Energy Systems: Your Complete Guide to Cardio Training

When it comes to Energy Systems Development (or Cardio Training), there are three energy systems that you use while exercising and they are split into two categories, with oxygen, and without oxygen. Aerobic exercise is an exercise that uses oxygen as a catalyst for energy production while anaerobic exercise is an exercise that creates energy without oxygen. There are two different types of anaerobic energy production, Lactic and Alactic.

All three energy systems are necessary for almost any form of sport or gym-based program, and the purpose of this article is to discuss each one and explain how they work together. But first, we should define each energy system, and look at what they do.

Anaerobic Alactic (ATP-PCr) System

Adenosine Triphosphate (ATP) is often called the “energy currency”, it is an organic chemical that is used to transfer energy between cells. Every action that you produce during the day requires ATP. The body does not store much ATP in cells but is excellent at replenishing it. Studies have found that the body can recycle an entire body weight’s worth of ATP each day [1].

ATP is made up of an adenosine molecule and three phosphate molecules (hence the “tri” in the name which means three). Through a process known as hydrolysis the body breaks down ATP by combining it with water, removing one phosphate molecule, this process creates the energy required to perform actions. You are now left with one adenosine molecule and two phosphate molecules.

This is a new organic chemical called Adenosine Diphosphate (ADP) and it is fairly useless when it comes to energy production. You need that third phosphate molecule. Luckily, your body stores phosphate molecules combined with Creatine – called phosphocreatine (PCr). The purpose of this article is to give you an idea of what is happening – rather than a complete scientific breakdown, so let’s keep this simple.

  • ATP + Hydrolysis = ADP + Energy
  • ADP + PCr = ATP

In a perfect world, this process would be able to continue indefinitely and we wouldn’t need cars anymore because we could just sprint 3 miles to work each day. But you only have a small amount of ATP stored in your bodies, and a small amount of PCr. Supplementation with Creatine monohydrate can help you store slightly more PCr in your muscles, but even so, it’s not going to make too much difference.

Because of the limited stores of ATP and PCr in muscle, the ATP-PCr system is only used for very short, intense bursts of exercise. It can last for around 5-10 seconds, but this will vary depending on your current fitness level, your diet (vegetarians have much lower amounts of PCr stored in their muscles), and the exercise type.

A sport that would use the ATP-PCr system almost exclusively would be the 100m sprint, or the 60m sprint (used for indoor athletics competitions). This is because most sprints are sub 30 seconds, and professional sprinters would be sub 11 seconds.

In a gym session, a one-rep max squat would be a perfect example of an exercise using the ATP-PCr system almost exclusively.

The ATP-PCr system is anaerobic because it does not require oxygen to create energy, and it is Alactic because it does not lead to increased lactic acid production.

Anaerobic Glycolytic System

Let’s say that you were performing a 400m sprint, for the first 5-12 seconds your body would use stored ATP to produce energy, using phosphocreatine to replenish ATP. At the same time, another ATP production method would be kicking into gear. This energy system uses dietary carbohydrates (stored in the body as glycogen) to produce ATP.

Anaerobic glycolysis is used by the body during prolonged high-intensity exercise. When oxygen demand outstrips oxygen requirements. Anaerobic glycolysis turns glucose or glycogen into ATP but while doing this it creates lactate (also referred to as lactic acid). It does this by turning glucose into Pyruvate through a process of 10 chemical reactions which break it down. This leads to 2-3 ATP molecules, but also produces lactate.

Lactate has a very useful purpose, it helps to recycle (or regenerate) glycogen in the liver. This helps you to keep exercising for longer. Sadly lactic acid has got itself a bad reputation among the general public – many of whom blame lactic acid for muscle pain and fatigue during prolonged exercise.

For a long time, scientists believed that lactic acid was responsible for muscle soreness, and this information was taught to most of us while at school. It’s an easy mistake to make because lactic acid is released at the same time that muscle soreness begins. You can understand why it was commonly believed that the lactic acid was causing this.

But correlation does not imply causation. While your body is producing lactic acid as a by-product of anaerobic glycolysis it is also producing hydrogen ions. The increase in hydrogen ions leads to a change in pH in the body – it is this that is responsible for acute muscle soreness and fatigue. So while it is fair to associate lactic acid with muscle soreness, it is unfair to blame lactic acid for it!

