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?

Your organism has some exercise energy systems to be able to obtain energy from different substrates, making energy production much more efficient.

In this post, we'll cover and explained all types of exercise energy system. Are you ready?

What are Exercise Energy Systems?

Exercise energy systems are metabolic proceses that relates to the flow of energy when your body needs it.

By using different substrate, your body is able to produce ATP, the main metabolic energy currency (source)

exercise energy systems meme

Under specific conditions, you'll be able to obtain energy from different exercise energy systems.

That way, one ensures to have enough energy in the form of ATP to perform physical exercise and the rest of daily actions.

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 Role of ATP in Exercise Energy Systems

Adenosin Triphosphate (ATP) is your body's energy currency.

It's an organic compound that provides energy to drive the majority of metabolic processes in your metabolism (source).

atp meme

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).

ATP and exercise energy systems

When a phosphate bond is broken, it produces ADP, which contains only two phosphate groups. If another phosphate is cleaved, AMP is formed, which is a marker of energy depletion (study).

Types of Exercise Energy Systems

Depending on the moment during the exercise or the substrate we use, your body uses different exercise energy systems.

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


The exercise energy systems are not binary, You can obtain energy from more than one system simultaneously. Your body has developed this ability to improve energy production and not rely on one simple system to survive and evolve.

Anaerobic Exercise Energy Systems

Anaerobic exercise energy systems are the first ones 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, you need to perform high-intensity training close to exhaustion. For testing the anaerobic capacity, it is often used the Wingate anaerobic test (study)

ATP Stores

ATP can be readily used for energy production through the cleavage of its high-energy phosphate bonds.

There's approximately 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).

This number can vary, depending on your ability to produce energy, the activity of your mitochondrias, or the adaptations you've built training in a prolonged period of time (study).

However, this system only provides energy for a very short period of time. When ATP is depleted, you can no longer rely on the direct ATP stores for energy production.

ATP/Phosphagens System

The ATP-PC system is the most genetic and the least adaptable of aññ the energy systems.

When you deplete ATP, phosphocreatine (PC) donates its phosphate group to replenish ATP from ADP (study).

ATP/PC system

Weightlifting and other strength disciplines largely depend on this exercise energy system to produce energy.

However, 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).

creatine meme

Lactate Energy 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

This exercise energy system yields 2ATP for every molecule of glucose used.

The accumulated lactate can then travel throuh the blood to the liver, 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.

Cori Cycle

Aerobic Exercise Energy Systems

The aerobic exercise energy systems mainly uses carbohydrates and fats to replenish ATP (study).

It needs oxygen to produce ATP, and it is usable for much longer. These exercise energy systems are the main energy source of aerobic exercise.

In the presence of oxygen, you produce energy by three coupled complex metabolic pathways,  called the "aerobic respiration".

It takes around 1-2min for this exercise energy system to start producing energy, and 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'.

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

The Krebs Cycle is the most important pathway of energy metabolism (source). It is a central part of the aerobic respiration, yielding direct and indirect forms of energy.

Besides its importance in energy metabolism, the intermediates of the cycle can also be used for other metabolic purposes (study)

Krebs Cycle

First, Pyruvate enters the mitochondria from the cytosol by a pyruvate transporter (study). Once in the mitochondria, the complex pyruvate deydrogenase forms 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 metabolism

Acetyl-CoA is regenerated with oxaloacetate, the last intermediate of the cycle. After one round of the cycle, you obtain redox potential (3 NADH + 1 FADH2) and energy currency (GTP).

GTP can be directly used as energy currency, and the redox factors drive production of energy in the 'oxidative phosphorylation'

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 drive the ATP synthase and yield ATP.

From one molecule of glucose, 24-28 ATP are formed only in this step (source)

Oxidative phosphorylation

Oxygen (O2) is the last acceptor of the electron transport chain. This is the reason why we need oxygen to survive.

Without oxygen, you wouldn't be able to produce energy efficiently, and your metabolism would simply crack.


The complete aerobic respiration yields 36-38 ATP from a molecule of glucose.

However, this is a large process that requires of time. 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? It comes from exercise energy systems.

In this post, we've seen all energy systems used during exercise to understand how energy metabolism work while we are performing physical activity.

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