Adenosine triphosphate (ATP) is the fuel that provides energy for all energy demanding cellular processes. ATP is the fuel that allows contraction of the muscles. Below is the equation which depicts the reaction occuring within the muscles and makes contraction possible. This reaction is reversible so that ATP supply can be resynthesized.

 ATP <> ADP + P + energy

The supply of ATP within the muscle is limited and can last for about 3-6 seconds of maximal contraction before it is depleted and all ATP is converted into adenosine diphosphate (ADP). Because a muscle can only contract with ATP as fuel, it has to be resupplied and this is first achieved with the aid of Creatine Phosphate. The produced ADP can bind with a phosphate group which was bound to the Creatine molecule, and ATP is resynthesized and the contraction can be sustained. This system can help resupply ATP 5-15 seconds, after which the anaerobic glycolysis takes over.

The anaerobic glycolysis breaks down carbohydrates without oxygen (anaerobic) and generates through a complex set of reactions ATP. This system resynthesizes ATP at a lower rate and consequently, the intensity of exercise that can be maintained is lower. When exercise intensity is not too high, after 1-3 minutes the aerobic system takes over ATP generation. The aerobic system makes use of, depending on the exercise intensity, a mixture of carbohydrates and fat, which are broken down with the aid of oxygen (aerobic). This system is even slower, but exercise can be sustained for prolonged periods.

References:

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