Energy Systems and their application to training principles


Exercise physiology is underpinned by the energy systems. When we exercise our body is constantly working to supply muscles with enough energy to keep going. The way energy is made available to muscles changes depending on the specific intensity and duration of exercise


  • Role of Adenosine Triphosphate (ATP)
  • Glycolysis – aerobic and anaerobic
  • Principles of training – intensity and duration
  • Energy system continuum
  • Energy systems application to sport

Overview of the Energy Systems

  • Adenosine tri-phosphate (ATP) is the only energy source for all bodily functions and activities (movements)
  • When ATP is used for energy production it must be replenished
  • The body can replenish (recreate) ATP aerobically or anaerobically
  • Where is one Aerobic Energy System which requires oxygen to replenish ATP and two Anaerobic Energy Systems that can recreate ATP to produce energy without the need of oxygen

These are the 3 Energy Systems

  • ATP-PC System or Alactic System – ATP and creatine phosphate (CP) are present in very small amounts in the muscle cells. The system can supply energy very quickly because oxygen is not needed for the process. No lactic acid is produced in the process (Alactic)
  • Anaerobic Glycolysis or Lactic Acid System uses carbohydrates (glucose) stored in the muscles as Glycogen. Because no oxygen is required to re-synthesise ATP, energy is produced quickly. Also because no oxygen is used in the process lactic acid is produced as an end product.
  • Aerobic System – This system uses carbohydrates (glucose/glycogen) and fats to replenish ATP. Because oxygen is required for the process, energy production takes a little longer but can continue for a much longer duration. Because of the presence of oxygen, no lactic acid is produced.

Intensity and duration

The energy systems are all working at the same time (See Energy Continuum). However the predominant Energy System used to re-supply ATP depends on 3 things: -

  • INTENSITY of exercise. (How hard you are working). The more intense the exercise the greater amount of (Anaerobic Energy) – Creatine phosphate and muscle glycogen will be used. Low to medium intensity exercise will use predominantly the aerobic system
  • DURATION of exercise. (How long you are exercising). E.G. If the exercise is high intensity and lasts over 2 minutes then both CP and Muscle Glycogen will become depleted and need repaying. Intensity of exercise will drop as the aerobic system becomes more dominant.
  • FITNESS LEVEL of the performer. Individual levels of both aerobic and anaerobic fitness will impact on the predominant energy system being used. A higher level of aerobic fitness will mean it will take a performer longer to reach the Anaerobic Threshold (The point at which the performer gets more energy from the anaerobic systems rather than aerobic). This is beneficial because when a performer begins to work anaerobically there is only a limited supply of energy available (PC and muscle glycogen - up to 2 minutes max). If the exercise continues to increase then the performer will run out of anaerobic energy and return to using aerobic whereby performance will then drop. This can be seen at the last few stages of the multi stage fitness test where the performer struggles to stay in time with the ‘beeps’ and eventually has to stop.
    The greater the anaerobic fitness the longer the performer can work in the anaerobic zone. In reality most people have anaerobic stores that last just over a minute. A trained performer can last up to 2 minutes, and also be able to ‘tolerate’ greater amounts of lactic acid in their muscles.

In practice, all these factors work together to determine which are the predominant energy systems being used during the activity.

% of maximum rate of energy production

Quick revision

  • The energy systems work together to replenish ATP.
  • The 3 energy systems are the ATP-PC, Anaerobic Glycolysis and Aerobic.
  • The energy systems all work together at the same time to keep replenishing ATP. At no point will only one energy system will be used, but there is often a predominant system.
  • The predominant energy system used during exercise will depend on the intensity and duration of the activity and the individual’s levels of fitness.
  • ATP-PC system is predominantly used during maximum intensity activities lasting no longer than 10 seconds.
  • Anaerobic Glycolysis system is predominantly used for high intensity activities lasting approximately 1 minute
  • Aerobic system is predominantly used during medium to low intensity activity.
  • The predominant energy system being used at rest is the aerobic system.
  • The predominant energy system used during exercise will depend on the intensity and duration of the activity and the individual’s levels of fitness.
  • ATP-PC system is predominantly used during maximum intensity activities lasting no longer than 10 seconds.
  • Anaerobic Glycolysis system is predominantly used for high intensity activities lasting approximately 1 minute.
  • Aerobic system is predominantly used during medium to low intensity activity.
  • The predominant energy system being used at rest is the aerobic system.

