ATP is the “energy currency” of the human body; in other words, all activities which require energy (such as beating of the heart, growing hair, breathing, jumping jacks, etc…) use ATP as the “fuel”. An explanation of the body’s “energy systems” actually discusses how ATP is created, rather than what is being created), which is always ATP.
This is similar to a house which uses electricity to power all of its appliances. There are many different ways to create the electricity (solar panels, nuclear energy, coal plants, etc…), but no matter which method, each with its positive points and drawbacks, is being used, the end product is always electricity. Similarly, the human body only uses ATP as its fuel source, but has a number of different energy systems from which to create this fuel source.
The ATP-PC (Adenosine triphosphate-Phospho creatine) energy system is the primary energy system in the human body. It is used for very short duration, maximal effort activities such as power lifting or short sprints. It is anaerobic, meaning that it does not require oxygen, which allows for an immediate energy delivery, but one that runs out shortly.
The ATP-PC system does not actually create any ATP. Instead, it uses what little bit of ATP is stored in cells. As this ATP becomes used, it is regenerated by the energy system and then reused. As stated earlier, this system can not last for very long, and after about 8-13 seconds of an all out exertion, the ATP-PC system begins to falter. At this point, the human body moves on to the next energy system, the Anaerobic System, which is used for longer lasting activities.