Energy Systems of the Body

When it comes to working out, or just every other activity, the human body somehow needs to release energy. Generally, during exercising activities, the body uses 3 main energy systems to fulfill the energetic demand and grant energy for continuous activity.

The 3 main energy systems, that the body uses to fulfill the demands of our physical activities, are- Anaerobic-Alactic, Anaerobic-Lactic and Aerobic.

Now, this may be a bit complex or slightly familiar for you, but odds are, you are not totally aware of these systems in-depth, especially if you are a beginner trainee who’s never had contact with sports literature.

So, worry not! We are here to explain all of this!

Of course, our Science topics provide structured knowledge, however, they also provide science-based information, written in layman terms making it very easy for anyone to understand.

Before we explain more about these 3 systems, let us give you a couple of terms to help you understand the names of the systems, as well as how they function.


  1. Aerobic processes
    These are energy-releasing processes in the body, that REQUIRE oxygen to function. Aerobic as a word is derived from the Greek words “Aero” and “Bios”, meaning “air” and “life” respectively. Hence, aerobic simply means “Requiring or using oxygen from the air”.
  2. Anaerobic processes
    These are energy-releasing processes in the body that DO NOT require oxygen to function (release energy). The word is derived from the same Greek words as “Aerobic”, but we have an affix- “An”, meaning “without” in Greek. So, anaerobic translates to “Without oxygen”.
  3. Adenosine Triphosphate & Adenosine Diphosphate (ATP & ADP)
    As we said just in the beginning of this article, every living being on earth, including plants, requires oxygen to function and sustain its functions. Even the slightest of moves like extending your leg under no resistance. Adenosine Triphosphate is that exact energy molecule that provides energy to living beings. It is often called “The molecular energy currency”.This compound grants energy momentarily and it is the purest source of biological energy. During intense workouts, the body uses ATP to fulfill the energetic demand of the intensity. However, the ATP supplies of our musculature are quite limited and only last for about 5 seconds of INTENSE physical activity. Then, the ATP breaks down to adenosine diphosphate, and the body needs to regenerate the ATP to allow muscle contractions to continue.
  4. Creatine phosphate (CP)
    To restore ATP, the body uses a phosphate molecule to join the ADP, rejuvenating ATP to allow further activity for up to 10 seconds. That is to say that creatine phosphate is an energetic RESERVE of the human body. However, CP is also limited. So, the whole process of ATP breakdown to ADP and then rejuvenation and second depletion of ATP, lasts for no more than 10 seconds.
  5. Glycogen
    As the ATP & CP storages are very limited as we already mentioned, the body needs to find different means of regenerating ATP. After ATP & CP are depleted at around the 10 second mark, the body starts breaking down Glycogen for energy. Glycogen is basically the end product of carbohydrate metabolism and is in fact, the stored form of glucose. It is stored primarily in the muscles and secondarily in the liver. The process of glycogen breakdown is called “Glycolysis”.
  6. Lactic acid
    As we know by now, intense exercise breaks down ATP. Every time an ATP molecule is split for energy, it is broken down to ADP. However, there’s also a hydrogen ion released- Hydrogen ions can simply be referred to as protons.These protons that accumulate in the muscles cause the BURN you feel during intense exercise. Up until now, it was thought that lactic acid is what causes that burn (Acidosis in science terms). However, at the final stage of glycolysis (glycogen breakdown), there is a substance released, called “Pyruvic acid”, also called “Pyruvate”.In order to neutralize the soaring accumulation of Protons and Pyruvate in the muscles, the pyruvic acid soaks protons and turns into LACTATE. Keep in mind, lactate is not like lactic acid and it is rather the conjugated version of lactic acid. So, bottom line is that lactate actually BUFFERS that burn and does not cause it. Furthermore, after the 2-minute mark, the body can actually turn lactate back into glucose to use it as energy.

The Systems

Before we explain the systems in depth, we would want to highlight this- The goal for ALL 3 systems is to regenerate ATP. They all do so, but with different substrates and at different rates.

Anaerobic-Alactic (ATP & CP system)

Now that we know the terms, we can say that the Anaerobic-Alactic system does not require oxygen to release energy (Anaerobic) and does not lead to a build-up of lactic acid (Alactic). This is the most powerful energy system in the body, as it grants energy momentarily.

However, the ATP & CP system is a very short-lasting one, though it is the quickest one to regenerate ATP. This energy system uses ATP & CP as its main sources of fuel and lasts for about 10 seconds.

The Anaerobic-Alactic system is used by the body to endure HIGH intensity physical activities that are short in duration, such as a 100-meter sprint.

Anaerobic-Lactic (Glycogen system)

Past the 10-second mark, the body starts breaking down glycogen, via the anaerobic-lactic system, which, intuitively does not require oxygen (anaerobic) and leads to an accumulation of lactate (lactic).

This is the second most powerful energy system in the body, and is the second quickest to regenerate ATP. However, it is still relatively short-lasting, as it does so for about 90-120 seconds.

The anaerobic-lactic system uses Glycogen as its main fuel source and is used by the body to endure moderate to high intensity physical activity, moderate in duration, such as a 200-400-meter sprint.

Aerobic (oxidative)

Last but not least, the aerobic system. Past the 2-minute mark, the body needs to find a way to sustain the activity for longer periods of time. That is when the aerobic system kicks in.

It is the least powerful energy system in the body, out of all 3. It is called oxidative, as it uses glycogen to grant energy, however, as opposed to the first 2 systems, it does so with oxygen.

The breakdown of glycogen with the use of oxygen is called “Aerobic glycolysis”. This system uses muscle and liver glycogen, as well as fatty acids. If these are not available, the system’s last resort are the proteins.

That is to say that if you do cardio, your muscles won’t be burnt, unless you severely deplete yourself of carbohydrates (glycogen). Even though it is the least powerful energy system and regenerates ATP pretty slowly, it can do so for HOURS.

The body uses the aerobic energy system to endure low intensity physical activities that are long in duration, such as a 5000-meter cross run.


Depending on the intensity and duration of the physical activity you’re doing, you will trigger different energy systems in the body and therefore, the end result will be different.

That means that the body will adapt in different ways to serve different functions.

Knowing this is important, as in order to train according to your goals, you have to set up your training variables accordingly, to trigger a certain system and hence, produce the desired result.

For further reading, read our article on Energy Systems – Anaerobic & Aerobic Processes.

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