Phosphocreatine allows for rapid production of ATP, boosting performance in high-intensity short-duration activities like sprints and weight training. It exists in limited stores in the body and is replenished through diet and endogenous production. The lactic acid energy system produces ATP anaerobically for up to 90 seconds of maximal work, but lactic acid buildup leads to fatigue. The aerobic system produces ATP slowly through 3 stages, but can sustain low-to-moderate intensity exercise indefinitely due to virtually limitless capacity. The intensity and duration of exercise determines which energy systems are utilized.