The aerobic system, also called the oxidative system (aerobic means a proces which functions with the aid of oxygen) is the most important source of ATP in rest and during lower intensity exercise. This system generates ATP from both carbohydrates and fats. When the carbohydrate stores are depleted protein can also be used. Depending on the intensity of exercise, a certain ratio of carbohydrates and fats is used. At rest this ratio is around 70% fat and 30% carbohydrates. When exercise starts, the use of fat as an energy substrate decreases and carbohydrate use increases with the increase in intensity. When intensity rises to 100% VO2max, almost all ATP will be generated from carbohydrates.

There are different aerobic systems active in generating ATP aerobically. Bloodglucose and glycogen are broken down in the anaerobic glycolysis in the cytosol of the cell. When oxygen is present, the products of this anaerobic glycolysis (pyruvate) can be used in the mitochondria. There the citric acid or Krebs cycle uses these products. In a series of reaction a small amount of ATP is produced and subsequently, the byproducts of this cycle are used in the oxidative fosforylation (see figure)

When fat is used as a substrate, it is transported by the bloodstream or the intramuscluar fat store is used. The freed up fat, also known as free fatty acids, enter the cell and enter the beta-oxidation where they are converted into acetyl-CoA. This can also enter the citric acid cycle and its’ byproducts can enter the oxidative fosforylation.

The aerobic system is very efficient and can produce large amounts of energy. However the rate of energy production is not as high as in hte anaerobic glycolysis. The steps in the different aerobic cycles take more time. This is the reason why you cannot sustain maximum anaerobic power aerobically. Training intensities are consequently lower during aerobic training. In other words, the speed you cannot sustain the speed you reach in an all out sprint for long. The anaerobic system depletes and the aerobic system can only support activities that require less power.

References:
-Baechle, T.R., Earle, W.R. (2008). Bioenergetics of Exercise and Training. Essentials of Strength Training and Conditioning. Third Edition. USA. Human Kinetics.