fuels activities from 2 minutes to hours using oxygen to break down fats and produce large amounts of ATP.