Energy Systems

As you are consistently running more than 1500m, you are an endurance runner. If you want to improve your running (further, faster, easier) then your focus should be on the development of the energy system that is most important to endurance. This is the aerobic energy system, aerobic meaning ‘requires oxygen’.

The other two main energy systems are anaerobic, ‘not requiring oxygen’.  The table below shows the source of energy, either aerobic or anaerobic for competition runs of various times.

Running Time (Mins)

% Anaerobic

%    Aerobic



















120 and +



It illustrates how important aerobic energy is to running in a competition of 10 or more minutes, which covers all Striders events.

The good news is that all of us have loads of fuel to create aerobic energy stored in our body, it is called fat. The less good news is that few of us are good at using it efficiently, so we get out of breath much sooner than we want to.

Since we were born, training is largely based on repetition. We repeat the same things until we can do them without thinking. That applies to both mental skills such as reading or counting as well as physical skills, like walking or running.

So there is more good news! Just like learning to walk all we have to do to develop the effectiveness of our aerobic energy system is to practice using it in the way we want it to work. If we want to run for a long way without getting out of breath, then our practice sessions are to run slowly enough so that we don’t get out of breath. When we start breathing more heavily, we are going too quickly. Simple, but actually surprisingly difficult to do. More on this later.

First we need to understand a little more about our three fundamental energy systems so we can better appreciate why we do what we do. Every one of us has these three energy sources available in our bodies, where we differ is how efficient our bodies are at using them. So what are they, how much do we have of them and what can they do?


Common name

Needs oxygen?


Can last for how long?

Rate of supply





~6 seconds

Very fast




Sugar called Glycogen

(i.e. a carbohydrate)

~ 2 mins





Mostly fat and some Glycogen



All three energy systems are working all the time when we are running, to a lesser or greater extent. The lactic and aerobic energy systems also have an important effect on each other.

The alactic energy system is what helps you to immediately react. A high jumper or a sprinter is specifically interested in enhancing this energy system. But after a very short time (seconds) it is exhausted, in fact a 100m sprinter uses up all of their alactic energy half way down the straight.  After some hours of rest or a longish period of low activity the store of phosphocreatine builds up again. So that is why a 100m runner can compete in both a heat and a 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. If the only energy system you used was this one, it would last for less than 8 seconds.

When you are running (say) a half marathon, if you have trained your body to use your aerobic energy system well, then you will not have exhausted your alactic energy supply and can use it for your final 50m sprint at the end.

The lactic energy system can be called upon fairly quickly when you exercise and is based on converting glycogen (carbohydrate/sugar) stored in the muscles to lactate energy and a waste product called lactic acid. If you are running quickly, you will start using lactic energy immediately. While you are running slowly (in fact even while you are standing still) your body is using lactate energy but at a very low level, but nevertheless it is doing so and using up your stores of glycogen. This is an important point as your stores of gycogen are also needed to create aerobic energy. 

Using lactic energy can be more complicated for a runner to deal with, because the lactic acid that is created does the opposite to providing energy. Your body does deal with lactic acid, recycling it at a rate determined by how well you have trained it to do so. It is the body’s ability to deal with lactic acid that affects the level of fatigue that feels like “heavy legs” or “hitting the wall’. Once the amount of lactic acid in the blood system exceeds the amount the body can deal with, legs become heavy and very hard to move. Recovery takes a relatively long time so once you reach that point in a race, your race is almost over.

The aerobic energy system will always be your main energy source on a run, by far, when the distance is over about 1500m. The fuel source of aerobic energy is the fat stored in your body. To convert your body fat to aerobic energy requires oxygen (i.e. breathing) and a little glycogen that has been stored in your muscles thyat is also being used by your lactic energy system.

This energy source does not create energy quickly, so a 100m sprinter uses almost none. It does not get time to switch on! When your body starts to use aerobic energy for running, your breathing will pick up a little and you will use a little glycogen as a catalyst. Run faster and your breathing will get heavier as more oxygen is needed to create more aerobic energy and you will use a little more glycogen. Run faster still and keep going and you will breathe very heavily to get in as much oxygen as you can, while your body extracts glycogen from your muscles to convert your fat to aerobic energy.

So if your aerobic energy system can give you energy for many hours, why do we get tired so quickly when running very fast? The answer is that by running quickly we have switched on our lactic energy system as well, which has used up all of the glycogen available in our muscles and also created more lactic acid than our bodies can deal with. Without glycogen, our bodies cannot convert fat into aerobic energy.

What can an endurance runner do about that? 

The Implications

  • Train to use as little lactic energy as possible until it is essential to do so by developing the use of the aerobic energy system. Long slow runs.
  • Train to deal with more lactic acid for when we do use our lactic energy system. Tempo runs.
  • Train to get more oxygen into the bloodstream to be able to better convert fat into energy. Interval runs.

Since aerobic energy is by far the most important energy source, it follows that training to make best use of that energy source is also the most important of the three.

One more thought. Most club runners can improve their ability to run longer distances in faster times by concentrating on their aerobic energy system. Elite athletes cannot ignore the possibility of the smallest extra gains, so they will also be seriously working on their anaerobic energy systems, particularly lactic. The conundrum is that training to develop the lactic energy system is generally intense and this can hinder the development of the aerobic energy system; training for the aerobic energy system is relatively gentle so will not help the development of the lactic energy system. The different training requirements for each energy system are why the best marathon runners are not the best 5k runners and vice versa. At high level, training is event specific.

Graeme Loudain – September 2019/October 2020

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