Anaerobic glycolysis is dominant immediately after the ATP PCr system runs out of steam, and will last for around 90 seconds max. The body cannot keep performing at such a high intensity for longer than that, and you will begin to slow down as you fatigue. This allows you to breathe in oxygen and your body will now predominantly use our final energy system.

The Oxidative (Aerobic) System

The final energy system that we utilize is the aerobic system. In many ways, it works similarly to the Anaerobic Glycolytic system as its primary purpose is to turn glucose or glycogen into energy (ATP). Just like the Anaerobic Glycolytic system, you start off with glucose being broken down via glycolysis into Pyruvate (also known as Pyruvic Acid).

But instead of turning Pyruvate into lactic acid, the oxygen present is able to break down the Pyruvate further, until it becomes Acetyl-CoA. This is then entered into the Kreb’s Cycle, then the Electron transport chain. Eventually, you end up with 36-40 molecules of ATP (compare that to the 3-4 that Anaerobic glycolysis produces).

If you kept on running for even longer you may begin to run out of stored glycogen (there’s around 2,000 calories worth in most people). After this, the body will begin to use stored energy (body fat) to keep you going. This process is less efficient than the glycolytic system but is very similar. Fat is broken down into fatty acids, which then goes through beta-oxidation, to become Acetyl-CoA and then the process from then on is the same.

People who run marathons tend to switch from using glycogen to using stored fat at around the 15-20 mile mark. They describe it as “hitting the wall” and try to prolong this feeling for as long as possible by carb-loading before a race.

Energy Systems During A 1500m Run

To give you an idea of how all three energy systems work together we’ll use the 1500m race as an example. Among athletes, a 1500m race will last around 4 minutes or so. This means that it will encompass all three energy systems.

During the first 5 seconds of the race, our runner is accelerating and trying to build a lead. At this point, he is predominantly using the ATP-PCr system to generate energy. There will be a small contribution from the Anaerobic glycolytic system as well as from the aerobic system (though it will be minimal).

At the 10 second mark the runner will still be using ATP-PCr but now it will only be contributing 50% of the energy, while 35% is coming from the Anaerobic glycolytic system, and 15% from the aerobic system. At the 30 second mark, most of the energy is now coming from Anaerobic glycolysis (65%) while the aerobic system has increased and ATP-PCr has almost completely run out. Between 60 and 120 seconds into the race, the anaerobic glycolytic system is dominant.

But after the 2-minute mark, the runner will begin to use his aerobic system for energy. By the fourth minute, he will be relying almost entirely on the aerobic system. This example works if you believe that a 1500m athlete would spend the entire race running absolutely flat out. Which is almost true at an elite level.

But the runner may start slow and reserve his energy for a sprint finish. Basically, energy system use is not as straightforward as we’ve made out here. But hopefully, you can see why a 1500m runner would need to train all three systems to get the best results. But middle distance running is not the only sport that involves all three systems, almost any sport will require you to use each one.

Tennis is a great example, lots of very short, intense rallies that require your ATP-PCr system, and short games that require the anaerobic glycolytic system. But a match can last for hours (particularly in Men’s tennis) which requires a good aerobic system. If a tennis player did not train his ATP-PCr system he would not be good at serving powerfully and then sprinting to the net to smash an overhead volley.

If the player did not train his anaerobic glycolytic system he would not be able to keep the rally up if his opponent managed to return both the serve and then the volley. If the player did not train his aerobic system then he would not be able to last the match. All three systems need to be improved for the tennis player to excel.

Recovery Capabilities Of The Energy Systems

One question that you may be asking yourself is how could a marathon runner sprint for the line at the end of a 26-mile run? Surely their ATP-PCr system would have run out after the first 15 seconds? Well, it doesn’t work quite like that, as you are probably aware if you have ever performed any type of exercise.

All three systems work together to replenish ATP. Baker, McCormick, & Robergs (2010) wrote: “The replenishment of ATP during intense physical exercise is the result of a coordinated metabolic response in which all energy systems contribute to different degrees based on an interaction between the intensity and duration of exercise” [2].

Glycolytic ATP production (either Aerobic or Anaerobic) can also lead to replenishment of phosphocreatine (PCr) which means that the ATP-PCr system is constantly replenished [3]. Obviously, this will diminish as exercise continues, there will come a point where you will be too fatigued to perform a sprint. But hopefully, this gives you an idea of how a long distance runner could suddenly put on a burst of speed after running for over an hour.

