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Understanding Energy Systems

Energy Systems & Endurance Sport

Introduction to how our energy systems work

This post serves as a brief introduction to an aspect of how the body produces muscular power, but specifically looking at the traits of the three energy systems that convert different “fuels” into “energy”.

Key Points:

  • Our body has three different energy systems
  • They are all active simultaneously
  • Only one energy system is dominant at any one time
  • They each have pros and cons so learning to adapt and manage them is key to improving our sport performance.

By way of introduction, our body uses three different ‘energy systems’ to covert types of ‘fuel’ into energy (broadly power). Each system is distinct in terms of the fuel it uses, it’s potential for creating energy and the speed at which it can deliver that energy. All three systems are active simultaneously, but depending on the energy demand from our muscles at a given moment, only one system will be dominant. However, it is important to appreciate that total energy output is delivered from a mix of the three systems, even if one is providing 99%. 

Our three energy systems in brief:

  • ATP – CP (adenosine triphosphate) provides an immediate source of energy, but exhausts itself after only about 10seconds or so (limited by the availability of ATP which it metabolises to create energy). Because it generates energy quickly, it is perfect for a quick response to a threat (powering fight or flight). In terms of fitness activities, we use our ATP system for plyometric (or explosive) actions; such as a standing jump up on to a box, Olympic lifts in the weight room, or the launching from the starting blocks at a track event. 
  • Lactic / Glycolytic system provides short term energy but typically will be exhausted after maybe 1 to 2 minutes (depending on the athlete). We would use our Lactic energy system during a short sprint activity, where we need a relatively large amount of power for a relatively short period of time. Considered an inefficient system that metabolises glycogen but crucially without the need for oxygen. The main disadvantage of the Lactic System is that it produces Lactate as a by-product in the muscles which then eventually starts to overflow into the bloodstream. Dependent on training, the human body cannot tolerate much Lactate in the blood and will protect itself by reducing power output such that a different energy system can be used and thus stopping of additional Lactate entering the bloodstream. The main limiting factor to the Lactic system is the individuals ability to tolerate Lactate and specifically the point at which more Lactate is being produced per second than the body can clear.

Both of these two energy systems are referred to as Anaerobic Energy systems in that they produce energy from fuel WITHOUT the need for oxygen. Anaerobic systems are capable of metabolising fuel into energy quickly, which in sport performance terms means that muscles can produce a high power output.

Two big draw backs (1) Both Anaerobic Energy systems are short lived (you are not going to run a fast 5k relying on them). (2) Anaerobic Energy systems (specifically the Lactic system) produces a by-product, Lactate Acid. You may have felt a burning sensation in your muscles when working at higher intensity? This is caused by Lactate Acid starting to be produced in your muscles. At that point you have invoked your Anaerobic Energy System to a material extent. 

As an endurance athlete you are likely to want a sustained performance for longer that two minutes! Thus you will mostly be relying upon the third energy system.

  • The Oxidative (or Aerobic) energy system. This is considered a long term energy system in that it will continue to function indefinitely, albeit that the amount of energy (read power) that it can produce will decrease at some point as muscles become fatigued. The Oxidative system initially metabolises carbohydrate, which offers a high energy potential but is relatively time consuming to metabolise. After carbohydrate, body fat will be used, this offers an even greater energy potential but takes longer to release than that from carbohydrates. Carbohydrate and Fat are plentiful fuel sources and the main by-products are carbon dioxide and water, which is why the Oxidative System can continue to run indefinitely. The limiting factor however is the amount of energy (power) it can produce per minute, and this is constrained by the amount of oxygen available.

The living human has all of these energy systems active 24 / 7. Even when we sleep there is a basic requirement for energy (your basal metabolic rate) that enables your nervous and other life supporting systems to function.

As we start to exercise and become active there is additional demand for energy from our muscles and our Aerobic energy system comes under greater stress to service that demand.

If we continue to increase the intensity of the exercise (run faster / cycle quicker / increase the gradient etc.) eventually our Aerobic system will cease to be dominant and we will start to use our Anaerobic energy systems to meet that demand. Why?

The VO2max Threshold

As you now know, the Aerobic Energy system needs the presence of Oxygen within the cells of the muscle to produce energy (power). The greater the energy demand from the muscle the more Oxygen is needed.

But our ability to deliver Oxygen is limited by some physiological factors; the capacity of our lungs, the amount of red blood cells in our blood for oxygen to attach to, the size of our heart which governs how much blood it can move per beat. The strength of the heart muscle itself will also determine how much blood it can be move. Finally the amount of capillaries around our muscles (capillarisation) will play a part in determining the volume of oxygenated blood available to the working muscle. Some of these factors can be improved over time (physiological adaption to aerobic training stimulus), whilst others are simply a result of our genetics and “we are what we are”! 

A useful marker in Endurance Training is VO2Max. This describes the maximum amount of oxygen that an individual will have available for muscles to utilize and therefore describes aerobic capacity or cardiovascular fitness. It is expressed in millilitres of oxygen consumed in one minute per kilogram of bodyweight (ml / KG / min). As muscular energy demand requires more oxygen than is available the anaerobic energy system (lactic) starts to take over and becomes the dominant energy system. This cross over point is known as our VO2Max Threshold and will be discussed in more detail when I cover Coaching Interventions in a future blog post.

The Lactate Threshold

The Anaerobic energy system will continue to provide energy (at a faster rate than the aerobic system) so for a short time performance will increase. But this is only for a short time. The Anaerobic System will produce Lactate as a by-product. This initially starts to accumulate in the muscle tissue and leads to the sensation of muscle burn. During the early stage of Anaerobic energy production the body can clear the Lactate by using it as a fuel. If we remain in the Anaerobic “zone” however, the Lactate will start to overflow into our bloodstream, and that becomes a limiting factor. Our bodies cannot tolerate a build-up of Lactate in blood and as such will protect itself by reducing the energy being produced to a level that can be met by the aerobic system, as it once again takes over dominance. The Liver works to clean the blood of Lactate. Once a safe level is reached (and assuming the further availability of glycogen as a fuel) then the Anaerobic system is said to have recovered and becomes available again. 

The point at which our Anaerobic Energy system is shut down, due to a high blood Lactate level, is known as our Anaerobic Threshold. This is a second key marker that is important to understand in developing Endurance Sport Fitness as it is very much linked to performance.

Energy Systems over time

The above diagram is a common visual representation of the relationship between our three energy systems. However, this misses the context of what energy demand the body is attempting to service. And certainly, this does not represent most Endurance Sports events in the real world, other than perhaps a snapshot close to the start! 

What is missing from this representation is that in the real world (during sport) there is much more ebb and flow between the three systems. They recharge at varying rates and work to meet the power demands of the event.

Where Next?

Knowing how our energy systems work is useful background for the Endurance Athlete. Similarly, being very clear about what our VO2max and Lactate thresholds actually mean is also important. But the next step is to adapt our energy systems to help us perform better and reach our goals. That is the purpose of a training plan and the personalised training sessions within it. In a future blog I will discuss “coaching interventions” which are the mechanics of how we do this.

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Ed Stivala

Coach & Founder

Founder BitFitter, an innovative easy to use Training App. 
L3 Personal Trainer, British Cycling Coach, England Athletic Run Coach & Swim England L2 Instructor
When not Coaching, he competes in Duathlon and plays golf very badly!