Aerobic System
For an individual to continue to exercise longer than a few minutes requires the presence of oxygen to provide a continual supple of ATP. This is supplied bu the aerobic energy system.

The oxygen rich blood usually takes a few minutes to arrive to the working muscle for use, so the system is usually the third to kick in.

This system also relies on the supply of glycogen, and fats as the fuel.
It is also known the aerobic glycolysis system.

As there is plenty of oxygen during the breakdown process lactic acid is NOT produced.

How this system works:
After two or three minutes of exercise your body is able to produce sufficient oxygen to the working muscles for the production of ATP. This process occurs in special cells of the muscle called the mitochondria.

In the presence of oxygen, muscle glycogen is broken down to produce pyruvic acid, ATP (energy), carbon dioxide and water.

Fuel for this system:
The main source of fuel is glycogen from carbohydrates.

But when these stores are low ( and blood glucose had been used), then fats can be used as a source of fuel.

In extreme conditions protein can also be used as an energy source.

Efficiency of ATP production:
This system is extremely efficient in both the metabolism of fuel and provision of ATP for energy.
Using oxygen to break down glycogen is very efficient, as not much glycogen is needed to create energy (therefore glycogen stores last longer).
This enables us to continue exercise for sustained periods of time.

Duration of this system:
At rest our glycogen stores can last 12 hours.
During intense activity they will last between 1 and two hours.
For intermittent activities (netball, football) glycogen stores will last about 4 to 6 hours.
After these have been exhausted fats can then be used.
Some athletes train their bodies to use a mixture of glycogen and fats during exercise, leaving some glycogen stores for fast finishes.

Causes of fatigue
Fatigue us caused when a