Energy Systems - Anaerobic & Aerobic Processes

Training, to a big extent is very subjective, as different people have different goals. The thing is that most of those people and we’d say 90% of them, don’t really know much about the task at hand.

30-day challenges, fad diets and celebrity workouts are a part of the things that people consider effective. And while they might be effective for some people, there is something else that we need to understand, in order to do optimal workouts. That is namely how the body works under different conditions.

For this specific article of our educational series, we will talk about how the body provides energy for physical activities of different kinds. Rest assured that even though the headline may look a bit confusing at first, the article won’t be full of complex scientific terms.

However, we will still make sure to establish common terminology, in order for you to understand things better.

Aerobic and Anaerobic processes

We’ve mentioned in previous articles that we are essentially living, biological systems, that require and use energy for every single motion and process that happens in the body and environment. If we follow up with this thought, we can logically conclude that the body uses certain energy-releasing mechanisms, in order to fulfill the energetic demands of the organism.

Now, when the topic is exercising, the body has two types of energy-releasing processes. Those are namely the Aerobic and Anaerobic processes.

To make it simple, let’s define each of those processes:

  • Aerobic processes

These are energy-releasing processes that require oxygen to release energy. The word is derived from the Greek “Aero” and “bios” meaning “Air” and “Life”.

These processes are used by the body to grant energy for low-intensity, long in duration physical activities, such as a 5-kilometer cross run.

  • Anaerobic processes

On the flipside, we have the anaerobic processes, which do not require oxygen to release energy, hence the name – Anaerobic. (Derived from the Greek ‘An’ – Meaning ‘without’ and again, ‘Aero’ and ‘Bios’) These processes are used by the body to grant energy for high-intensity, short-burst movements that are short in duration.

A classic example here would be a 40-yard sprint dash. Now, with this we explained the energy processes very briefly, but you now start to understand a bit more – The body can release energy both in the presence and absence of oxygen.

The type of process used, depends on the type of activity. Now let’s dig in a bit more.

Energy systems of the body

As already mentioned, releasing energy is one of the inevitable processes that occurs in the body during a workout. When there is a demand for energy, we can logically conclude that we utilize different energy pathways to fulfill that demand.

The 3 main energy systems of the human body are the following:

  1. Anaerobic-Alactic
  2. Anaerobic-Lactic
  3. Aerobic

We know that many of you are unaware of this terminology, which is why we are going to give you some reference terms, before we define and explain each system individually.

Reference terms

  • Adenosine triphosphate & Adenosine diphosphate (ATP & ADP)
  • Creatine phosphate (CP)
  • Glycogen
  • Lactic acid

Let’s define each one.


As we should know by now, each and every move of the body, and each process inside of the body, require energy. For every living being on our planet, that key energy molecule is called Adenosine triphosphate (ATP).

It is also known as the energy currency of the living organisms, as it is essentially the most powerful energy substrate in the body, as it grants energy momentarily.

However, once ATP is broken down for energy during the first 5-10 seconds of intense activity, we get its byproduct, namely Adenosine diphosphate (ADP).

When that happens, the body needs to somehow regenerate ATP

Creatine phosphate (CP)

To regenerate ATP, the body conjugates its secondary energy reserve, creatine phosphate, with the byproduct of ATP, namely the ADP. A phosphate molecule from CP joins the ADP and so, the body almost instantly regenerates ATP and allows muscle contractions to further continue for up to 10 seconds more.

In other words, as we said, creatine phosphate is the secondary energy storage of the body.


Once our immediate energy reserves (ATP & CP) are broken down for energy and depleted, intensity naturally drops and the body starts looking for other means of energy provision. To allow the activity to continue, the body starts tapping into blood glucose & its stored form, namely glycogen. Glycogen is essentially the final product of carbohydrate metabolism and as we just said, it is the stored version of blood glucose.

To put it simply, when we metabolize carbs, the body uses as much glucose from it as needed and the rest, it stores as glycogen (Or fat, if the excess of energy is consistently in a surplus). That is simply because the body is smart and won’t allow energy resources to go to waste.

Glycogen is mainly stored inside the muscles and secondarily in the liver and is used during moderate to high intensity physical activities, lasting more than 10-15 seconds. After the 15th second, the body starts breaking down glycogen to regenerate ATP, in a process known as glycolysis.

Lactic Acid

Last but not least, it is important to remember that the breakdown of energy molecules (ATP, CP & Glycogen), leads to the accumulation of more byproducts. Those byproducts are called metabolites, and in the case of intense resistance training, we accumulate pyruvate and hydrogen ions (protons) in the muscle cells.

These two causes that nasty muscle burn and fatigue that we experience when we hit our anaerobic threshold. It was previously taught by many sports universities that the fatigue and burn is caused by the lactic acid.

However, what technically happens is that lactic acid (actually lactate), BUFFERS the burn! Furthermore, it gets used as an energy source and gets thrown into the energy cycle, in a mixed anaerobic-aerobic metabolism, at about the 90th second.

And so, lactic acid is not as guilty of fatigue as people used to think.

The 3 Energy Systems

Now that we have established some common terminology, we can get on to explaining the 3 energy systems.


Knowing the theory, we just learned, we can say that this system does not require oxygen to release energy (Anaerobic) and does not lead to an accumulation of lactic acid (Alactic). This is the first and most powerful energy system, that the body uses to endure high-intensity, short burst movements. The primary sources of fuel for this system are the ATP & CP which we just talked about.

