Energy Systems, and why every cyclist should know about them.

Energy Systems, and why every cyclists should know about them.

All energy used by the body, whether it is sprinting for the line on the Champs-Elysees or sleeping comes from the breakdown of a molecule inside the body called ATP (adenosine triphosphate). The more energy required for an activity, the more ATP is needed and consequently the faster this energy is needed the faster it is produced.  However very little ATP is stored with in the body at any one time, meaning that it has to be made from resources within the body to match the demands required to achieve our required workload.  To meet this demand the body has a three ways, or three energy systems, to produce enough ATP to both match the speed and amount of energy required.  These systems are the Aerobic, Glycolytic and ATP-PC (Phosphocreatine) systems.  Each of these systems is specially adapted to match different requirements of ATP production, but work simultaneously to meet these demands.

This article will discuss briefly how each system works and what they mean for cyclists.


Aerobic Energy System

The aerobic energy system is the most important energy system in cycling, as it produces the majority our energy on any given ride. The aerobic system uses oxygen combined with fat or carbohydrates to produce ATP.  It is also the system that provides the majority of energy for everyday living.

ATP production occurs in the working muscles where the oxygen from the air we breathe is delivered via the blood and is used to break down molecules of glycogen (stored carbohydrate in the muscle) or fat to release the building blocks for ATP. This means that as long as there is a sufficient supply of both oxygen and a fuel (carbohydrate, fat, and if these run out protein from the surrounding muscle can be used) the body can produce energy continuously.

The reason the aerobic system is our bodies preferred energy is that it is able to create large amount ATP from very little fuel, 37-39 ATP from one molecule of glycogen and a massive 129 ATP from one molecule of fat. This is due to the length of the chemical reaction each of the fuels has with oxygen takes. However although this system produces great amounts of ATP this production takes a relatively long time compared to the other systems with fat taking even longer to break down than glycogen.  But for an activity like endurance cycling, e.g. cycling 100miles, where a large amount of energy is required but is released slowly over a long period this system is perfect.

Aerobic system is used in the majority of our cycling, from long training rides and sportives to cruising in the bunch in a race. It is only when the speed at which we require energy exceeds the rate at which this system can produce ATP that another energy system is required.

A good analogy would be to compare this energy system to your bank account, where fuel and oxygen are your wage, hard earned over a long time, and the energy you need for cycling is what you spend. As long as these balance everything keeps ticking along nicely.  It is only when our spending exceeds our income that we need to seek other means to match the demands to seek alternative means, a different energy system.


Glycolytic (Lactic Acid System)

Although the aerobic system provides the majority of energy for endurance cyclists, when it comes to racing, whether the Tour de France or beating your club mates to the top of the hill, the glycolytic system makes all the difference.

When the rate of energy requirements exceeds that which can be produced aerobically the body can produce ATP without the use oxygen or anaerobically. The glycolytic energy system, like the aerobic system, breaks down glycogen to create the building blocks to produce 2 ATP molecules per molecule of glycogen.  This break down of glycogen happens very rapidly allowing for it to repeat several times to produce high amount of ATP.  However this very rapid breakdown of glycogen to produce ATP leaves a build-up bi products, such as lactic acid, which change the chemical environment of the muscle.  Once these bi products reach a certain threshold level they effect the muscles ability to contract, giving the glycolytic energy system only around 5 minutes in well trained individuals to meet the increased energy demand. This build-up of bi-products is the burning sensation you feel after a hard effort.

Once this threshold has been reached the body needs to return the chemical environment of the muscle to its natural state, using the aerobic system to clear lactic acid build up. Because of this energy production must be reduced to allow for the amount of time it takes for the aerobic system to turn these bi-products into ATP, always taking longer to remove these bi-products than produce them. This removal of bi-products by the aerobic systems begins immediately, but as their production occurs much faster than removal there is a net build up.  Once the waste has been cleared the muscle will be able to work at the higher intensity again, however as the glycolytic system uses glycogen as its only fuel source this is only possible if glycogen is still available in the muscle.  This is why if you don’t keep your carbohydrate in take up during cycling you eventually bonk when it comes time to climb that hill late in your ride.

To use the money analogy again you can think of the glycolytic system as a high interest credit card to supplement your bank account. When your wage won’t cover that desperately important bill or bike repair, then the credit card gives us that instant money to cover this cost.  However the card has limit, and once we reach this limit we have to stop spending until the debt is paid off.  Unfortunately because this is borrowed money we have to pay a bit extra back on top to return our balance to zero before we can spend again.

ATP-PC System.

The ATP-PC system is another anaerobic system, it does not use oxygen to create ATP. Unlike the other two system the ATP-PC system is less important to cyclists.

The muscles contain a substance called phosphocreatine (PC) which can be rapidly broken down to almost instantaneously create ATP, with one molecule of PC creating one molecule of ATP. This rapid production of ATP allows the muscle to crate the energy required for high powered explosive movements, such as the kick in a sprint, or push away from the gate on the track.  However there is only enough PC available in the muscle for 6-10 seconds of ATP production, after which CP need to regenerate, taking around three minutes to do so.  Once the ATP-PC systems has been used up the glycolytic system takes over to produce the high energy, but as the glycolytic system cannot produce ATP as fast maximum power will slightly decrease.  This is why when you go for that sprint on your bike the initial acceleration is the fastest part of your sprint.

If the aerobic system is your bank account and the glycolytic system your credit card the APT-PC system is a pay-day lender. Giving you one big lump sum to pay for your immediate need, but leaving you with a massive interest bill to pay off.


Understanding how the body produces energy allows the cyclists to better understand what is going on as they ride. Knowing that we couldn’t stay with the fast guys because our glycolytic system couldn’t produce enough energy, or that we bonked because we’d used all our glycogen up earlier.  Once we understand that our energy system have their limitations and we identify them we can train each one to improve them by doing training that works at the right intensity to get the system working better.


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