Everything You Need to Know About Energy System Fitness

  • In my recent post, The Components of Physical Fitness, I broke down physical fitness into it’s bits and pieces and provided a brief introduction of each part.
  • On Monday of this week, I wrote in more detail about Proper Body Alignment and how it impacts overall physical fitness.

Today I will try to explain the concept of Energy System Fitness and how it applies to overall physical fitness.

What is Energy System Fitness?

Energy system fitness refers to the efficiency of the human body to supply energy (specifically ATP – the main source of cellular energy) to all of the it’s cells.

Depending on the intensity and duration of the activity that you are performing (sleeping, working at your computer, jogging, running from the bulls in Pamplona) your body will withdraw or synthesize ATP from one, two or even all three of it’s energy system pathways.

For maximum efficiency, the body is usually drawing from all three, but depending on the activity, one energy system pathway will usually predominate.

The three sources or energy systems are the:

  1. ATP-PC System (Phosphogen System),
  2. the Anaerobic System (Lactic Acid System)
  3. and the Aerobic System.

In this article, I will throw some science your way. If that is your thing; enjoy. If not, skip it. You can get all of the benefits of this article by sticking to the layman’s terms.

The ATP-PC Energy System Pathway

The acronym ATP-PC stands for ATP (Adenosine triphosphate) and PC (Phosphocreatine). ATP and PC work together as part of a coupled reaction to provide very rapid, very pure but ultimately short lived energy to the brain and muscles.

A good way to picture the function of ATP-PC energy is to look at at the performance of an Olympic sprinter. In the 100 metre distance, sprinters explode from the blocks, build their speed up to their personal best and then try to hold off the inevitable loss in speed as their available energy begins to drop. As their explosiveness and high rate of speed relies on their ATP-PC energy system, so does their inability to hold onto that speed past the 6 to 7 second mark.

As ATP-PC begins to fail around the 5 to 7 second mark, the body has to call upon the Anaerobic/Lactic energy system to make up for the lost ATP-PC.

Anaerobic Energy System Pathway

Like the ATP-PC energy system, the Anaerobic energy system does not require oxygen to produce ATP for energy. However, unlike the ATP-PC system, it is more complex than a simple coupled reaction.

First; pyruvate is produced from glucose. Then the pyruvate is converted into lactate. Along the way, two molecules of ATP are produced for every one molecule of glucose that is used.

Simple right? This is where the controversy about lactate and lactic acid comes in.

  • The mainstream belief is that a build-up of lactic acid causes the famous “burn” that we have all felt when we pushed ourselves to our limit. The lactic acid was thought to cause pain and fatigue and eventually cause the muscles to stop working altogether.
  • A more recent theory doesn’t blame lactic acid for the pain and fatigue. This theory says that in very intense physical activity, large amounts of ATP is being produced and hydrolysed in a very short period of time. When ATP is is hydrolysed, a hydrogen ion is released. These hydrogen ions cause the muscle cells to become acidic. This acidity manifests itself as the “burn”.

Aerobic Energy System Pathway

Merriam-Webster defines Aerobic as “living, active, or occurring only in the presence of oxygen”

The Aerobic system produces 38 molecules of ATP from each molecule of Glucose that is used. It is used throughout the body for all metabolic processes. In athletics, the Aerobic Energy System is primarily used in sub-maximal exercise such as long distance running.

The Aerobic system can be divided into 3 separate stages:

  • Glycolysis,
  • The Krebs Cycle a
  • nd Oxydative Phosphorylation.

For the purposes of this post, I will not be delving that deep into the science. Way too boring for most readers. If you are interested, look here.

Not interested in the details? Here is the abridged version…

The Aerobic system produces a stable, long lasting source of energy. However, it can not respond as quickly as the Anaerobic or ATP-PC systems. Therefore, it is the dominant system during sub-maximal activities, while the other two systems dominate the more intense athletic endeavors.

And What Does All of This Mean?

Any person that wants to develop his/her physical fitness to the best of their abilities MUST address all 3 Energy Systems.

And How Do We Do That?

Fitness programs can easily be adjusted to focus on the three different Energy Systems.

In my practice, I usually lump the ATP-PC and Anaerobic systems together. For most people, the difference between the two is negligible. So now we are left with 2 systems – Anaerobic or Maximal effort and Aerobic or Sub-Maximal effort.

Anaerobic / Maximal

Anaerobic energy system training is best addressed by High Intensity Interval Training and High Intensity Resistance Training.

Aerobic / Sub-Maximal

Aerobic training has been popular in North America since Dr. Ken Cooper published his book, Aerobics in 1968. There is a ton of info online produced by people who love aerobic training. Me…I find it boring.


Basketball is an anaerobic sport as it alternates short duration, high intensity sprints with periods of lower intensity movements around the basket. These lower intensity activities allow the anaerobic system to recharge. This sport would improve the functioning of the anaerobic system at the expense of the development of the aerobic system. The same could be said for hockey and football and football.

Long distance running is the perfect example of an aerobic sport.

NoteAn excellent illustration of different sports & how they rely on different energy systems can be seen here.

Effect on Body Composition

A common misconception exists that aerobic exercise is the best exercise to encourage fat loss. Current research is showing that theory to be false. In this study, 17 subjects were split into 2 groups, and over a 20 week period of time, performed either aerobic exercise or anaerobic exercise (in the form of H.I.I.T.). At the end of the 20 weeks, the HIIT group had lost over 3 times as much subcutaneous fat while expending less than half as many calories.

Like this article???

If you like this article, don’t forget to subscribe to @healthhabits. When you subscribe, my friends at MailChimp will make sure to send you an email every time I post something new here at the blog.

As well, you also get access to the series of Supplement Reports that I am publishing this year.

button subscribe


  1. until you edit “it’s” to “its” and promise to never repeat that glaring error I refuse to read this post :p

  2. I suppose a lot depends on what your goals are. If you are looking for muscle growth, you will focus more on using your ATP-PC. If you are looking for stamina, general fitness and a slimmer look, you will focus more on using your Aerobic System.

  3. I used to do heavy weights, eat a high protein diet, and was pretty big, but now I prefer the look and feel of being slimmer, so I focus more on aerobic type exercises.

Comments are closed.