Energy Systems Explained – A Guide

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Energy systems Explained - What are the three energy systems in sport

Energy Systems – A Guide for Sports Coaches and Players

Energy systems are so important in sport and all coaches and players should know what they are and how they work. By understanding the different uses and advantages to each energy system, coaches and athletes will be able to tailor and adapt their training programme to suit.

In this post, we will explain the three main energy systems along with their advantages and disadvantages. 

Before we start, you also be interested in our article on the different styles of leadership or check out our free Sports Coaching Learning Centre for more articles on sports coaching techniques and advice. 

What are the three Energy systems used in sport?

The three main energy systems used in sport are:

  • ATP-PC System (Adenosine Triphosphate and Phosphocreatine System) 
  • Lactic Acid Energy System
  • Aerobic Energy System (Aerobic Glycolysis, Kreb’s Cycle and The Electron Transport Chain (ETC)

Why do we need the energy systems?

Adenosine Triphosphate (ATP) enables our muscle to contract and move. Without it, we would not be able to move our muscles. The downside of this is that our body has a limited to supply of ATP (1-3 seconds).

Therefore, to continue to function after our body has used up the ATP reserves, the body needs to create more ATP. This is where the three main energy systems come in. The book, High-Intensity Interval Training by Paul Laursen and Martin Bushheit provides some great examples on how sports coaches can use HITT training in coaching sessions and programmes (view price on Amazon here).

Energy systems Explained - What are the three energy systems in sport

What is the ATP-PC system?

When ATP is broken down, it releases energy for the muscle to contract. The by-product of this reaction is adenosine diphosphate (ADP) and one phosphate (Pi). The energy systems enable ADP and Pi to join back together so it can be broken down again and enable the body to continue to move.

Phosphocreatine (PC) can be broken down to enable the ADP and Pi to join together again. 1 PC makes 1 ATP.

PC is broken down by an enzyme called Creatine Kinase to produce Creatine and Pi. The energy from this reaction is enough to rebond 1 ATP.

What are the advantages and disadvantages of the ATP-PC System?

The advantages of the ATP-PC system are:

  • Oxygen is not needed (Anaerobic)
  • The ATP-PC system is quick 
  • Provides another 5-8 seconds of activity
  • There are no negative by-products

The disadvantages of the ATP-PC system are:

  • The body only has a limited amount of PC in the body (5-8 seconds)
  • The body needs to rely on another energy system after the PC has been depleted
  • It can take up to 2 minutes rest to replenish the used PC stores.

After the PC stores have been depleted, the body will then either use the Lactic Acid Energy system or the Aerobic Energy System. This is dependent on whether the athlete is able to get oxygen to the muscle (Aerobic Orr Anaerobic).

For example, a 100m sprinter is likely to have depleted their PC stores towards the last quarter of the race and will most likely be unable to provide the body with enough oxygen to continue at the current pace. Therefore, the body is likely to use the Lactic Acid system for most of the race.

Whereas, a Marathon runner is likely to predominantly use the Aerobic system for most of their race. This is because the runner is able to provide the body with enough Oxygen to continue to replenish their ATP.

What is the Lactic Acid Energy System?

The body is able to store energy as Glycogen. When needed, Glycogen can be broken down by Glycogen Phosphorylase and this creates Glucose (Our most predominant energy source). Glucose can then be broken down by Phosphofructokinase (PFK) and this reaction creates enough energy to replenish 2 ATP.

If the body does not have sufficient oxygen, Glucose is broken down in Pyruvic Acid. As too much Pyruvuc Acid can be dangerous to the body, the body can transform Pyruvic Acid to Lactic Acid by using Lactate Dehydrogenase (LDH).

Therefore, once a 100m sprinter has used up their PC stores, the body can replenish 2 ATP without oxygen to enable to sprinter to finish the race. Yet, this has a cost, Lactic Acid. I am sure those of you who have completed a 100m before know how ‘heavy’ your legs feel at the end of the race.

What are the advantages and disadvantages of the Lactic Acid Energy System?

The advantages of the Lactic Acid Energy system are:

  • Oxygen is not needed (Anaerobic)
  • The process is quicker than the Aerobic Energy system
  • Provides enough energy for up to 8-20 seconds

The disadvantages of the Lactic Acid energy system are:

  • Lactic acid is produced
  • Can be painful to an athlete
  • Their is a longer recovery stage than other energy systems

What is the Aerobic Energy System?

With the aerobic system, the body still uses energy from Glucose which can be stored as Glycogen.

As stated above when there is no Oxygen, the body will use the Lactic Acid energy system to reproduce ATP. When there is Oxygen available, the body still produces 2 ATP after PFK breaks the bonds of Glucose and Peruvian Acid is created as a by-product, yet the body can continue to produce more ATP through three stages of the aerobic system. In total, the Aerobic energy system produces 38 ATP in three stages.

The three stages of the aerobic system are:

  1. Aerobic Glycolysis (2 ATP)
  2. Kerb’s Cycle 2 ATP)
  3. Electron Transport Chain (34 ATP)

Aerobic Energy System Stage 1: Aerobic Glycolysis

After the 2 ATP has been released by breaking down Glucose, the pyruvic acid combines with Coenyme A to form Acetyle CoA. This is the first stage of the aerobic system.

Aerobic Energy System stage 2: Kreb’s cycle

Step 1: Acetyl CoA combines with Oxaloacetic Acid to form Citric Acid. 

Step 2: This citric acid goes through a cycle of 4 stages:

  1. CO2 is produced and exhaled through the lungs
  2. Hydrogen Atoms are produced
  3. Energy is released to resynthesise 2ATP
  4. Oxaloacetic Acid is regenerated and the cycle is able to begin again

Aerobic Energy System Stage 3: Electron Transport Chain (ETC)

Step 1: The hydrogen atoms that were created in Kreb;s cycle combines with CoEnzymes NAD and FAD to form NADH and FADH

Step 2: These then pass into the ETC

Step 3. Hydrogen is then split into H+ and e- and these pass through the ETC

Step 4: As a result of this, enough energy is released to resynthesise 34ATP

Step 5: Hydrogen combines with Oxygen to form H2O and this is then released in the body

As a result of the three stages of the Aerobic energy system, 38 ATP are able to released.

The Advantages of the Aerobic Energy system are:

  • 38 ATP are able to resynthsise which is a higher number compared to other energy systems
  • The body is able to to work continuously if the body is replenished with oxygen, water and glucose

The disadvantages of the Aerobic Energy system are:

  • Oxygen is required whereas the ATP-Pc and Lactic acid system does not require Oxygen
  • Glucose is needed
  • Body is unable to exercise at a higher intensity compared to the other energy systems
  • The process to resynthesise is slower than the other methods.

If you want more on the energy systems, here is link to a YouTube video that goes into the three different energy systems further. Also you may want to check out the book High-Intensity Interval Training.

View Price on Amazon here


Having an understanding of the limitations of each energy system will help sports coaches to help plan their training programmes to suit the need of each energy system. Our next article will be on how coaches can adapt their training sessions to suit the needs of the athlete using the energy systems.

You may also be interested in our article on our favourite football coaching books or our free downloadable sports session planning template.

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