Bioenergetics for Dummies


CAUTION: It’s about to get sciency in here…

So here’s a big fancy college word: Bioenergetics

Which basically means the study of sources of energy and how they are ultimately utilized to help you kick ass in the gym and at life.

This is kind of an important subject when it comes to training for optimal performance, surviving in the field for extended periods of time, and just overall feeling great and kicking ass (Oh, also pretty important for just being alive…).

Keep in mind, having a basic understanding of this stuff is pretty important when evaluating a training program…

So, let’s dive in.

The food you stick in your gullet is basically chemical energy that your body absorbs and stores as glycogen, fat, and protein. Then, when needed (i.e. Your next workout) it is released to provide energy to produce adenosine triphosphate (More commonly known as ATP).

Now, I’m sure you’ve heard of ATP but if you haven’t…

ATP is that little high-energy molecule for things like muscular contraction and a variety of other metabolic and life-sustaining functions in your body.

ATP is created in your body when adenosine diphosphate (‘di’ = 2 phosphates) (ADP) is combined with an additional inorganic phosphate (Pi). Unfortunately, this process (combining ADP and Pi) take a whole bunch of energy so we need to eat food to make this happen (Or it will be taken from the body by breaking down what you have).

I tell you all this because when that ATP bond is broken (bond between ADP + Pi) a bunch of energy is released that provides you with the ability to perform physical activity.

So at this point, we can all agree that ATP is important (even if we really don’t care how we make it), right?

Cool, so let’s keep going…

We’ve got two primary chemical reactions (facilitated by enzymes — Not going to get into this) that are classified as either exergonic reactions, a chemical reaction that give off energy (i.e. The reaction that breaks the ATP bonds and gives off energy for muscles to contract) or endergonic reactions, the chemical reaction that absorbs energy from its surroundings.

An example… If you are sprinting 400m, ATP is being broken down and energy is being released in your muscles (in order to power the required muscular contraction), this is an exergonic reaction. At the same time, the muscles are utilizing this energy to power each contraction, which is endergonic.


Chemical energy comes from the food we consume.

That chemical energy is then transformed (by a number of processes in the body) to mechanical energy in the form of muscular contraction.

You lift heavy weight over your head and put it down.

Pretty simple, right?

Now, let’s talk about energy systems

I just mentioned that chemical energy is transformed into mechanical energy and allows you to PR your back squat but now we need to dive into the primary energy systems that actually provide that ATP.

There are 3 primary energy systems that provide ATP for your awesome physical activities: The phosphocreatine system, the glycolytic system (anaerobic), and the oxidative system.

Each of these systems can be characterized by the duration and/or intensity of the workout task you are performing.

Here is a quick reference: Energy systems & their approximate contributions to various durations of exercise at high/maximal intensity

Phosphocreatine system (PC)
PC & glycolytic system (slow)
Glycolytic system (fast)
Glycolytic & oxidative system
Oxidative system
0 – 10s
10 – 30s
30s – 2mins
2 – 3mins
3+ mins (and when at rest)

So, from this table above a few things should be apparent.

  1. Higher intensity, shorter duration exercise is powered by the Phosphocreatine & Glycolytic systems
  2. The Oxidative system powers lower intensity (including rest) long duration stuff
  3. Your body does a pretty good job blending the capabilities of all of these systems to ensure you are getting the ATP you need to be a badass.

Pretty cool, huh?

Let’s dive a bit deeper into each of these systems

The Phosphocreatine System

When ATP is needed for energy FAST and for a very short duration then this is your bodies go-to system. It is very efficient at producing ATP quickly for a short period of time such as when you are performing a 1RM power clean.

A few other characteristics of the phosphocreatine system:

  1. It involves one chemical step which = very fast chemical reaction
  2. It is catalyzed (sped up) by an enzyme call Creatine Kinase
  3. One ATP is generated by every creatine phosphate molecule consumed (By the chemical reaction)
  4. Lasts about 5 – 10s
  5. It is anaerobic (Which means oxygen is not required in the chemical reaction)
  6. You’ll fatigue when you run out of creatine phosphate
  7. It is the primary energy system in speed & explosive power

The Glycolytic System (a.k.a. Anaerobic Glycolysis)

This system breaks down carbohydrates anaerobically (without oxygen) in order to produce energy. This carbohydrate will come from either glucose in your bloodstream or from glycogen (stored glucose in your liver and muscles).

Glycolysis can be broken down into fast & slow glycolysis based on the amount of oxygen available which will talk about in a second.

Fast glycolysis – Breaks down glucose to pyruvate (an acid used in the oxidative system) and eventually lactic acid and produces 2 ATPs. However, if the substrate being used is glycogen (instead of glucose) then it will produce 3 ATPs.

Slow glycolysis – Basically refers to the path that pyruvate takes when oxygen is present (i.e. It starts to feed the oxidative system)

A quick overview of the characteristics of the glycolytic system:

  1. It requires 18 chemical reactions (6 of which are repeated)
  2. It’s fast, but not as fast as the phosphocreatine system
  3. It produces 2 ATPs if glucose is the substrate and 3 ATPs if glycogen is the substrate
  4. It is anaerobic (Doesn’t require oxygen)
  5. Lasts about 2mins
  6. Predominate energy system in high-intensity, non-maximal exercise

The Oxidative System

This system “burns” carbohydrate aerobically (with oxygen) and is a very complex system that involves the Krebs cycle and the electron transport system. Both of these systems combined, which I’m not going to cover in detail, are complex aerobic metabolic processes that produce a whole bunch of ATPs, especially compared to the anaerobic systems mentioned above.

Here is a quick overview of the oxidative system:

  1. It involves 124 chemical reactions!
  2. Is is very slow compared to the other 2 systems
  3. 36 ATPs are produced from one molecule of glucose (37 from glycogen)
  4. It is potentially limitless at low intensities
  5. Fatigue is associated with glycogen depletion
  6. It is the predominant energy system in endurance events and longer duration exercise.

Why is this stuff important to know?

In my opinion, a lot of the nitty-gritty details aren’t very useful to you as an athlete. However, when you’re training for performance (not aesthetics) you should probably be spending some time training in each of these pathways. You all probably know “that runner” guy who can crush a 5k in 15 minutes but is then crushed by a 45# ruck in a few short miles.

He probably spent the majority of his training time in the oxidative pathway, build up a robust aerobic engine but neglected the higher intensity (i.e. heavier/faster) stuff.

To perform well during any number of given tasks you need to be able to produce maximal explosive force, maximal strength, high-intensity efforts, and long duration movements at any given time.

Just running won’t get you there.

Just jacking weight in the gym won’t either.

So when designing or evaluating a training plan you are going to use to train you for your required activity (such as being a strong, durable, and deadly military athlete) you should make sure the program is designed to make you better across all of these energy system domains and doesn’t favor just one, to the exclusion of others.


Chandler, T. J., & Brown, L. E. (2012). Conditioning for strength and human performance. Baltimore, MD: Lippincott Williams & Wilkins.

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