The human body has chemical, electrical, radiant and mechanical energy systems. In the human body we can measure energy in terms of heat using a kcalorie. The phosphagen system supplies ATP to the muscle cell for immediate energy. All other systems work to support this, however their rate of ATP production is much slower, thus explosive bursts of strength or speed deplete ATP within 30 seconds. However, PC's content is also limited in the cell so the combined available energy from these two products may last for around two minutes.
Glycolysis is the primary energy system used for power and strength.
The result is muscle fatigue and the inability of the athlete to continue to perform. This system is a multi -fuel engine, at rest it relies on fatty acids and glucose as its' primary fuel sources. However, during prolonged endurance activity, this system may also utilize amino acids when carbohydrate reserves become low. See the lipids unit for an in-depth look at aerobic energy pathways. Systems One benefit of training is that it improves both the level of intensity and length of time an athlete can perform a particular activity.