Energy Systems used in Triathlon

April 10th 2020

By Matt Hill
Categories: Triathlon Training

Energy Systems used in Triathlon

The more knowledgeable we become the more confusing everything is – especially when it comes to energy systems! With so many studies into biology, physiology, biomechanics, etc, you’d have thought that we’d have more conclusive and concrete conclusions. However, it’s just not the case.

This is explained well by Christie Aschwanden, a science journalist, in her recent book (Good to Go) and podcast/interview with Ross Tucker, Professor of Exercise Physiology with the School of Medicine of the University of the Free State, S Africa. Sport science studies are often poorly constructed and are not replicable outside the “conditions” required for the study. This is due to the many varying factors in the real world (sleep, nutrition, etc). Additionally, the vast majority of studies are biased towards the objective – to find the “right” answer for marketing purposes. Why? Because the studies are funded by those with a vested interest in showing positive results!

To a large extent, this carries across into the numerous theories of training the human energy systems, in spite of the science behind them being quite solid. At a simple level, pretty much everyone knows that there is the aerobic and the anaerobic energy system, but it is even at this level that the initial errors are made. It is generally assumed that the aerobic system uses oxygen while the anaerobic system doesn’t, and training regimes promoted accordingly.

But what is actually going on and what sub-factors come into play?

Well, there are three areas that we need to address to explain what is happening in the body when we move, train or race.

  1. Mitochondria

Mitochondria are the “power houses” of energy production in the body; they specialise in efficiently creating adenosine triphosphate (ATP) from fatty acids (fat), glucose (sugar) or protein (as a last resort). ATP is the energy “currency” the body uses for creating ANY task.

The mitochondria adapt what they do best, that is to oxidise fat/glucose or protein, to the rate of  energy demand; at low intensity, or aerobic levels fat oxidation is by far the most efficient means of providing energy. Increase the intensity and the body will need to find energy from other means.

Mitochondria are located in most cells in the human body, with high concentrations in slow twitch muscle fibres; the fibres associated with endurance.

  1. Muscles fibre types

Before delving into the energy systems, we first need to discuss the types of muscle fibres and how they require energy.

Muscle fibre types are known as:

  • Slow twitch (Type I) muscle fibres – contract slowly, are less powerful but are relatively slow to fatigue. They are able to sustain power output for a long time; these are the endurance muscles. These muscle fibres are densely populated with mitochondria, and more can be “grown” by training in the low intensity training zone
  • Fast twitch muscle fibres
    • Type IIA – have high oxidative capacity (relatively large number of mitochondria) and glycolytic capacity and fatigue relatively slowly. They are partially adaptable and can be trained to be more like Type I or Type IIB fibres, depending on your sporting needs
    • Type IIB – have low oxidative capacity (low number of mitochondria) and high glycolytic capacity and fatigue quickly (and are recruited for anaerobic sprints / lifting sports)


Note that “slow twitch” and “fast twitch” refers to the speed in which they fatigue, not the speed that they contract.

As Dr Iñigo San Millán, Ph.D. presents, the different energy systems exist to directly supply the energy needs of the type of muscle fibre being recruited. Low intensity activities (as in triathlon) predominantly use Type 1 muscle fibres and so the need for energy is constant and at a steady, slow rate. Whereas for sprinting or weightlifting, the need for energy is rapid but for short periods.

The human body has evolved to efficiently provide energy to meet the demands of the muscle types being used, and can do so simultaneously where Type I and Type IIA, and even IIB (for short periods) muscle fibres are used.

  1. The energy systems

In essence, training for any sport should be aimed at developing the efficiency of your energy (supply) systems and movement efficiency. The key is to develop the energy systems most appropriate to the sport you do; combining the right mix of training intensities to give you the most “bang for your buck”.

The human body has evolved a number of energy systems that allow it to perform activities at varying intensities from short hard sprints through to long steady efforts.

Whilst all of these energy systems are always working (more on this later), there will always be one that is dominant. Your training priority should therefore focus on the energy system that is most used. Triathlon in this case.

So, what are these energy systems, and which one is appropriate for a triathlete?

The Aerobic System

This is the key endurance energy system – applicable to all activities lasting longer than 2-3min.

In order to develop this system, the training intensity MUST be controlled and kept below 75% of maximum heart rate (MHR). Below 75% MHR is low enough for the body to create energy with oxygen. At this low intensity, the body prioritises oxidising fatty acids in the mitochondria within Type I and Type IIA muscle fibres to yield large amounts of ATP (around 120 ATP molecules).

It is important to note that training at 75% MHR is NOT necessarily going to provide most benefits. Working at intensities down to 60% MHR have show to provide similar benefits.

This energy system allows us to enter a fat-adapted state, simply due to the increased ability to burn fat. Fatty acids produces significantly more ATP per molecule than glucose and proteins.

Aerobic training (and racing) intensity subjects the body to (relatively) low stress; the impact on the body’s structure (muscles, ligaments, tendons and bones) and hormonal response are low.

The Glycolytic System

This is the short to medium term energy system that applies to activities lasting between approximately 10sec through to 3minutes. 50m – 100m swim events, short track cycling and 200m to ~1500m track running are examples of the sports using this energy system.

This energy system also uses Type I and Type IIA muscle fibres, but due to the increased requirement for energy, mitochondria are no longer the primary producer of energy.

Glycolysis starts to take over; this literally means the breakdown (lysis – Greek for “a losing”) of glucose through a series of enzymatic reactions, not through oxidation. Glycolysis occurs within the cytoplasm of the muscle cells (cytoplasm = everything within the cell membrane excluding the nuclei and mitochondria).

