The energy systems

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Even at rest the body needs to produce energy. During exercise, energy is needed at a faster pace – muscles contract more frequently, the heart beats faster, etc. This energy comes from the food that is eaten:

  • Carbohydrates are stored as glycogen in the muscles and liver, and as glucose in the blood.
  • Fat is stored under the skin and is known as adipose tissue. It helps prevent heat loss and serves as a fuel store. Fat is made up of triglyceride molecules that are broken down to fatty acids, releasing energy in the process.
  • Protein is used for growth and repair of the body’s tissues. It is not stored by the body unless there is too much (it is then converted to fat). Protein can be broken down to release energy by amino acids. This only takes place, however, during long-lasting endurance events.

Energy is released in the body by the breakdown of carbohydrates, fats and proteins to produce a chemical called adenosine triphosphate, or ATP. ATP is made up of one molecule of adenosine and three phosphates. These are held together by high energy bonds, which when broken, produce energy. ATP is stored in small amounts in the muscle fibres and when one of its phosphate bonds is broken, adenosine diphosphate, or ADP, is formed together with energy and heat production (this is why people get hot when exercising).

ATP → ADP + energy for contraction

Muscle stores of ATP are limited. Once exercise has started, the demand for energy increases and the stored supply of ATP is used up in a couple of seconds. If continued muscular contraction is to take place, ATP has to be reformed from ADP once it has been used up. This is done by reversing the breakdown. The body has three systems for doing this, which are collectively known as energy systems – there are two anaerobic and one aerobic system. These three energy systems do not provide energy for physical activity – only ATP can do this – they provide energy to restore ADP into ATP.

Anaerobic systems

(anaerobic = without oxygen)
These systems produce energy when muscles need to move quickly or forcefully, but do not need to keep going for very long. There are 2 anaerobic systems:

  1. CP system
  2. Lactic acid system.

The CP system

(also called the creatine phosphate or PC or phosphocreatine system)
Muscular contractions need ATP (stored in muscles). Energy is immediately available but stores are very small and will only give enough energy for a few seconds. The CP system remakes ATP as quickly as the muscle stores use it up. ADP is turned back into ATP by using another chemical found in the muscle fibres called creatine phosphate (CP). CP is also stored in the muscle fibres in small amounts, and although ATP can be reformed very quickly using this method, it cannot be kept up for very long, as the stores of CP are used up in 6-10 seconds.

ATP → ADP + P + energy (for movement)
CP → C + P + energy (to resynthesize ATP)
ADP + P + energy → ATP
CP system
The CP system is used for activities that need bursts of explosive speed or power such as sprinting, jumping and throwing.

The lactic acid system

It takes some time for oxygen to get into the blood and then to working muscles. The lactic acid system of energy production is used when oxygen is not available and when stores of creatine phosphate run out (after approximately 10 seconds). The ADP uses energy formed from the breakdown of carbohydrates (glycogen or glucose) to produce ATP. Lactic acid is produced as a by-product. As lactic acid accumulates in the muscle, muscular contraction becomes more difficult.
lactic acid system
ATP → ADP + P + energy (for movement)
Carbohydrate → lactic acid + energy (to resynthesize ATP)
ADP + P + energy → ATP

This energy system does not last long and will sustain energy production for intense activities that last up to 3 minutes. With continued high intensity exercise, there is a build-up of lactic acid in the working muscles. If the rate of build-up is greater than the rate of removal then the muscles become fatigued and muscle contraction becomes impaired. Exercise must be stopped or intensity reduced to remove lactic acid.

The aerobic system

This system produces energy when muscles need to keep moving over a prolonged period of time at a steady pace. It is used when there is a plentiful supply of oxygen for the muscles to use while working. Aerobic means ‘with oxygen’. This system starts to ‘kick-in’ after about 3 minutes and theoretically will go on for ever. The ADP formed during energy production uses energy from glucose/glycogen (carbohydrates), fat or protein breakdown to reform ATP. This system allows the body to work in a steady state – the muscles work below maximal effort and keep this going for a long period of time. Carbon dioxide and water are produced as by-products. The aerobic process takes place in mitochondria – specialised structures within the muscle cell. They are like factories containing special enzymes which work on the oxygen.
aerobic system
ATP → ADP + P + energy (for movement)
Carbohydrate/fat/protein → carbon dioxide + water + energy (to resynthesize ATP)
ADP + P + energy → ATP

The aerobic system yields a large amount of ATP and is used for all light continuous exercise and activities. This system produces energy at a much slower pace than anaerobic systems, and is much too slow for intensive or explosive activities. Regular aerobic training increases the size and number of mitochondria making the body more efficient at burning fat. By improving the utilisation of fatty acids, the body is able to use less glycogen which is in much shorter supply. Although the aerobic energy system is far more efficient than the anaerobic systems, it is much slower to take effect.

Energy systems and exercise

The energy needed for different activities varies considerably. The energy system used is determined by the:

  • Intensity of exercise
  • Type of exercise
  • Duration of exercise

Some activities use 1, 2 or even all 3 energy systems at different times during the activity, with the emphasis changing according to the intensity relative to fitness levels.
energy systems table

Oxygen debt

When anaerobic energy systems are used, an oxygen deficit is produced – muscles need more oxygen than they can get at that time. If the activity is continued, lactic acid is produced. After the activity has been finished, the performer needs to have a rest and take in the extra oxygen that is needed. This extra oxygen is known as the oxygen debt, and it helps to remove the build-up of lactic acid, replenish the stores of oxygen in the body, and build up the ATP and creatine phosphate stores in the muscles.