The way that our bodies convert calories into fuel is through a chemical called ATP. If you wish to perform any activity, you have to use ATP; it is the energy currency of the body. There are three systems when forming ATP:
Lactic Acid System or Glycolytic Energy System
Aerobic or Oxidative Energy System
The ATP-CP and glycolytic energy systems are both anaerobic systems, meaning they operate without the use of oxygen. The aerobic system uses uses oxygen. The anaerobic systems are limited and inefficient, whereas the aerobic system is very efficient because it uses an abundant supply of fat and carbohydrates to create ATP to fuel any activity. While all three energy systems function together at any one time to provide the energy the body requires, there are times when certain systems become more predominant.
This system provides fuel for up to 10 seconds at maximal intensity. ATP and CP are high energy compounds stored in limited amounts (5 calories) in muscles and other cells. For the start-up of intense activity, and especially for brief, very intense activity, this system provides the ATP for muscle contraction to occur. The total amount of stored ATP and CP is limited to the 10 seconds of intense effort. Once the storage is depleted, the cells will no longer be able to provide ATP at the same high rate, and the body will have to slow down so that metabolism can match energy needs through glycolysis, or switch to another energy system.
This system provides fuel for up to 2 minutes at maximal intensity. The glucolytic energy pathway causes the breakdown of carbohydrates in the form of glycogen, which is stored in the muscle cell or uses glucose found in the blood. This method of metabolism creates energy in the form of two to three ATP molecules, depending on the source. From glucose, it produces 2 ATP molecules, and from glycogen it produces 3 ATP molecules. The reasons for the difference are due to 1 ATP being used in one of the reactions of the glucose breakdown. Without more oxygent it is an incomplete chemical reaction and a three-Carbon compounded call pyruvate is formed, the more glucose that is metabolized, more lactic acid is produced. As the lactic acid gathers in the cell, it begins to lower the PH in the muscle this is felt as a burning sensation in the muscle and begins to slow contraction speed and strength. This will eventually lead to temporary muscle failure, until the body can flush out the lactic acid. The more you train this system the less Lactic acid is produced.
When sufficient oxygen is available to the muscle cells for a given intensity of exercise an abundance of ATP is produce in the cells. In this case the supply of oxygen is delivered by the cardio respiratory system to meet the demand for oxygen in the exercising muscles so that ATP is now being made in the presence of oxygen. There are two subcategories of aerobic metabolism that can create ATP aerobic glyoccis and fatty acid oxidation.
This system provides fuel for more than 2 minutes of exercise at moderate intensity. Aerobic metabolism beings in the same way as glycolytic pathway in that glycogen or glucose are broken down into pyruvate. However because oxygen is present, instead of pyruvate being converted to lactic acid, the oxygen is made available to the muscle cells from the nearby capillaries and the cell is able to take in that oxygen. The muscle cell can now begin convert to use the available oxygen with glucose or fat, to produce ATP aerobically through the oxidative system.
Fatty Acid Oxidation
This system provides fuel for over 2 minutes during low-intensity exercise. The muscles cell is also capable of using fatty acids to make ATP. This occurs when there is a continued supply of oxygen that is present. This aerobic metabolism of fat produces a very large amount of ATP. Few fatty acids that are burned off because they are a high energy fuel,and they are difficult to metabolize because of the large amount of oxygen required for the reaction to occur.The chemical reasons produce carbon dioxide and water as well as heat and these by-products are easily disposed of elsewhere in the body.
Interaction of the Energy Systems
Only small amounts of energy are needed, and they are supplied almost excluvtily using aerobic metabolism of fatty acids.
At the beginning of exercise
Depending on the difference between resting state and the level of exercise during the warm- up, the energy system used may vary. If the energy demand is only slightly higher than at rest, the aerobic system will be used. If the energy demand is immediate or high, stored energy will be used.
During steady-state exercise
Once the supply of oxygen meets the demand, the muscle cell creates ATP using the breakdown of glucose through the oxidative system. This systems works as long as needed, provided enough glucose is available. Fat metabolism can occur if there intensity is low enough and there is enough oxygen.
During strenuous exercise
When energy demand is rapid and is expected for an extended period, the energy demand will require the anaerobic system to provide ATP. Once the ATP-CP system is fatigued, the glycotic system stakes over to produce ATP. The production of lactic acid dictates the longevity of this system.
As the need for higher supply of energy is reduced or eliminated, the body continues to take in more oxygen than it needs, thereby making it available to pay off the debt of oxygen that occurred at the beginning of exercise.