Energy Systems

This topic may well sound boring. Why should you bother to read it? Because it is the basis of why you do the volume, frequency and type of training that you do. A basic understanding can also help you determine what is the wrong sort of training for your objectives.

While not strictly scientifically accurate, it’s helpful and relevant for a runner to think of having three sources of energy available to them.  As endurance athletes (i.e. we are not sprinters) we use all three energy systems throughout a run, to a lesser or greater extent, so we should not ignore any of them. We cannot decide on which one we use, but we can train our bodies to enhance how much energy we can get from them.

Also, when we don’t train our bodies to deal with the ‘Lactic Acid’ produced from the lactic energy system, it will hinder us rather than help.

Here is a summary of each type of energy system.

Common name


Rate of Energy Supply


Alactic (Anaerobic)


Very fast

3-15 secs

Lactic (Anaerobic)



15 secs to 3 min


Carbohydrates and Fat

(combined with oxygen)


Over 3 min

The duration of each energy system in the above table is a guide. In reality, we will have use of and will use all three systems throughout a long run, but the amount of alactic and lactic energy we use will be relatively small, though significant.

The alactic energy system is what helps you to immediately react. A high jumper or a sprinter is specifically interested in enhancing that energy system. But after a very short time (seconds) it is exhausted. After some rest or a longish period of activity the store of phosphocreatine builds up again ready to go. So that why a 100m runner could compete in both heat’s and final in the same day. You might use alactic energy in the final sprint for the finish, or for a short burst up a steep hill. Oxygen is not required – anaerobic.

The lactic energy system is more complicated for a runner to deal with, because as lactic energy is used, a waste product commonly (but incorrectly) referred to as lactic acid builds up and it is the body’s ability to deal with that waste product that affects the level of fatigue that feels like “heavy legs” or “hitting the wall’. Tempo runs are a means to improve our ability to effectively use the lactic energy system. You can be sure that you will use the lactic energy system and create the waste throughout a run – no choice. Oxygen is not required – anaerobic.

The aerobic energy system will always be the main energy source, by far, when the distance is over about 1500m. The source of the energy is carbohydrate and fat stored in your body, but it needs to be combined with oxygen from your blood supply to release energy, using glycogen. To get more oxygen you will breathe harder.

The Implications

On a very slow jog, over 90% of the energy used will come from your aerobic system. Little energy is required and your body has enough time to allow oxygen to break down the carbohydrate and fat to create aerobic energy.

But in contrast (to illustrate the point) on a 5k race, energy is needed more rapidly and so only about 75% will come from aerobic, most of the rest being lactic. As the waste product from lactate builds, the duration that the 5km speed can be maintained is restricted, which you do not want.  

Ideally you want to do two things better:-

  1. Deal with the waste product created by the use of the lactic energy system more effectively, called increasing the Lactate Threshold, so it takes longer for the waste products to build up and slow you down. Interval Training is used to increase your lactate threshold.


2. Make more use of the aerobic energy system (because it does not create waste products that hinder you) by enabling more oxygen to get into your bloodstream, called increasing  VO2 max*, and to make best use of that oxygen when it’s there called Increasing Endurance.

You will find separate sets of notes that will explain more about the Lactic Threshold, also  how we can make better use of the lactic and aerobic energy systems with Lactate Threshold (Tempo) Runs, VO2 max (Interval) runs and Long Runs for Increasing Endurance.

  • VO2 max – a physiological measure of maximum oxygen uptake. The highest amount of oxygen you can breathe in, transport to your muscles and change into energy to create movement. Hard to measure outside of a lab, but the principles to enhance it are relatively well understood.

Graeme Loudain – September 2019

These notes are published as a general aid to all endurance runners. They are guidelines based on current best practice, intended to illustrate the points in the articles rather than provide a precise training schedule. They are written in a way to be of help to runners. Individuals will have their own requirements and the content of the notes will not suit all. Please use them in the spirit in which they are intended and modify them to suit your own needs. If you have suggestions for improvement, amendments or comment, please email