Glycolysis can occur aerobically (just below anaerobic threshold level) and anaerobically (above threshold level):

  • Aerobic Glycolysis occurs when there is plenty of oxygen available. The outcome of the enzymatic reaction is pyruvate along with 8 ATP molecules.
  • Anaerobic Glycolysis occurs when there is little oxygen available or the intensity is too high for the body to create energy quickly enough. An increased amount of lactate is produced as a by-product, along with 2 ATP molecules.

The anaerobic glycolytic system produces ATP from glucose much faster than the aerobic system or aerobic glycolytic system. Its down side is the limited glucose availability and not being “efficient” (producing only 2 ATP molecules). There is approximately 2000kcal worth being stored in muscles and the liver.

Feeling the burn?

This system causes the “burn” which was originally thought to be caused by lactate: However, a recent  study has shown it to be caused by the extra hydrogen ions that are released from the breakdown of ATP occurring outside of mitochondrial respiration (basically, above aerobic intensity / >75%MHR). Lactate actual slows the onset of the burn sensation!

It is not possible to use this system for long periods of time. Attempting to use it for longer than 3minutes WILL result in a significant drop of speed to allow for recovery.

In triathlon, this system will be used in short bursts for running into T1, efforts sprinting out of corners, a “kick” at the end of a race but should be limited.

Very often, many endurance athletes spend far too much time training this system, hindering aerobic and strength development.

 The ATP-PC System

The energy system used for extremely short events where Type IIB muscle fibres are responsible for fast and forceful contractions. Sometimes known as the “fight or flight” system – it’s the system we’d use to instantly fight or escape from danger! Powerlifting, Olympic lifting and Strongman, 100m sprints, throwing sports and golf swings use this energy system.

The body uses ATP that is stored directly in the Type IIB muscle fibres to create movement. Under normal resting condition, there is only enough stored in the muscles to perform around 2-3seconds of work. If activity exceeds 3 seconds, ATP is resynthesised with phosphocreatine (PC), extending the working duration to 8-10 seconds.

This energy system doesn’t require oxygen or produce lactate as a by-product but does require extensive rest to restock levels (VERY low intensity or complete rest for at least 2minutes).

All sports use all energy systems

All sports recruit all energy systems at various points. Endurance events for example will use the ATP-PC system in the first few seconds as the body gets moving.

Effective management of effort during training and the accumulation of training dictates how the body develops. The more suitable the training the better the race day performance will be. Table 1 indicates the relative use of the different energy systems for different sports.

Table 1: Energy system utilisation

As we’ve discussed many times, all distances of triathlon are aerobic / endurance events. Therefore, it is essential to prioritise the development of the Aerobic System.

The main objective for endurance athletes should therefore be to enhance the aerobic systems. The more oxygen that is taken on board by the blood and delivered to the working muscles, the more ATP can be produced per fat / glucose / protein molecule in the mitochondria and the longer you will be able to continue.

Adaptations to aerobic training

To maximise your body’s aerobic system, you need to train at an intensity that allows the system to prioritise oxidising fat, which can only be achieved when the intensity is low (<75% MHR) or EASY!

This low intensity training is very beneficial in many ways, and the physiological changes, or adaptations, that occur, include:

  • Enhanced heart functionality – training it (like any muscle) to grow the musculature, enlarge the chambers (enhancing stroke volume), improve its elasticity and contractibility, etc
  • Improved and increased blood vessels (capillaries) surrounding the working muscles; more capillaries surrounding the muscles means the muscle fibres and cells receive more oxygen
  • Increases the blood haematocrit (the ratio of red blood cell volume to total blood volume) and reduces blood viscosity (to enable easier blood flow); there are more red blood cells to carry more oxygen to the muscles with less resistance in the blood vessels
  • Increase the efficiency of the alveoli in the lungs allowing better delivery of oxygen to the blood. Note, however, that lung capacity cannot be enlarged
  • Increase the function and number of mitochondria in the muscle’s cells; these are the “power houses” or factories in the body that create ATP for all body functions. More along with more efficient mitochondria = more performance.
  • Fat adaptation – training your body to use fat as its primary energy source, oxidising fat rather than glucose – for more ATP production per molecule of fuel source.

Training above ~75% of maximum heart rate will stunt your body’s ability to improve all of these factors. Therefore, training MUST be sufficiently easy to allow the body to make the changes.

The combination of these aerobic benefits all contributes to an improved threshold levels, VO2Max and race day performance.


The human body has evolved to provide energy to the working muscle fibre types. Simply, for endurance athletes, who partake in longer durations at lower intensities, the aerobic system is the one to train and use.

Triathlon is an aerobic sport, and this means triathletes need to prioritise training their aerobic system to become more efficient.

At the lowest level, this mean increasing the number of mitochondria in the slow twitch muscles, enhancing the body’s ability to remove by-products of the energy production, increases the number of red blood cells, enlarges the heart for more pumping power, increases the number, and efficiency of blood vessels, and the ability of muscles to use this energy.

These adaptations can ONLY occur when activity levels are below 75% of maximum heart rate. Above this intensity your body uses a different energy system and elicits different physiological adaptations; that are not suited to triathlon.

Keep it simple and keep it EASY

In our next article, we will discuss race pace training for endurance athletes. We’ll delve into how the body can benefit from cleverly but SIMPLE higher intensity aerobic training while explaining why too much of it and at too high an intensity WILL limit your chances of actually enhancing your race pace!