The three major cycling energy systems the human body uses for energy are:
- Anaerobic – Phosphocreatine (PCr) System (ATP; triphosphate, as in three phosphates)
- Glycolytic or Lactic Acid System
- Aerobic System
The body draws on all three cycling energy systems, regardless of the type of effort, never closing one off completely. They merely change in the percentage and amount of energy the cycling energy systems contribute depending on the duration and intensity of the effort.
Let’s get an understanding of how they are used in cycling by looking at Dr Andy Coggan’s Power Levels. If you’re not familiar with these levels, they are a way of categorising how intense an effort is, which dictates how long the effort will last and which cycling energy systems will predominantly supply energy for the effort.
There are a few variations out there, but for cycling with a power meter – Coggan’s Power Levels is the most comprehensive.
There are 7 levels or zones each representing an intensity and time frame and now an energy system.
Zones 1-3 represent the aerobic system.
Z1 / <55% Active Recovery / 70-80 years
Z2 / 56-75% Endurance / 2.5 hours to 14 days
Z3 / 76-90% Tempo / 2.5-8 hours
Zones 4-6 represent the lactic system.
Z4 / 91-105% Lactate Threshold / 10-60 minutes
Z5 / 106-120% V02 Max / 3-8 minutes
Z6 / >120% Anaerobic Capacity / 30 seconds to 2 minutes
Zone 7 stands out on it’s own and represent the ATP system.
Z7 / N/A Neuromuscular Power / 5-15 seconds
Let’s begin with Zone 7:
Energy System 1: Anaerobic – Phosphocreatine (PCr) System
The first phase is called the ATP- CPr (Adenosine Triphosphate)- (Phosphocreatine) system. ATP is stored in all cells, particularly muscles. It is the only system that doesn’t require a blood supply and has no by products. It functions in the absence of oxygen.
There are not many steps in the chemical reactions that make up the ATP-PCr system in cycling energy systems. The reactions can take place in the absence of oxygen and phosphocreatine is a relatively high energy molecule. As a result, the ATP-PCr system can provide a lot of energy quickly but only for immediate and short (10s) maximum intensity efforts.
In a sense, it is free energy because the body stores ATP to make it available for immediate use, however, you can only use it once and it needs recovery time to restore the storage. Once you have depleted you Phosphocreatine stores in a sprint it can take as long as 5 minutes to restore them to their resting levels, ready to sprint again. Making it a high rate – low capacity system compared to other facets of the cycling energy systems.
Energy System 2: Anaerobic – Lactic Acid System (LA)
The next major phase is called the Lactic (LA) system. After the 20 seconds of the ATP-PCr system, the body requires another ingredient – muscle glycogen (glucose) to be added to continue.
This system breaks down carbohydrate, a fuel in limited supply in the body, to produce medium amounts of power for medium amounts of time. The energy is produced without oxygen using carbohydrate > sugar > glucose > glycogen > ATP.
The body’s stores around 500 grams worth of carbohydrate in the tissues of the liver and muscles in the form of glycogen. This amount of energy would fuel approximately 2000 Kilojoules of mechanical work on the bike, as recorded by a power meter.
Regardless of how long an effort is, carbohydrate is always initially broken down through a chemical reaction called anaerobic glycolysis. Oxygen is not required for this reaction and whilst only about 5% (2 ATP molecules) of the energy potential of a glucose molecule can be realised the energy is liberated quickly, so this energy system is well suited to high intensity efforts greater than 10 seconds to 2 minutes.
Because anaerobic glycolysis can only supply short efforts, it only makes a small dent in the 2000 Kilojoules of stored carbohydrate available, so the time limitation is related to the chemical processes involved in anaerobic metabolism and their interaction with the body, rather than a lack of availability of carbohydrate.
Its by-product, lactic acid, comes from the breakdown of the glucose released from the muscles. Most cyclists have heard of lactate or lactic acid. Lactate is not a waste product, but is actually an important part of anaerobic and aerobic metabolism.
During high intensity efforts lactate is produced in greater amounts than can be removed and contrary to popular belief, fatigue may not simply be the result of lactic acid accumulation – there is a lot of misunderstanding around this molecule. For one, lactate does not cause muscle soreness. Another, fatigue from exercise is not due simply to lactate accumulation.
Energy System 3: Oxidative (Aerobic) System
The first or third system is the Oxidative phase. In this phase, as the term indicates you are using oxygen to fuel the breakdown of carbohydrates first, free fatty acids second and if the exercise continues long enough -protein. Whereas, the previous systems have related to higher intensity work (or power) the aerobic system is more for moderate or low intensity work, but of longer duration.
It can draw on your stores of glucose but only for ~90 minutes at max. This is why you need to replenish your glucose stores with CHO during your ride.
The oxidative system should be developed to aid in lactic system. The development of the aerobic system aids in lactate removal, so that you can tolerate more lactate.
It is only able to produce relatively small amount of energy, so cannot produce enough energy for any sprinting, but can produce power for extended periods of time, making it the predominant system used during any endurance ride.
What Does Mean For You?
No matter if you’re a road rider or an MTB racer you use all their energy systems to ride in all types of terrains. Individually you have strengths and weaknesses for specific durations and intensities relative to others. Whether it’s being a better sprinter than long climbs, or hammering short-steep hills your energy systems can be improved through training.
As mentioned before when on the bike all cycling energy systems are providing a portion of energy depending on the intensity of work being done. This can actually be tested in a lab. Similar to the equipment used to analyses the Fatmax test.
If you were to do a 120 second sprint test. The first 10-15 seconds is fueled almost entirely by the PCr system, producing a huge burst of power, but very quickly fatiguing. After around 10 seconds the Phoscreatine system is completely exhausted and the lactate acid system starts to kick in. By 30 seconds, the LA system has fully taken over, but rapidly starts to fatigue as lactate acid accumulates. By 40 seconds, the aerobic system has begun to kick in as oxygen has made it to the working muscle and begins to assist with the aerobic contribution of energy production.
You can test this without a lab. But really you’re just better off doing a Power Profile Field Test. This will highlight the main areas that need working on because you aren’t producing enough power or you’re predisposed to one type of event.
What about training? Training should address all of your cycling energy systems, combining efforts from sprints to long rides over multiple hours.
The awareness of your cycling energy systems may also come in handy when racing. Any time you are on the bike ask yourself ‘where is my energy coming from’ or ‘what energy have I burnt recently?’ Knowing where you are drawing energy from will help you to know how much you might have in reserve. Or how long you should rest before you can go again at max capacity.
This is still a guessing game at this stage and links in with the idea of ‘matches’ and the work on W Prime / Functional Reserve Capacity. These are only new ideas (relatively) and there isn’t much literature around yet but there’s no harm in trying to work out what works for you, and how you can best optimise your cycling energy systems.