And, remember all three energy systems are operating simultaneously whether at rest or in a marathon. The only difference is one system will predominate depending on the demands of the activity.

Power Capabilities Of Each Energy System

The benefit of the oxidative (aerobic) system is that it can continue for a long time – sometimes many hours at a time, compare this to the ATP-PCr system where you have only 12 seconds before it runs out. The downside of the oxidative system is that it can’t produce much power.

Let’s imagine that you were performing a bench press of your bodyweight. For the first rep, you have a lot of energy at your disposal and your chest drives the bar up explosively, but then you have to repeat the movement. Your second rep is still decent, but rep three is lacking the same amount of power. This is because you have used up your fast released energy and are now relying on the anaerobic glycolytic system. Still powerful, but not quite as powerful as before.

Reps 5-12 are much more difficult. You are no longer driving the bar explosively, you are just pushing the bar. You’re working harder now than you were on rep 1 but you’re noticing a drop in power, an increase in muscle soreness, and your technique is beginning to suffer.

You re-rack the bar and lower the weight before immediately starting again (a drop set). But even though the weight is much lighter, you are still struggling. This is because you are now using the oxidative system almost exclusively, and your power has dropped to almost nothing. Your technique has declined significantly, and your muscles are on fire (thanks to all those Hydrogen ions right?).

The above example uses a resistance exercise (bench press) but it could also apply to running, jumping, or any exercise. The longer an exercise continues without rest, the less power you will have due to using different energy systems.

Using Heart Rate & The RPE Scale To Train Each Energy System

There are two ways to use your heart rate when training. The first and definitely most accurate method is to wear a heart rate monitor like the My Zone system as it will calculate your heart rate training zones for you. The other way is to use your Rate of Perceived Exertion (an RPE Chart is included in the 6-Week Cardio Program below) to estimate your intensity for you. Since the first method would require a monetary investment, let’s take a look at the RPE Method.

The rate of perceived exertion is a scale of how hard you feel while you are working. There are a few different scales out there, but the one we’ll use goes from 1 to 10. At 1 you are basically lying down doing nothing, while 10 is exercising so hard that you think you might pass out.

Active recovery (oxidative system) would be a 1–2.5 on the scale with a low heart rate (i.e., 50-59% of your Max Heart Rate). Then you have aerobic extensive (i.e., Aerobic Capacity) which is 3-4.5 on the scale and a slightly higher heart rate (i.e., 60-69% of your Max Heart Rate). Next comes aerobic intensive (i.e., Aerobic Power) which is 5-6.5 on the RPE scale and medium/high heart rate (i.e., 70-79% of your Max Heart Rate). After that is Anaerobic Threshold which is 7-8.5 and high heart rate (i.e., 80-89% of your Max Heart Rate), followed by Anaerobic Capacity/Power training which is maximal heart rate work (i.e., 90-100% of your Max Heart Rate) and a score of 9-10 on the RPE scale.

For your convenience, I have created an in-depth 6-Week Cardio Program (enter your details below and I will send it to you) utilizing a three-session weekly rotation so you can train across the Energy Systems continuum. I’ve also included a sample training schedule and noted when you could do your strength training sessions as well as an RPE chart in case you don’t have a Heart Rate Monitor.


CLICK HERE To Get Your 6-Week Cardio Program

Finally, I just want to thank Dr. Tony Ricci for reviewing this article and giving it his stamp of approval. Make sure you follow Dr. Ricci on Instagram as puts out awesome content.


Törnroth-Horsefield, S., & Neutze, R. (2008). Opening and closing the metabolite gate. Proceedings of the National Academy of Sciences of the United States of America105(50), 19565–19566.

Baker, J. S., McCormick, M. C., & Robergs, R. A. (2010). Interaction among Skeletal Muscle Metabolic Energy Systems during Intense Exercise. Journal of Nutrition and Metabolism2010, 905612.

Forbes, S. C., Paganini, A. T., Slade, J. M., Towse, T. F., & Meyer, R. A. (2009). Phosphocreatine recovery kinetics following low- and high-intensity exercise in human triceps surae and rat posterior hindlimb muscles. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology296(1), R161–R170.


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