Energy Systems in Action — PT Direct (2023)

'Failure' during exercise is usually due to energy system fatigue. Read this page for an overview of the three human energy systems and how they 'fuel' different activites

As you read through the energy system pages here at ptdirect you will may well encounter some unfamiliar terms, or terms that aren't crystal clear. To help you out we've provided some 'lay' definitions of these terms which you can use as a reference as you read though the energy system pages.

Energy systems key terms




Means production of energy through processes that do not require oxygen. When applied to exercise, anaerobic refers to all exercise that does not rely on oxygen to help produce energy


Means production of energy through processes that require oxygen. When applied to exercise, aerobic refers to all exercise that requires oxygen to help produce energy.


Stands for adenosine triphosphate. ATP is essentially the energy currency of the body. It is the breakdown of ATP that releases energy which the body’s tissues such as muscle can use to create movement.


Stands for adenosine diphosphate. ADP is the by-product of the breakdown of ATP for energy. It is the remaining adenosine molecule and two (di) phosphate molecules


Stands for one phosphate molecule, such as the phosphate that is released when ATP is broken down to provide energy for the body to use


Stands for phosphocreatine and is also known as creatine phosphate. It is stored in muscle cells and used as a fuel to create or 'synthesize' ATP


ATPase are a group of enzymes which contribute to either the breakdown of ATP or the manufacture (synthesis) of new ATP


Glycolysis means the breakdown (lysis) of glucose and consists of a series of chemical reactions controlled by enzymes


This refers to a 'building' process where two different elements are brought together to create a new element, such as ADP and Pi being brought together to create or manufacture ATP

Krebs cycle

The krebs cycle is part of the aerobic energy system and creates ATP through a series of chemical reactions involving oxygen

Electron transport chain

The electron transport chain is part of the aerobic energy system and also creates ATP through a series of chemical reactions involving oxygen

Energy Systems in Action — PT Direct (1)What is energy?

All energy starts as light from the sun. Plants convert sunlight into chemical energy through the process of photosynthesis.

We humans then eat the plants, or we eat the animals that have eaten plants, and in turn this stored chemical energy is passed on to us.

In food, energy is stored as either; carbohydrates, fats or protein.Energy Systems in Action — PT Direct (2)

The ability to run, walk, lift weights, play sport and in fact sustain every bodily function depends on the ability of the body to extract chemical energy from the breakdown of the food nutrients that we consume.

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Once the foods we have eaten are digested they can be stored in the body, and/or transferred into chemical energy for immediate use as fuel for the body.

What are the energy systems?

In order to extract the energy from the foods we eat and turn it into the chemical energy that our bodies can use, we have three separate energy production systems, these are the:

  • ATP-PC system
  • Anaerobic glycolytic system
  • Aerobic system

What do the energy systems do?

Energy Systems in Action — PT Direct (3)

The three energy systems work together in order to ensure there is a continuous and sufficient supply of energy for all our daily activities.

Each system differentiates in the way they produce chemical energy (ATP) from different sources and at different speeds.

The ATP-PC system and the anaerobic glycolytic system are both anaerobic systems, meaning that oxygen is not used by these systems to synthesise ATP. These systems are quicker at producing energy, however they do not last very long (they fatigue quickly).

The aerobic system on the other hand relies heavily on oxygen to synthesise ATP. Because the chemical processes that use oxygen to produce energy are more complex than the anaerobic processes, the aerobic system is slower at making energy, but it can keep making energy for a very long time without fatique.

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Why are the energy systems important?

The energy systems are what enable every cell, tissue and organ of our bodies to function and survive. Without sufficient energy being continuously supplied through the energy systems our bodies would literally shut down, cease to function and die!Energy Systems in Action — PT Direct (4)

Essentially the body is like a machine and like any machine it needs energy to power it. For example a car without petrol in the tank is just a piece of metal that can't do anything. With fuel the car can come to life and drive you from 'A to B'.

Understanding how the energy systems work and interact with each other will help ensure that you are advising the right type of fuels for your clients to consume, as well as designing and prescribing the correct type of training, and applying the variables (sets, reps, rest intervals etc) correctly to ensure your clients achieve their specific goals by design rather than accident.

The energy systems and fitness

When you workout in the gym, go for a run or play football with your friends there are many body systems involved that work together in order for this to be possible.Energy Systems in Action — PT Direct (5)

For example going for a run on the treadmill for 20 minutes requires the following:

  1. Nervous system – memory of running movement patterns, action potentials to skeletal, cardiac and smooth muscle
  2. Muscular system – contraction and force production of leg muscles to run
  3. Respiratory system – inhalation of O2 and exhalation of CO2
  4. CV system – heart and blood vessels transport O2 and nutrients to muscles and remove CO2 and waste products

Energy is constantly needed by all of these systems in order for them to function. For this reason the three energy systems work constantly in conjunction with each other to achieve this.

Depending on the intensity and duration of the exercise one particular energy system may be more influential than the others. With higher intensity exercise over a short time period the bodies reliance for energy will be placed on the anaerobic energy systems (ATP-PC system / anaerobic glycolytic system), whereas lower intensity exercise over a longer time places greater reliance on the aerobic energy systems.

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If you train someone in the wrong way, it can be very detrimental to them achieving their goal or performing well in their chosen sports or activities.

Energy Systems in Action — PT Direct (6)For example if someone wants to run a marathon in a time of 3 hours and you design a programme that has them doing 6 x 100meter sprint repetitions, will they be ready to do the marathon or will they just be quick over a short distance?

Yep you guessed it; they will be quick over a short distance and have no nope of completing a marathon, let alone in 3 hours!

When it comes to working with clients and developing appropriate exercise programmes that help them achieve their goals, it is vital to consider the energy systems.

Understanding which energy system they will predominantly be using during their training is crucial to ensuring that you prescribe the correct duration and intensity of exercise.

Energy Systems in Action — PT Direct (7)

Energy systems used in sports

Energy Systems in Action — PT Direct (8)

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It is important to understand that while the energy systems have unique characteristics, they do not work independently of one another.

From very short intense exercise through to very light prolonged activity, all three energy systems make a contribution. However one or two will usually be most dominant.

Energy Systems in Action — PT Direct (9)To get an understanding of how the systems work together we can look at various sports and think about what is happening and why certain energy systems are more or less active.

Try to remember that duration and intensity are the two variables that will determine which system is most active at any given time.

Following is a list of sports and approximate percentages of how much each of the energy systems contributes:



Anaerobic Glycolytic






Field events (shotput, discuss, javelin)



Golf swing















Running (distance)












Swimming (50m freestyle)








Each of the three energy systems can generate power to different capacities and this varies within individuals.

The best estimates suggest that the ATP-PC system can generate energy at a rate of roughly 36 kcal (calories) per minute. Glycolysis can generate energy only half as quickly at about 16 kcal per minute. The oxidative system has the lowest rate of power output at about 10 kcal per minute.

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The capacity to generate power of each of the three energy systems can vary with training. The ATP-PC and glycolytic pathways may change by only 10-20% with training. The oxidative system seems to be far more trainable although genetics play a limiting role here too.

Aerobic power can be increased by as much as 50% but this is usually seen when regular training is undertaken by initially untrained, sedentary individuals.


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