Energy Systems: aka how we Power Performance …

We eat to fuel our body for work. For any fitness enthusiast and athlete this work includes training and performing in their chosen sports. In this post I focus on the three different energy systems the body has to power your workouts, which systems dominate in different types of workout, and the implications of this for an athletes diet.

The energy systems …

The three systems we possess to power our performance are summarised below. Even from this simple graph it should be clear that there are fundamental differences between them that will impact how much we rely on them in different activities and sports.

The phosphagen pathway (blue in the graph above) is our rapid response team! It reacts almost instantaneously to a large increase in demand for energy, such as an explosive tuck jump, or heavy power lift. It runs off the phosphocreatine stored in our muscle cells; when the phosphocreatine (PCr) is broken down to phosphate and creatine, a huge burst of energy is released that is used to produce adenosine triphosphate (ATP), which is the energy currency of our cells, i.e. what powers muscle contraction. But there is only a limited amount of phosphocreatine stored in our muscle cells and so this source of energy is short lived – the pathway is typically exhausted within 10 seconds of work.

The glycolytic pathway is our next responder. It kicks in relatively quickly (seconds) and powers high intensity movement for up to around 3 minutes. The limiting factor here is typically not the exhaustion of the fuel (glucose), but the build-up of acidic hydrogen ions in the muscle that are the waste product of this pathway. This is what causes that lovely burn feeling you accumulate in an all out 400m sprint, tabata or equivalent!

The oxidative pathway is our long haul energy pathway. The rate of energy production is limited by the rate at which oxygen can be supplied to the muscle and so it typically produces energy at a slower rate than the phosphagen and glycolytic pathways. However, it is the least fussy pathway and can use glucose, fatty acids, and amino acids to produce ATP … meaning it can produce a consistent energy output for a long period of time, powering your marathon, ironman, or similar!

Feeding to fuel the energy systems …

Apart from – obviously – training the energy systems appropriately, athletes in different sporting disciplines should consider how their nutrition supports the energy systems they are going to want to rely on in training and competition.

I will leave the super high intensity explosive athlete powered by the phosphagen pathway until last, and focus first on the use of the glycolytic and oxidative pathways. Both the glycolytic and oxidative pathways run off carbohydrates (glucose). The oxidative pathway can also run off fats (fatty acids) and protein (amino acids); it should be noted that running off protein is a stressed state that occurs when other fuels are exhausted, typically involving breaking down muscle protein to release amino acids … aka not desirable!!

The higher the exercise intensity the greater the reliance on carbohydrates as a fuel in the oxidative as well as glycolytic pathway. In lower intensity (<50% max heart rate) and longer duration (>60 min) exercise, the oxidative pathway is the primary energy source and relies on fat as the primary fuel. Whilst fat has the capacity to release more energy per molecule than carbohydrates, the process of converting stored fat to fuel for muscle cells takes time and oxygen. This means it is typically an effective fuel source only for lower intensity activity. I say ‘typically’ because training and certain dietary protocols enhance the efficiency with which we can utilize fats to fuel exercise – although carbohydrates remain the default fuel for high intensity exercise.

What does this mean for an athlete’s nutrition?

It means that if your sport requires sustained high intensity, you want to ensure you have the carbohydrates available to power this; the more stored glycogen you have at the start of their exercise, the longer you will be able to perform at the highest intensity. Typically, we have the potential to store enough glycogen in our liver and muscles to sustain moderate to high intensity exercise for up to 2 hours. So, how do you maximize your glycogen stores?

First, in the 2-4 hours before exercising consume 2-4g/kg bodyweight of low glycemic index (i.e. slow digesting) carbohydrates. This gap between eating and exercising is necessary to allow time for digestion, release of blood glucose, and generation of glycogen from the consumed carbohydrates. It also reduces the risk of rebound hypoglycemia (low blood sugar levels) occurring because of the combined impact of muscles removing glucose from the blood to power the exercise and insulin released in response to feeding reducing blood glucose levels.

Second, within 60 minutes of completing exercise consume 1g/kg bodyweight of carbohydrate. This is to take advantage of the phenomenon of glycogen super compensation whereby glycogen levels are replenished to above pre-exercise levels if carbohydrates are eaten within a short enough timeframe after exercise ceases i.e. when the enzymes for glycogen synthesis are still hyperactive. The specific timeframe varies from individual to individual and is influenced by gender, age, genetics, training and physiological state – however 60 minutes appears to be a short enough window for most individuals to successfully load glycogen levels. And more glycogen means a greater ability to exercise at high intensity in your next bout of exercise!

The final consideration is feeding during exercise. In endurance exercise lasting more than 60 minutes, i.e. where there is the potential that glycogen stores will be exhausted, you are recommended to consume up to 70g of high glycemic index carbohydrates (i.e. sugar) per hour. This helps provide a constant source of blood glucose for the muscle cells to use as fuel.

At this point I can potentially see hands in the air asking ‘What about athletes on a ketogenic diet? They can be successful’. The ketogenic diet for those who are not sure is a very low carbohydrate diet such that the body runs principally off ketone bodies rather than glucose. Inducing this state typically requires consumption of less than 50g carbohydrates per day. On such a diet individuals do still have glycogen stores and circulating blood glucose, however these are much lower than on a higher carbohydrate diet. There is evidence that in appropriately trained individuals on a ketogenic diet fat metabolism becomes more efficient to help maintain higher intensity exercise. However, it is also clear that there are genetic differences in the ability to adapt in this way. There are many nuances and subtleties in athletic performance when in ketosis that are beyond the realms of this particular post – but I will come back and discuss these and more in a later post.

Okay, so having addressed the glycolytic and oxidative pathways let us now consider fueling the phosphagen pathway for short high intensity explosive movement.

Although creatine is present in animal protein in the diet, much is typically broken down in the cooking process; most of our creatine we make ourselves, in the liver. Despite being able to make it ourselves, it seems that even if we do train regularly our muscle cells are never naturally at full capacity, i.e. they could hold more (see Kreider et al 2017). Supplementing with creatine is a recognised ergogenic aid (enhances athletic performance) as numerous studies have shown that by supplementing, you increase the amount of creatine and phosphocreatine in muscle cells, and the energy you can produce by this pathway. This post does not discuss the evidence behind the studies or the potential pro’s and con’s of supplementing with creatine. For further information, see the resources in the ‘Read more’ session of this post. Over the coming months I will also be addressing various popular supplements in this blog – so check back later for further discussion!

In summary …

Different sporting disciplines rely differentially on the different energy systems of the body. Appropriate training strengthens the efficiency, capacity and efficacy of the relevant energy systems. Appropriate nutrition can support this, through helping ensure the appropriate substrates are available for use by the body in training and competition!

Read more …

McArdle, W, Katch, F, Katch, V. Exercise Physiology, Energy, Nutrition and Human Performance. 8th Edition. Wolters Kluwer Health, Philadelphia, USA. 2015.

Wilmore, JH, Costill, DL. Physiology of Sport and Exercise. 3rd Edition. Human Kinetics Publishing. 2005.


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