The human body uses carbohydrates, fats and proteins to supply itself with the energy need to stay alive and perform tasks. In the human body, ATP, adenosine triphosphate, is broken down to create energy for muscle contraction. The human body creates ATP aerobically and anaerobically. There is one aerobic energy system and two anaerobic energy systems. Most activities of the body use a compilation of all three energy systems to generate the energy needed.
This is the simplest energy system. It is the anaerobic energy system that utilizes ATP stored in the muscle to create energy. Phosphocreatine is then used to rephosphorylate ADP into ATP using creatine kinase. This newly formed ATP can then be used to create more energy. It is the system used for short bursts of high-intensity work lasting approximately 10 seconds or less.
The anaerobic glycolytic energy system utilizes carbohydrates to create ATP for energy. This is a two-phase energy system where glucose is broken down to form ATP and pyruvic acid or lactic acid molecules. There are more than 10 steps to this energy system. It is the system used for relatively short periods of high-intensity work lasting only a few minutes. After a few minutes of work, the accumulation of lactic acid will reach a point where pain and fatigue will begin to hinder performance. This is referred to as the lactate threshold.
The most complex energy system, the aerobic oxidative energy system utilizes carbohydrates, fats and proteins to generate ATP for energy. This is a three-stage system comprised of many steps in the preparation of Aceytl-CoA, the Krebs cycle and the electron transport chain. The Krebs cycle and electron transport chain are both complicated stages and each is made up of many steps. The end result of the oxidative system is the production of ATP and water molecules. The complexity of this system, along with the fact that it relies on the circulatory system to supply oxygen, makes it slower to act than the anaerobic systems. It is the system used for long-term, low-to-moderate-intensity work lasting more than just a few minutes. It can be relied upon for long periods of work, making it the primary system used for endurance activities.