There are three basic biological energy systems our bodies use to function and exercise each day. Phosphagen System , Glycolytic System, & Oxidative System The phosphagen & glycolytic systems are used primarily in anaerobic activity (high intensity/short duration exercise). The Oxidative System, uses aerobic mechanisms (i.e. Krebs Cycle) to provide energy over longer durations of exercise at a lower intensity (think long distance running). It is important to note that all en