exercise energy systems

Exercise Energy Systems – Where does Energy Come From?

Physical exercise requires of high amounts of energy, but how do we obtain that energy? Through evolution, our metabolism has acquired different exercise energy systems to be able to obtain energy from different substrates, making energy production much more efficient.

Understanding the principles of energy metabolism during exercise will make you improve in your discipline and understand what's happening inside you when you are training.  In this post, we'll cover and explained all types of exercise energy system. Are you ready?


Exercise energy systems are metabolic proceses that relates to the flow of energy when our body requires of it. By the use of different substrates, our body is able to produce ATP, the main metabolic energy currency (source)

Through evolution, our body has acquired different exercise energy systems. Under specific conditions, we'll be able to obtain energy from different exercise energy systems. That way, one can ensure to have enough energy in the form of ATP to perform physical exercise.

The use of exercise energy sytems depends on the intensity and duration of the activity (study), as well as the fitness level of the user and the preference to use specific systems (study).

An endurance athlete will show a totally different energy metabolism when compared to a weightlifter. The principle of specificity comes here into play, shifting the graph shown above to improve and optimize energy production for their specific discipline.


Adenosin Triphosphae (ATP) is an organic compound that provides energy to drive the majority of metabolic processes in our metabolism (source).

Often referred as "molecular energy currency", ATP is the main molecule used for energy production. It contains three phosphate groups bound to adenosin. These are high energy bonds, and when broken, it releases a large quantity of energy (study).

When a phosphate bond is broken, it produces ADP, which contains only two phosphate groups. If another phosphate is cleaved, AMP is formed instead, which is a marker of energy depletion (study). High concentrations of AMP in our organisms activates the breakdown of substrates such as carbohydrates or triglycerides to produce more energy (study)

ATP and exercise energy systems


Depending on the moment during the exercise or the substrate we use, we can classify the different exercise energy systems to optimize our energy metabolism.

The traditional classificiation, however, classifies the energy systems depending on whether they use oxygen (aerobic) or not (anaerobic).

Before getting into each of them, I want to you to note that they are not binary, meaning that we can obtain energy from more than one system simultaneously. Our body has developed this ability to improve energy production and not rely on one simple system to survive and evolve.


Starting from second zero, anaerobic exercise energy systems are the first ones to be used for energy production. The anaerobic system responds to high-intensity training with biochemical, neural, and anatomic adaptations (study).

To activate the anaerobic energy system, we need to perform high-intensity training close to exhaustion. For testing the anaerobic capacity, we often use the Wingate anaerobic test (study)


The first source of energy we have in our organism is ATP. Considered the 'metabolic energy currency', ATP can be readily used for energy production through the cleavage of its high-energy phosphate bonds.

Estimately, there's 100g ATP in our body, most of it found in the muscle, since it's the main tissue to take and use the energy (study). But this number can vary, depending on our ability to produce energy, the activity of our mitochondrias, or the adaptations we've built training in a prolonged period of time (study).

The main drawback here is the short duration of this system as source of energy. Because of the high demand of ATP in our body and its low-solubility in water, ATP is quickly depleted (study). When ATP is depleted, we can no longer rely on our direct ATP stores for energy production, and we'd need to regenerate ATP to be able to perform physical activity.


The ATP-PC system is the most genetic and the least adaptable of the energy systems. When we deplete ATP, phosphocreatine (PC) donates its phosphate group to replenish ATP from ADP (study).

Weightlifting and other strength disciplines largely depend on this exercise energy system to produce energy. However, in physiological levels, 50-70% of phosphocreatine is depleted within the first 5-30s, and it takes around 3-5min for the ATP/PC system to regenerate (study).

To optimize energy production from this sytem, we can increase phosphocreatine levels by creatine supplementation (study).

For more about creatine supplementation click HERE

ATP/PC system


Also called anaerobic glycolisis, lactate is generated from the oxidation in glucose under the absence of oxygen in the muscle (study). First, glucose is used to synthesize two molecules of pyruvate through a metabolic pathway called glycolisis.

The pyruvate formed, in the absence of oxygen, produces lactic acid, and consequent lactate, through what's called anaerobic fermentation.