This system is known as the immediate energy system, as it is able to instantly fulfill the energy demands for high intensity activities. The resources of this system however are relatively limited and we deplete ATP & CP at around the 10th-15th second. The anaerobic-alactic system is used by the body for high-intensity, short duration activities.


Once we deplete the ATP & CP, the body starts using up the glycogen and at about the 30 second mark, we also have an accumulation of lactate, hence the name of the system – Anaerobic-Lactic.

This is the second most powerful system, which uses glycogen and glucose as the main sources of fuel. The system quickly regenerates ATP, but then again, just like the first system, the duration is relatively limited.

This system was designed to overcome moderate to high-intensity work that are moderate in duration (About 2 minutes)


The more the activity continues, the more the intensity naturally drops and the more we start relying on a mixed anaerobic-aerobic metabolism. Eventually, past the 3-4-minute mark, the body starts looking for more sustainable means of fulfilling the energy demands.

The aerobic energy system as we learned, requires oxygen to function and that’s why it is also known as the oxidative system. Its main sources of fuel are glucose, muscle and liver glycogen, fats and if those are not available, the last resort is protein.

Even though this system is the least powerful, when it comes to the rate of ATP regeneration, it is the most sustainable one. Think about it – You can practically jog (Aerobic work) for hours if you practice enough, right? But you can’t sprint (Anaerobic work) for hours.

That is to say that the aerobic energy system can regenerate ATP for hours on end. This system is used by the body to overcome low intensity, long duration activities, such as a 5-kilometer run.

Why do you need this?

Well, to simply be able to structure your own plan of action. Now that you have a bit more knowledge about how the body works, let’s see the difference between the two types of processes and the energy systems.

First off, we’ll mention that the training parameters and their ratios are the prime determinant for the triggering of the different energy systems. And so, a more intense workout would activate the first two systems, while a less intense workout with a bigger volume will trigger aerobic processes too.

In simple words – Heavier weight triggers the anaerobic processes and the fast-twitch muscle fibers. These fibers were designed for short, explosive work and have the highest hypertrophy potential, which is exactly why bodybuilders have to shoot for their optimal development. On the other hand, if we do an extensive cardio activity, we will trigger the aerobic processes and the slow-twitch muscle fibers.

That second type of fiber, was designed to sustain long in duration activities and has a lesser potential for growth, as opposed to the fast-twitch fibers. On top of that, muscular development is way back on the priority least during aerobic work. That is because the body doesn’t need to carry the oxygen-demanding muscle and its weight, when we’re working extensively.

Instead, it is looking for other means of optimizing the energy delivery, which is why we achieve adaptations of the cardiovascular & respiratory systems.

Which is better?

As we know now, the anaerobic energy systems are triggered during intense physical activities and the main adaptations we achieve are the following:

  • Muscle growth
  • Explosiveness
  • Strength gains

What this means is that if one or all of the above are your goals, you should aim for anaerobic workouts. Again, those are bouts of intense, powerful burst movements. If we’re talking about weight training, that would be bouts of 15-20 seconds for 6-10 repetitions.

Of course, we can do 1-5 repetitions, but that would primarily result in adaptations of the nerve paths and the muscle fibers. This is not bad, but if your goal is developing a bodybuilder’s physique, you can focus on the 6+ rep range and occasionally get into the 1-5 rep range for strength gains.

And so, to conclude the part about anaerobic workouts, we can say that moderate to high intensity workouts, utilizing the anaerobic-lactic system, will mainly yield bulk muscle growth.

If we use very high intensity, utilizing the anaerobic-alactic system, we will mainly achieve strength increases, accompanied by an increase of the volume of the muscle fibers. With the first, strength gains will be a secondary adaptation and with the latter, bulk muscle growth and glycogen content will be a secondary adaptation.

Aerobic workouts

Aerobic workouts, or, cardio as we know it, can actually benefit your muscle growth, though not directly. As we learned, this type of workouts mainly engages the heart and lungs to improve and optimize oxygen & blood delivery.

Knowing that these workouts have a benefit on heart & lung function, we can conclude that the improvement of those parts of our system, will ultimately make our anaerobic workouts better as well.

Every self-respecting athlete includes aerobic work in their regimen, due to the vast variety of health benefits.

So, which one is better…?

The answer is … None.

Both have their function and benefits, which is why it is best to combine them and tilt the scales in favor of what we want to focus on.

To help you understand the information thus far, let’s see a table that says it all!

Level of Intensity
Energy System Used
Fuel Used
Primary End Result
Secondary End Result
Very High
Anaerobic-Alactic ATP & CP Strength gains Muscle growth
Anaerobic-Lactic Muscle glycogen Muscle growth and strength endurance Strength increase
Aerobic Muscle and liver glycogen, fatty acids, protein if the first 2 are unavailable. Improved cardiovascular endurance and function, numerous health improvements. Slow-twitch fiber development.


What we essentially learned during this short read, is that depending on the ratios of the different training variables, we will trigger different energy systems in the body. Therefore, the end result will be different.

We have two modes of training – Anaerobic and Aerobic.

The first mode uses the most powerful energy substances in the body – ATP & CP, as well as glucose and glycogen (Stored glucose). The main adaptations we achieve with anaerobic training are expressed with an increase of strength, muscle size and muscle glycogen content (Strength endurance). This type of training mainly helps us improve upon our physical parameters.

On the other hand, aerobic training is primarily focused on the development of the cardiovascular & respiratory organs, to make the blood & oxygen delivery more efficient. Finally, we will say this again – Combine both types of training, if your goal is for every aspect of your body to be developed!

Use the body to the best extent possible!

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