Energy Systems Explained

This article was researched and written by Callum Stewart, my colleague from Stewart Athletic Development and reproduced here with kind permission. Check out his website and follow him on Facebook here

What it is about? – What the energy systems are responsible for, what the different energy systems are & how they relate to specific activity, and how to target each systems.


I will preface this here – This article will not be a lesson in physiology, physics or biochemistry, so don’t worry. There are some areas which may appear a little more technically explained, however all should become clear by the end of the article. If you do have any areas you would like further explanation of, or questions looking for more detail then feel free to shoot me an email / DM on social media and I will do my best to help you out!

Energy systems overview

For human movement to occur, the body requires energy. The energy is in the form of a molecule known as adenosine triphosphate, more commonly referred to as ATP. The energy systems in the body generate or degrade substances which are converted into ATP for movement. The amount of ATP required and rate of ATP production is dependent on 1) the activity performed and 2) the intensity of the activity. For example, a 100m jog would require less energy than a 100m sprint. The type of activity performed dictates which energy system is the primary source of energy, however all the energy systems work together as an integrated unit. The energy systems can be broadly classified as either aerobic (with adequate O2 supply) or anaerobic (with inadequate O2 supply) as highlighted below.

ATP-PCr – Also known as the Phosphagen/alactic anaerobic system. ATP-PCr is stored intramuscularly, meaning there is a finite supply of available energy. Intramuscular Phosphocreatine (PCr) is produced in 3 ways. 1) PCr can be produced from creatine in the liver, 2) PCr can be produced from intramuscular creatine 3) PCr can be made from the enzymatic reaction between ATP-PCr intramuscularly. This energy system is the predominant fuel source in explosive activity such as sprinting. This system can provide energy very quickly, as there are only 2 reactions required to produce ATP. However, the finite stores of ATP-PCr mean that this energy substrate is depleted extremely quickly (approximately 10s of maximal activity) and takes 3-5 minutes to resynthesise depending on your fitness levels. Sports such as weightlifting, 100m sprinting, and throwing events are predominantly fueled by this system. Team sports such as rugby, football, basketball etc will rely on this during line breaks & sprints during the game.

Anaerobic glycolytic – also known as anaerobic lactic system. Anaerobic glycolysis also produces ATP very quickly from intramuscular glycogen stores & from pyruvate produced in glycolysis when O2 supply is inadequate. This pyruvate is then used in the lactic acid cycle, which converts pyruvate to lactate. Lactate accumulates in the blood as exercise intensity and duration increases; however, this is not a bad thing! A common misconception is that blood lactate is a negative thing, however in the lactic acid cycle it is converted into a fuel source and is actually incredibly important for performance! This system is responsible for moderate to high intensity exercise. Resynthesis occurs via oxidative pathways and from exogenous carbohydrate consumption i.e. eating carbs. Sports such as 200-400m running, rugby, football and swimming will rely predominantly on this energy system for performance.

Aerobic Glycolytic/Beta oxidation – Also known as the oxidative system, aerobic glycolysis & free fatty acid (FFA) oxidation. These systems are responsible for producing energy at low intensity. They produce high amounts of ATP, however the chemical reactions required to do so are more complex thus take more time. Firstly, we will examine Aerobic Glycolysis. This occurs during glycolysis when there is adequate O2 (unlike anaerobic glycolysis). Similarly, this process produces pyruvate, however this pyruvate is used within the tricarboxylic (TCA) cycle (also known as the Krebs cycle) and not the lactic acid cycle. This cycle then produces molecules called acetyl-CoA & NADH which are then used in the Electron Transport Chain, producing ATP. FFA’s can also be used for energy via Beta Oxidation. FFA’s are derived from intramuscular triglycerides (intramuscular fat stores), adipose tissue (body fat tissue) and circulating high-density lipids and low-density lipids (HDL / LDL’s). Beta oxidation also produces acetyl-CoA & NADH for the Electron Transport Chain, however this is the process which takes the longest.

Whilst both processes yield large amounts of ATP, there are lot of rate limiting enzymatic steps, thus it takes a long time to produce energy. As a result, these energy systems are responsible for basal and low energy activities such as walking and recovery and are not particularly efficient for moderate to high intensity exercise.

That’s great, but what the f*ck does that mean for my training?

You may be wondering how any of this information can be applied in a useful context for your training, and that’s where I am here to help you. I will explain how each of these systems can be targeted via training & why you might want to target them.

ATP-PCr – To target this energy system you need to exercise at a maximal (or supramaximal) intensity for the appropriate period of time. Rest periods for this energy system are also very important. Looking at sprinting activities, you want to be sprinting for 5-15s. Sprints can either be done from a stationary start in a 2-point (Figure 1), 3-point (Figure 2,) or 4-point (figure 3) stance or from a rolling start where you run into the start, building up your pace. All stances have their advantages for different purposes in sprint training, which will be discussed in a later article.

Looking at resistance, plyometric, and power training the principles are the same. This energy system will be predominantly utilised in high effort sets, such as max effort power cleans, squats and repeated bounds. Regardless of the exercise performed, sufficient rest is required to allow ATP-PCr stores to resynthesise. This can take 3-5 minutes, depending on your level of aerobic fitness. As recovery is an aerobic process, the more aerobically fit you are the quicker you will recover.

An example of a pre-season sprint session for a rugby union player, which targets the ATP-PCr system can be seen below. It should be noted this is not inclusive of other training sessions they would be undertaking during a pre-season phase. These sessions would likely be incorporated with other training sessions such as a skills session.

Anaerobic glycolytic system – This energy system requires high (not maximal) effort input. Anaerobic glycolytic training can be quite uncomfortable in nature due to high metabolite (waste product) build up. Efforts can range between 30s to 5 minutes of high intensity exercise. Recovery for this system is driven partially from aerobic pathways, and also from nutritional intake. From an aerobic recovery standpoint, you are looking around 2-4 minutes recovery dependent on the length, mode, and outcome goal of the training. Typically, recovery is 90s-4 minutes in length. From a cardiovascular training standpoint this system can be worked by running, cycling, rowing, swimming, or similar activities. This energy system will also be predominant during higher rep exercise sets where there is more of an endurance focus.

An example of a week of watt bike training which predominantly utilises the anaerobic glycolytic system could be;

Oxidative system(s) – The oxidative systems are trained at a low to low/moderate exercise intensity. The sport which you compete in will determine the emphasis of these systems for performance, however even for higher intensity sports the oxidative systems still play a huge role in recovery. Endurance sports such as long distance running and cycling will use oxidative pathways, in particular aerobic glycolysis, to provide energy for performance. Sports such as rugby union and football will depend on oxidative pathways to a lesser extent, however interplay and post-match recovery will be influenced by oxidative systems, therefore are still an important consideration. The oxidative systems can be stimulated through high intensity interval training (HIIT), however there are caveats within exercise prescription of HIIT (which will be discussed in a later article) so for the purposes of this article, traditional oxidative training will be discussed.

Traditional aerobic training which stimulates oxidative pathways is typically viewed as endurance training. Long distance training will typically tax the oxidative pathways until there is a shift from aerobic to anaerobic metabolism for energy. This occurs when the rate of blood lactate clearance is exceeded by the rate of blood lactate production. This physiological phenomenon is referred to as the onset of blood lactate accumulation (OBLA) and is often viewed as one of the most important factors in predicting endurance sport performance. Delaying the point in which OBLA occurs is crucial to endurance sport performance and is increased by improving oxidative metabolism pathways and aerobic capacity. An example of a traditional endurance training week for a triathlete may look as follows;

Until next time and as always, stay strong.


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