The Muscles and the Energy Systems
All the muscles within the body require energy to perform any movements or activities. As the body can call upon 2 different types of systems to supply the energy; the aerobic (requires oxygen) system and the anaerobic (does not require oxygen) system. It is the intensity and duration of the movements that determine which energy system is utilised by the active muscles to acquire the energy they require. However before it can be explained how the body generates energy through these 2 very differing systems, it is essential that it is understood what energy actually is.
What is Energy?
Adenosine Triphosphate (ATP) is the source of energy for all muscular contractions. Energy (and Adenosine Diphosphate [ADP]) is produced when ATP is broken down. Although it is the integral element within energy production, ATP is not stored within the skeletal muscles in large volumes, in fact there is only enough ATP stored to allow an individual to undertake maximal effort for up to 4 seconds (Matthews and Fox, 1971) before their entire store in diminished. Therefore the body relies on different processes to exact energy from ATP to ensure that it is able to meet the demands of the activity being undertaken.
Anaerobic
The anaerobic (without oxygen – so breathing during activity is not essential) system of supplying energy to the muscles can be further categorised into 2 processes;
Creatine Phosphate (CP): CP is a chemical compound stored within muscles, and can be used as a source of and to restore supplies of ATP (when combined with ADP) during periods of explosive, high intensity and short duration movements and activities. The CP system can provide energy to up to 20 seconds of movement but then requires up to an additional 5 minutes to become fully replenished. As the burst of activity undertaken is so small in time, the amount of lactic acid produced is minimal so it does not inhibit the individual’s ability to perform, although it can exhaust the muscles employed.
Anaerobic Glycolytic (AnG): This system becomes the dominant one after 20 seconds or when the CP system is depleted. The AnG system breaks down glucose (glycolysis) to pyruvate, as this conversion is not complete a by-product is produced – lactic acid. Glycogen is stored and broken down into glucose within the muscle fibres. Although it is often assumed that AnG is limited by the level of glycogen stores within the active muscles this in not the case. Instead it is limited by the production and accumulation of the lactic acid. The removal of lactic acid is usually assisted by oxygen, so when there is no oxygen present (as is the case with anaerobic activities) the muscles cannot remove sufficient amounts of lactic acid quick enough. This then results in the pH level of the muscle lowering stimulating the free nerve endings to experience a burning sensation (pain) and the muscle experiencing impaired contractions. The AG system can allow high intensity movements and activity for, usually, up to 2 - 3 minutes (although not at such as high intensity as the CP system) before the lactic acid build up becomes too significant. Once the AG system is fully exhausted and the build of lactic acid too great the individual may take up to 1 hour to fully recover.
Aerobic
Aerobic Glycolysis (AG): AG requires the presence of oxygen, which breaks down pryuvate, which preoduces ATP through chemical reactions. This process is similar to the AnG process as it obtains glucose from the glycogen stores, however unlike AnG, the breakdown is slower and complete, meaning that lactic acid is not created. Therefore there is nothing to prevent the abundance of glycogen stores being utilised and continually converted to ATP over long periods. Although the glycogen stores will eventually deplete, this usually occurs after the individual has undertaken low intensity activities for longer than 90 minutes. Recovering and replenishment of this form of energy is greatly dependent on the diet and nutritional intake of the individual.
How is energy gained to complete ‘at rest’ activities?
The most readily available energy source for the muscle fibres, within the majority of individuals, is fat. The term used to describe the breakdown of fat to produce ATP is lipolysis. Although the availability of fatty acids to use for energy is essentially unlimited, the rate which lipolysis can occur is a significant limiting factor. Lipolysis is the main energy source for resting muscle activity but its usage as an energy source will significantly decrease as the intensity of the muscular contractions increase.
Football and the 3 Energy Supplying Processes