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The Aerobic System

The aerobic system accesses a massive store of virtually unlimited energy. On this page you'll learn how this system will keep you chugging along forever without ever letting you get out of 2nd gear!

The aerobic energy system utilises fats, carbohydrate and sometimes proteins for re-synthesising ATP for energy use. 

The aerobic system produces far more ATP than either of the other energy systems but it produces the ATP much more slowly, therefore it cannot fuel intense exercise that demands the fast production of ATP. 

personal training double decker busThink of the aerobic system as the big diesel bus with a massive fuel tank as opposed to the V8 car of the ATP-PC system and the V6 car of the anaerobic glycolytic system. 

While the aerobic system doesn’t produce nearly as much power as the other systems, a major feature is its capacity which is virtually limitless, as it just keeps on producing ATP. 

Think of this capacity as the fuel tank of the diesel bus – it is so big that it’ll hardly ever run out of fuel. 

personal training 3 stages of aerobic systemThe aerobic system consists of three processes or ‘stages’ each of which produce ATP.

These stages involve more complex chemical reactions than the other energy systems which is why ATP production is much slower. (The more complex the process - the longer it takes to produce ATP)

The three stages which will be discussed in greater detail are:

1.      Aerobic glycolysis (slow glycolysis)

2.      Krebs cycle (also known as the citric acid cycle)

3.      Electron transport chain

Aerobic (slow) glycolysis – Stage 1

Aerobic glycolysis is exactly the same series of reactions as anaerobic (fast) glycolysis, except it just has a different outcome because sufficient oxygen is present. 

personal training steps of aerobic glycolysisSteps of aerobic glycolysis:

  1. Initially stored glycogen is converted to glucose.  Glucose is then broken down by a series of enzymes.
  2. 2 ATP are used to fuel glycolysis and 4 are created so the body gains 2 ATP to use for muscular contraction.
  3. Pyruvate is created as the end product of the breakdown of glucose.  As oxygen is present pyruvate is converted into a substance called 'acetyl coenzyme A'.
  4. Acetyl coenzyme A can then be synthesized in the second and third stages of the aerobic system to create more ATP.


The second and third stages of the aerobic energy system continue the breakdown of glucose that was started by aerobic glycolysis and result in the formation of the by-products carbon dioxide (CO2) and water (H2O), and the synthesis of more ATP. 

personal training mitochondriaThese later stages occur in the mitochondria which are tiny bean shaped structures located inside the muscle cells (pictured adjacent). 

Mitochondria are known as aerobic ‘power plants’ as it is within these power plants that the majority of aerobic ATP is produced.

The second stage of the aerobic system also deals with the acetyl coenzyme A that is produced by aerobic glycolysis.  This second stage is known as the Krebs cycle.

Krebs cycle (Stage 2)

Fatty acids (from fats) and amino acids (from proteins) are converted to acetyl coenzyme A through a series of complex chemical reactions.  Along with the acetyl coenzyme A from glycolysis they enter the Krebs cycle and are broken down.  This results in ATP production and the by-products of carbon dioxide and hydrogen are produced.

personal training krebs cycleThe hydrogen produced in the Krebs cycle plus the hydrogen produced during glycolysis, left unchecked would cause cells to become too acidic.

Acidity in the muscle is what causes the anaerobic glycolytic system to fatigue. 

So in the aerobic system hydrogen combines with two enzymes and is then transported to the electron transport chain. 

The main purpose of the Krebs cycle is to generate hydrogen to transfer to the electron transport chain where it can be ‘dealt to’ in a way that will control acidity and enable the aerobic system to keep synthesising ATP.

Steps of the Krebs cycle:

  1.  Acetyl-coenzyme A enters the Krebs cycle.
  2.  Acetyl-Coenzyme A is broken down into carbon dioxide (a waste product which is expelled through breathing) and hydrogen.
  3.  2 more ATP are synthesised during this process and made available to fuel further muscle contractions.
  4.  Hydrogen is transferred to the electron transport chain. 

Electron transport chain (Stage 3)

The electron transport chain is the most complex and productive pathway of the aerobic energy system. personal training electron transport chain

It produces 34 molecules of ATP for every molecule of glucose that is used.  Its complexity however makes it very hard to understand – we don’t expect you to be an expert!

