The oxidative energy system is the primary source of ATP at rest and during low intensity exercise. Fats and carbs are the primary substrates that are used . After about 90 seconds of continuous physical exertion this energy system will kick in. The other energy systems (ATP-PC and Glycolysis) provide energy for shorter duration and higher intensity exercise. If you are running, swimming, biking, or walking long distances you will be using this energy system.