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  • Writer's pictureAlexa

Energy Systems

When we exercise our cells burn a chemical called ATP (adenosine triphosphate), there is very little of this in each cell, so they are used up very fast (in a second or less) when we exercise so need to be replenished quickly. There are three (or four depending on how you count!) energy systems that our body uses to do this. These systems also tie in with how we train and pace ourselves.

1. ATP-PC System (Alatic System)

2. Glycolytic System (Lactate System)

3. Oxidative System (Aerobic System)

ATP-PC System (Alatic System)

This system lasts for 6-10 seconds, so it’s used for explosive, high power sports like sprinting and weightlifting. It’s a simple process that allow for the muscles to react and contract fast. After the 6-10 seconds is up you’ll notice performance and power drop. The system refills back to 85% capacity in around 3 minutes - so that’s why the recoveries need to be longer for sprint training!

Glycolytic System (Lactate System)

This system gets it’s name from the chemical glycogen that’s circulating in our blood stream and stored in our liver and muscles. Glycogen is created from glucose so carbohydrates are needed to create it efficiently. For exercise from 10-90 seconds in length with are pretty much purely using this system for fuel, so for fast effort we need to be well fuelled with carbohydrates to get the best performance out of the sessions.

During the process of burning glycogen for fuel lactate and hydrogen ions are produced. It’s actually the hydrogen ions that make the blood more acidic and produce that muscle burn sensation. Lactate is a useful part of the fuelling system and got the blame for this wrongly in the past!

Oxidative System (Aerobic System)

This system gets it’s name because it uses oxygen as part of the process to produce ATP. There are a couple of mechanisms it uses, one is the Kreb’s cycle which you may remember from school science?! These processes can’t produce ATP as quickly so this is the fuel system for slower, endurance training and it’s why doing long runs slow enough is key. This system can use carbohydrates, fat and protein as fuel, but fats and protein take longer to break down than carbs. Training your body to burn more fat by taking these sessions more slowly is a great way to manage fuelling on longer runs, especially when training for endurance events like the half marathon, full marathon and beyond.


These three energy systems work together as a team, very rarely are we only using one. We do want to be aware of planning training and executing sessions in the right zones, so we are working on the right aspect of our fitness in each run. For example it’s important to run your long runs slow enough so you are fuelling yourself aerobically, and also slow enough to allow the body to be using more body fat as fuel. One of the most common training mistakes I see in runners is doing their aerobic and long runs too fast!

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