Once in the electron transport chain the hydrogen ions from the Krebs cycle undergo further chemical reactions.  Here they are combined with oxygen to form the end product of water.

The process of transferring hydrogen ions from its carrier molecules to oxygen and having the hydrogen ions move across a chemical gradient produces the energy required to combine ADP and Pi to form ATP.  In summary the electron transport chain works as follows:

Steps of the Electron transport chain:

 1.      Hydrogen ions from Krebs cycle are carried to the electron transport chain by carrier molecules.

 2.      Hydrogen ions are transferred to carrier molecules embedded in the electron transport chain where they go through a series of chemical reactions.

 3.      A hydrogen ion gradient is created.  As hydrogen ions move across this gradient another form of ATPase phosphorylates ADP (adds another phosphate group) to form ATP.

 4.      Water is created as a by-product as hydrogen combines with oxygen.

 In summary the ATP gained from the complete breakdown of 1 glucose molecule in the aerobic system is as follows:


            Glycolysis                                            2 ATP

            Krebs cycle                                         2 ATP

            Electron transport chain                     34 ATP

            Total                                                   38 ATP


From this we can see how the aerobic energy systems capacity to generate ATP is virtually limitless. 

Where the anaerobic glycolytic system synthesises only two ATPs from the breakdown of one glucose molecule, the aerobic system can synthesise 38 ATPs from one molecule, albeit very slowly in comparison.  This also explains how our capacity for low intensity endurance activities is so large.

Training the Aerobic Energy Systempersonal training time trial

This energy system can be developed with various types of training.

  • Interval training – Interval training for the long term aerobic energy system would have a work-rest ratio of 1:1 or 1:2.  The work periods would usually exceed several minutes and the rest periods would be active but at a lower intensity that could be sustained.
  • Continuous training – Training that maintains a constant intensity and lasts for a prolonged period of time (usually longer than 15 minutes)
  • ‘Fartlek’ training – A type of interval training whereby the exerciser varies the speed and effort throughout the training session according to how they feel ensuring that they can continue to exercise at all times (i.e. no rest intervals).

Examples of training that is primarily focused on the aerobic system are:

  • Run of two minutes at mod/high intensity, followed by two minutes at low intensity (active recovery) repeated for 30 minutes.
  • 30 minutes low/moderate intensity cycling, swimming or jogging without change in intensity.
  • 30 minute jog over some hills requiring bursts of extra effort every now and then but never stopping throughout the jog.
Madhusudan says:
Jul 05, 2020 11:55 PM

I have a question- so HIIT exercises involves Aerobic system or Anerobic system?

Steven Gourley
Steven Gourley says:
Jul 06, 2020 01:08 PM

Hi. Both. When ATP is broken down to release energy in the cell, if oxygen is available to pick up the bi-products it's aerobic. If no oxygen is available at that moment, it's anaerobic.

The bi-product is mainly 'pyruvate' and if there isn't oxygen available it is bound to Hydrogen ion and makes 'lactate'. This is what burns. When you have lactate your pH balance changes (hence the burn) and you start to accumulate CO2 as well. More CO2 requires more breathing - that's a negative feedback loop - the higher the CO2 the higher harder you breathe to 'blow off the CO2'.

So, when you do HIIT, the rate at which you burn energy in the cell will mean most of the time oxygen isn't present, but some of the time it will be. As your HIIT progresses and your aerobic system reaches full capacity, it's trying hard to supply as much oxygen as possible and get the CO2 out. Your capillary beds in the working muscles are dilating fully to increase blood flow to try and clear the lactate. Given the rapid depletion of glucose stores your body is rapidly releasing catecholamines to access fat stores. You won't burn much fat during this workout (RER is over 1) but in recovery fat will be a primary engery source (RER around 0.7). Research shows a different fat utilisation post HIIT more related to belly fat stores they think due to the catecholamine release and drop in insulin / increase in glucagon.

HIIT is both aerobic and anaerobic. The work is predominantly anaerobic but to get there you are beyond lactate threshold which is the ceiling of the aerobic system too. So both energy systems are being conditioned.

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