ATP- Adenosine Triphosphate
The energy source- Adenosine Triphosphate (ATP) is the most immediate source of chemical energy for muscular activity. If you suddenly jumped two feet into the air from a standing start, the energy source would be ATP.
The energy available from ATP is very limited. If you ran 100 meters as fast as you could, you'd exhaust all of your ATP.
The usefulness of the ATP system lies in the rapid availability of energy -- rather than quantity. Only about 30 seconds of ATP is stored in the body.
ATP is stored in most cells, but, particularly in muscle cells. It is the most important anaerobic fuel source available. In fact, other forms of chemical energy, available from foods, must be transformed into ATP before they can be used by the muscle cells. ATP is the only source of fuel or energy the body accepts. Therefore, it is important to understand that all remaining fuel systems are simply re synthesizing and rebuilding ATP. There are two ways the body restores energy to ATP without the presence of oxygen.
Free Fatty Acids
Just as carbohydrate provides basic usable form of energy in the body (glucose), so does fat. This usable form of energy in the body is called Free Fatty Acids (FFA).
Fats taken through the diet are first digested to produce fatty acids. After the fatty acids are absorbed they are converted to triglycerides. Triglycerides are the stored form of FFA. Stores of triglycerides are found in the adipose(fat) tissue and in the skeletal muscles.
Early in the exercise the blood fatty acid concentration falls as the muscle begins to draw on the available fatty acids. But, if the exercise continues for more than a few minutes the hormone epinephrine is called into play. Epinephrine signals the fat cells to break apart their stored triglycerides and to liberate more fatty acids into the blood. After about 20 minutes of exercise the blood fatty acid concentration rises and surpasses the normal resiting concentration.
It is during this phase of sustained, sub maximal exercise-beyond 20 minutes- that the fat cells begin to shrink in size as they empty their lipid stores.