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*1.1.c. Energy for exercise GapFill
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Adenosine triphosphate (ATP) is the energy of living cells. Hydrolysis (breakdown) of ATP is catalysed by the enzyme ATPase, releasing the energy required to drive the process of cells. In the case of muscles, this is to enable contraction. As well as the energy produced, other components of its breakdown are adenosine diphosphate (ADP) and (P). These components are used to resynthesise ATP in the reversible reaction: ATP = ADP + P + energy. The energy used in this reaction comes from the three main energy systems. These can be placed on an energy , which characterises the different rates at which and duration for which they are able to resynthesise ATP. The predominant system used depends on the intensity and duration of exercise (i.e. the demand for ATP at the muscle and the capacity of each system to provide this).
The ATP- (PC) system provides the most rapid generation of energy for the cell, so is used at the onset of exercise regardless of intensity. If intensity is maximal, then this system will be the predominant supplier of energy for up to ~ seconds. Once the limited store of PC in the muscle is depleted, the ATP demands at the muscle exceed the rate at which it can be supplied, and so exercise intensity must reduce as a consequence.
The system takes over as the predominant supplier of energy. The reactions taking place result in the production of as a by-product, which is associated with the reduction in in the cell and inhibition of the processes involved in muscle contraction. Fitter individuals are able to resist fatigue due to this harmful by-product longer than their untrained counterparts. This can generally last for up to ~ minutes, depending on the intensity of exercise, before the resynthesis of ATP can no longer meet its demands at the muscle. As a consequence, exercise intensity must be reduced even further.
The aerobic energy system uses both and as main sources of fuel for exercise. The former has an average maximum storage capacity of ~600 g in the , but because of the large storage capacity of the latter, this energy system can be used almost indefinitely. Therefore, it is the most predominant energy system used in long-duration events.
Many sports requires activity of an intermittent nature (varying intensities), which instigates a dynamic interplay between all three energy systems. The roles of each energy system during intermittent exercise depend on the following factors:
- Intensity and duration of exercise
- Recovery periods - The duration at which energy supply can last with each system is correlated with the length of time each takes to recover. It is also dependent on the intensity and duration of each exercise bout, but it is often stated that around 50% of initial PC stores are resynthesised after ~30 seconds of recovery, with this system making an almost full recovery in ~3-4 minutes. The system would be unable to make a full recovery during intermittent exercise, due to the removal of harmful waste by-products taking roughly between 20 minutes and 2 hours. During intermittent exercise, there is enough fuel availability present with the aerobic energy system to maintain supplies of ATP.
- Fitness levels - Fitter individuals tend to have higher stores of and a greater resistance to lactic acid, allowing them to use these energy systems for longer. Fitter individuals are also able to utilise the aerobic energy system at higher intensities, which will assist them in sparing their reserve of energy for the later bouts of intermittent exercise, maintaining their work intensity.