The Aerobic energy system or family car is the predominant energy system used by endurance athletes. For example long distance runners, road cyclists and triathletes all have predominant use from the aerobic energy system. It is all responsible for the majority of ATP resynthesis at rest, meaning right now your aerobic energy system is probably your predominant one. It has an unlimited capacity to resynthesise ATP but this is a complex process so it can only be replenished during low intensity exercise. It plays a critical role in the recovery of both anaerobic energy systems.The aerobic energy system is activated at the very start of high intensity exercise and becomes the predominant system when the effort exceeds about 1 minute. Most sports and exercise last longer than 1 minute so it is the major provider of energy for exercise between sub-maximal and VO2 max intensity. It is also particularly important for performance in team sports that are stop-start in nature like soccer, netball and basketball. The aerobic energy system has the unique ability to use two fuel sources. Aerobic glycolysis or the use of carbohydrates broken down into glucose to fuel the system is the predominant fuel in the beginning but aerobic lipolysis or the use of lipids (fats) to fuel the system is used for longer periods of exercise. The by-products from the aerobic energy system are water (H2O) and carbon dioxide (CO2). The body also produces heat. The body will only fatigue and no longer be able to sustain exercise when sufficient levels of blood glucose, muscle glycogen or stores of energy run out can no longer be used in energy production.