It is important to be able to provide practical sporting examples of when each of the energy system is being predominantly used. Good knowledge and understanding is demonstrated by being able to highlight the link between intensity and duration of exercise as well as the individual’s level of fitness to the predominant energy system being used.

Practical Examples From Different Sports

Example 1. In netball the ATP-PC system would be the predominant energy system used when a Centre sprints at maximum intensity or 100% to get into space to receive the ball.

The candidate has therefore demonstrated with the example that there is a direct link between the ATP-PC system and maximum or 100% intensity.

Example 2. The Anaerobic Glycolysis or lactic acid system would be predominantly used in netball when a centre works at a high intensity for duration of up to 40 seconds. This could occur if a team fails to score, resulting in a prolonged period of play. Also during this time most of the CP stores would have been depleted, therefore the body would rely on the anaerobic glycolysis system for energy.

The candidate has demonstrated with the example that the anaerobic glycolysis system is high intensity and is used over longer periods of anaerobic exercise. The answer also shows further knowledge through the link between the depletion of CP as the predominant energy source and the use of muscle glycogen as the predominant source.

Example 3. The Aerobic System would be predominantly used by a centre in netball in medium to low intensity phases of play when the ball is out of play or when returning for a centre pass when a goal has been scored. An increased level of aerobic fitness would be mean the centre would take longer to reach anaerobic threshold and therefore would maintain anaerobic energy stores for longer which means a higher intensity can be maintained throughout the duration of the game.

The candidate has used an appropriate example to make the link between low to medium intensity exercise and the aerobic system. There is also understanding of how aerobic fitness levels influence the predominant energy system used.

Key points Covered

  • Energy Continuum is the interchanging of the energy systems during exercise
  • The energy system being predominantly used is dependent on the type of activity being performed.
  • The predominant energy system being used will depend on the intensity and duration of exercise and the fitness level of the performer.

Practical Application/explanation

In reality, the energy systems never work in isolation and are ALL working at different percentages at different times. E.G. when jogging the body will still be using a very small proportion of the ATP-PC system and when sprinting the Aerobic system will also be used albeit in very small amounts. As stated the percentage use of each of the energy systems will be constantly changing particularly in games type sports where the intensity and duration of the exercise being performed is constantly changing. For example during a game of football a wide player may sprint down the line beating defenders using approximately

  • 85% ATP-PC
  • 13% Anaerobic Glycolysis
  • 2% Aerobic

Then when he stops to prepare to cross the ball when confronted by a defender the proportion of each system will change because the intensity of the exercise will drop and hence the proportion of the energy systems being used will change:

  • 30% ATP-PC
  • 50% Anaerobic Glycolysis
  • 20% Aerobic

At the end of a match, competition or activity the proportion of each energy system will be totalled. Examples of this can be seen in the table below.

Quick Check

  • All the energy systems work together in varying proportions to replenish ATP
  • The factors that determine the proportions of the energy systems used are intensity and duration of the exercise and the fitness levels of the individual.
  • Threshold - The point at which the predominant energy system being used cannot provide sufficient ATP to maintain the current intensity of exercise. E.G The threshold for the CP/lactic acid system is approximately 10 seconds (after very high intensity exercise) after this the stores are depleted and the Anaerobic Glycolysis system will become the predominant system to provide ATP.
  • Anaerobic Threshold – The point that which the aerobic system cannot sustain energy production (ATP replenishment) at this intensity and the anaerobic glycolysis (lactic acid) system becomes the predominant system.

Practical Application/explanation

A common misconception with energy systems and the energy continuum is that during exercise the body will replenish ATP using one system (e.g. aerobic) then as intensity gets harder the body switches completely to another system (anaerobic Glycolysis)then as it gets harder again, only the ATP-PC is used. In practise, as the intensity increases there will be a gradual move towards the anaerobic glycolysis and ATP-PC systems from the aerobic system, but the aerobic system will still be helping to replenish ATP.

As with the energy systems the candidates must link the energy continuum to the intensity and duration of the exercise and fitness level of the performer if appropriate within the answer (see Energy Systems top tips).

Exam Style Questions

  1. What is the ‘Energy Continuum’ and explain the factors that determine the energy systems used during exercise. (5)
  2. Compare the energy demands of 2 different sporting activities of your choice. Estimate and justify the contribution of each of the 3 energy systems for each activity