Ultra-endurance athletes can push their bodies to extraordinary extremes, but even they run into a hard biological wall. Researchers tracked ultra-runners, cyclists, and triathletes over weeks and months, discovering that no matter how intense the effort, the human body maxes out at about 2.5 times its basal metabolic rate when measured long-term. Short bursts of six or seven times BMR are possible, but the body quickly pulls energy away from other functions to compensate, nudging athletes back toward the ceiling.