Energy Systems. Taking a look under the hood.

While undertaking a sporting activity seems like a simple process, what’s going on “under the hood” is anything but! Our bodies use a diverse number of pathways to deliver energy to the working muscle to make us pedal harder, run up that steep incline or make that last sprint for the line.

These pathways can be referred to as energy systems and the energy they set about creating is known as adenosine triphosphate or ATP for short… a collective groan as we remember leaving cert biology and some terrible memories come haunting back! But don’t worry, we’re not going to dive in-depth into the physiology for this post.

The energy system we tap into for sporting activity depends directly on the intensity and duration of that sport. Essentially, we have 3 to choose from:

  1. The anaerobic alactic or ATP-PCr system.
  2. The anaerobic glycolytic system.
  3. The aerobic oxidative energy system.

All sound a bit complicated? Essentially we can break these up into 3 distinct domains: sprinting, intense exercise lasting up to 60 seconds and prolonged exercise lasting over 60 seconds. Before we get wrapped up in the which does what though it is worth acknowledging that these energy systems and their corresponding sporting endeavours all lie on a continuum where overlap often occurs. See the graph below for a simple visual on the contribution of each energy system over time during maximal exercise. Important to keep in mind this is an example of the energy systems used in a continuous maximal exercise, like running 1km as fast as possible. Most of the time, the ATP-PCr system is not just used once, but is used in the initial stages when we are fully recovered, so in the example of a football match, when we get a chance to recover, this energy system can be replenished and used again.

1) The anaerobic alactic system or ATP-PCr

This energy system produces energy for all exercise lasting up to 10 seconds or so depending on the athlete. When we refer to this system think track sprinting for cycling, chasing a competitor on the pitch for that last-minute block or sprinting 100M. This system is also targeted in explosive weightlifting movements such as those seen in Olympic Weightlifting. These exercise modes all have one thing in common – invoking maximum strength and power for a short duration.

2) The anaerobic glycolytic system

This energy system produces energy for exercise lasting up to 60 seconds, again this is dependant on the athlete. Here, we can think of sports such as 400M running, track cycling or multi sprint work such as that involved in field sport.

3) The aerobic energy system

This energy system produces energy for those exercise types lasting over the limit of maximum aerobic glycolysis. So, if we take the above example, any exercise lasting over 60 seconds. This system has a huge range in terms of time when we look at sports such as Ironman Triathlon versus an easy 20-minute jog. One stark difference with the aerobic energy system is it is not solely reliant on carbohydrate for fuel like the previous two mentioned, rather, the less intense the aerobic exercise is the more we rely on fat as a primary fuel source which is much more abundant in even the leanest of athletes!

Making sense of it

Most sports contain some degree of all these energy systems within their remit. What’s important for us as coaches and competitors is we understand the predominant energy system within our sport and spend time training it, using the other pathways as a backup or foundation to facilitate performance. See the tables below for a sport by sport breakdown of the contribution each energy system makes.

For those sports which involve a diverse crossover in all these systems understanding the complex way in which they interlink, when and where to plan in your season and how to play to your strengths is equally as important and a topic in it’s own right! But, for now, I hope you enjoyed this short insight into what, exactly, is going on “under the hood” while we exercise.



Jamie is an endurance specialist and head coach for Premier Endurance, which can be found at

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