This system is the main engine for endurance athletes; distance runners or triathletes. These athletes use a 'pay as you go' system meaning that as the athlete performs and uses energy the body keeps breathing in oxygen to fuel the energy system. The aerobic energy system has a slower rate of resysthesizing ATP than the other systems. Although this limitation means that there is a reduction power output and speed (overall intensity), the aerobic system is characterized by an unlimited capacity to resynthesize ATP. This system is responsible for the majority of ATP resynthesis at rest. The system is activated at the on-set of high intensity and becomes the main system after 1 minute of exercise. As well as being the major provider of energy for exercise that ranges between sub minimal and VO2 Max intensity, this engine also plays a critical role in the recovery of both anaerobic energy systems. This is very important in team sports that are intermittent (stop & start) in nature. The aerobic system has the ability to break down more than one type of fuel source. The fuels being fats, glucose, carbohydrates (CHO), protein, this system also relies on the circulatory system to transport oxygen to muscles before it creates ATP. Aerobic means that the energy system uses oxygen to function. Waste products of this system are H2O (water), CO2 (carbon dioxide) and heat.