Lactate energy system

The net energy balance of this coupled pathways is 2 ATP, which can be used for energy. The accumulated lactate can then travel throuh the blood to the liver, in where it can be transformed into pyruvate, and consequently glucose. The so called Cori Cycle spends 6 ATP (study). After combining both the production of lactate with the resynthesis of glucose, we end up with a negative energy balance of -4 ATP, but more glucose to synthesize much larger quantitities of ATP through aerobic metabolism.

This is a great example of the synergy and perfect mechanisms our metabolism has to produce energy efficiently.

Cori Cycle


The aerobic exercise energy systems mainly uses carbohydrates and fats to replenish ATP (study). Because it requires of oxygen for producing ATP, this system takes a bit more time, but it can also be kept for much longer times, being the main source of energy in all aerobic exercises (study).

In the presence of oxygen, we produce energy by three coupled complex metabolic pathways, also called the "aerobic respiration". It takes around 1-2min for this exercise energy system to start producing energy, but it can be mantained as long as your body has available carbohydrates or fats for oxidation.


Glycolisis is the firts step of the aerobic respiration. In this metabolic process, glucose is oxidized to yield two molecules of pyruvate (source). The process is formed by ten coupled reactions, three of them being critical regulatory steps.

Steps 1, 2 and 7 are highly regualted by markers of energy levels, substrate, or other allosteric regulators (study)


The process is divided in two 'halves'. On the first one, also called the 'energy-requiring phase', two molecules of ATP are needed. On the second half, starting with glyceraldehyde-3-phosphate (GAP), two molecules of ATP are generated per molecule of GAP. This is called the 'energy-releasing phase'.

Since two GAP are yield from one molecule of glucose, the second half of the pathway will yield a total of four ATP. Thus, from one molecule of glucose, one can yield four ATP, two pyruvate and reduction potential in the form of NADH, which can be used for energy production (source)


The Krebs Cycle, now called Citric Acid Cycle or Tricarboxylic Acid Cycle, is probably the most important pathway of energy metabolism (source). Being a central part of the aerobic respiration, the krebs cycle yields direct and indirect forms of energy.

Through evolution, this pathway has become more and more efficient at producing energy, now being a cycle and being able to produce large quantities of energy without the need of a lot of metabolic work. Besides its importance in energy metabolism, the intermediates of the cycle can also be used for other metabolic purposes (study)

Krebs Cycle

The Krebs Cycle starts with Pyruvate entering the mitochondria from the cytosol by terms of a pyruvate transporter (study). Once in the mitochondria, the complex pyruvate deydrogenase catalyzes the reaction between pyruvate and CoA to yield Acetyl-CoA, the substrate of the Krebs Cycle (study). Acetyl-CoA can be formed both from carbohydrates and fats, being a central molecule of energy metabolis.

Through a series of catalyzed reactions, Acetyl-CoA is regenerated with oxaloacetate being the last intermediate of the cycle. After one round of the cycle, we obtain redox potential (3 NADH + 1 FADH2) and energy currency (GTP).

While the GTP can be directly used as energy currency, the redox factors must now be used to drive production of energy, in the so called 'oxidative phosphorylation'


Oxidative phosphorylation is the last step of aerobic respiration. During this step, redox cofactors produced in the Krebs Cycle are used to create a proton gradient in the intermembrane space. The proton gradient will then be used to drive the ATP synthase and yield ATP. From one molecule of glucose, 24-28 ATP are formed only in this step (source)

Oxidative phosphorylation

During this step, oxygen (O2) acts as the last acceptor of the electron transport chain. This is the reason why we need oxygen to survive. Without oxygen, we wouldn't be able to produce energy efficiently, and our metabolism would simply crack.

After all three steps of aerobic respiration, 36-38 ATP are formed from one molecule of glucose. But, as you can see, is a large process that requires of time. Thus, this exercise energy system will prime during aerobic exercise, but it won't have too much effect on anaerobic disciplines.


Physical exercise requires of large amount of energy, but where does that energy come from? As we saw today, the answer to that question is exercise energy systems.

Through evolution, we have acquired different exercise energy systems to be able to produce energy under different conditions. In this post, we saw all energy systems used during exercise to understand how energy metabolism work while we are performing physical activity.

If you have any doubts, leave it below in the comments section!

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