Stewart Athletic Development

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What fuels performance? A beginners guide to the energy systems

I will preface this here – This article will not be a lesson in physiology, biology or biochemistry, so don’t worry. There are some areas which may appear a little more technically explained / technical jargon, 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!

In the previous article I wrote about how athletes can deal with Covid 19 in terms of maintaining & improving some of their physical fitness parameters. This article will look in more detail at the energy systems role in movement & performance and how it relates to your training. It will also look at how the energy systems can be emphasised, thus relating more sport specific adaptations from a physiological perspective.

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 is 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 walk requires less energy than a 100m jog, which requires 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 or a power clean. This system can provide energy very quickly, as there are only 2 enzymatic 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 aerobic capacity. Sports such as Weightlifting, 100m sprinting and throwing events are also predominantly fuelled by this system. Team sports such as Rugby, football, basketball etc will rely on this during line breaks & sprints during the game etc, usually in the components which can be game changers

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!  Lactate is required for the lactic acid cycle , where the lactate is converted into a fuel source and is actually incredibly important for performance!. This system is responsible for moderate to high intensity exercise.

Blood lactate clears over time, and the more the adapted to using lactate as a fuel source you are, the quicker the lactate will clear. Resynthesis of intramuscular glycogen 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 a longer time to do. 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 Acetyl-CoA & NADH which are then used in the Electron transport chain, producing ATP. Free fatty acids (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 HDL / LDL’s.  Beta oxidation also produces Acetyl-CoA & NADH for the electron transport chain, however this is the process takes a considerable amount of time relative to anaerobic processes.

Whilst both processes yield large amounts of ATP, there are lot of rate limiting enzymatic steps, thus 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 / Alactic – To target this energy system you need to exercise at a maximal (or supramaximal) intensity for the appropriate period of time. Rest period for this energy system is also very important. The work; rest ratio can range from 1; 12-20. Looking at sprinting activities, sprinting typically lasts for 5-15s which ensure the Alactic energy system is targeted. Sprints can either be done from a stationary start (2,3- or 4-point stance) or from a rolling start where you run into the start, building up your pace. Both stances have their advantages for different purposes in sprint training, which will be discussed in a later article. Sprinting can also be completed on a bike / cycle ergometer, rowing machine battle ropes and other modes of cardiovascular training. The important factor in stressing the ATP-PCr system is the intent in which you exercise. You need to push yourself to maximal efforts to emphasise this pathway for substrate metabolism. If you do not push it to maximal effort, you will yield the desired adaptations.  

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 modality 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. During these type of movements there is also involvement from other mechanisms such as the stretch shortening cycle, however that will be discussed in a later article.

An example of a pre-season sprint session, for a team sport athlete which targets the ATP-PCr system can be seen below. It should be noted this is not inclusive of other training sessions, such as skills and gym sessions, in which they would also undertake during a pre-season phase.

DayExerciseSetsRepetitionsTime / distanceRecovery time
MondayAcceleration sprints255s sprints60 – 90s recovery between sprints, 4 mins between sets
ThursdayMax velocity sprints3410s @ max velocity120s rest between reps, 5 minutes between sets
SaturdayAcceleration & Change of direction sprints2410s acceleration120s rest between reps, 5 minutes between sets

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 in length 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 dependant 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 activities. This energy system will also be responsible during higher rep exercise sets where there is more of an endurance focus. 

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

DayExerciseSetsRepetitionsTime / DistanceRecovery period
MondayWatt bike session412km as fast as possible90-120s off
Wednesday 612 mins on60s off
FridayWatt bike session10160s on60s off
SaturdayWatt bike session414 mins on2 mins off

Oxidative system (s) – The oxidative pathways can be trained from both long endurance style training, however HIIT training has also been shown to increase aerobic capacity despite the activity being supramaximal in nature. However, this will be discussed in greater detail in another article. For the purpose of this article, we will look at traditional “endurance” training.

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 utilise 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 for performance, 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 HIIT training, 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 for a triathlete week may look as follows;

DayExerciseSetsRepetitionsTime / DistanceRecovery period
MondaySwim (Focus on pacing)315 lengthsAim for each set to be within ±10s of the previous.3 mins
WednesdayRun (Focus on pacing)42km runsAim for each set to be within ±20s of the previous.4 minutes
SaturdayRun & Swim (Focus on effort)1 each1 eachSwim 25 lengths (1.25km) as fast as possible. Run 10km as fast as possible30-60 minutes between sessions. Refuel with exogenous carbohydrates between


I hope this article has been clear enough in the explanation of the energy systems and how they 1) are utilised in sports & 2) how they can be trained. As a rule of thumb, to train a specific energy system you 1) need to train for as long as the energy system is the primary fuel source (e.g 10s sprint for ATP-PCr) & 2) allow adequate rest to allow this energy system to recover. If you have any questions, then do not hesitate to get in touch via email or on the social media DM’s.

Until next time and as always, stay strong.


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