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  • Writer's pictureDavid Wadsworth

Cycle Physiology

Cyclists like to talk about “having a big engine”, meaning they can push big power all day long. It’s one of the variables that determines the outcome on race day – who was the strongest? Why don’t cyclists simply ride harder for longer? What makes them slow down?

To answer these questions we need to dive into Cycle Physiology - how does the body respond to exercise and how do we use this information to drive improved performances?

Like an engine, cyclists’ muscles need fuel to run, and when the body cannot continue to produce “fuel” or energy fast enough, then you slow down. Understanding how you fuel your cycling efforts helps make better choices in training and racing. In this post I’ll examine the different types of muscle “fuels” and why they matter for cyclists.

ATP: The Energy “Currency” of Muscles

The power for every muscle cell to contract comes from a single molecule known as adenosine triphosphate (ATP). Muscle cells break ATP down to release energy for contraction. Muscles cannot obtain ATP from the blood so they must manufacture it inside the cell. We need to continuously produce ATP since a muscle cell only stores enough ATP for about 1-3 seconds of work, which makes ongoing replenishment of ATP essential during any ride.

Given that the rate of energy consumption during intense exercise can increase up to 100-fold over the energy required at rest, muscles need several different methods of producing ATP depending on how much is required and how quickly.

The 3 Fuel Systems for Cycling:

The body has 3 primary methods of producing ATP from the food you eat. These are known as:

1. The phosphagen system (also called the anaerobic alactic system);

2. The glycolytic (also called anaerobic) system;

3. The oxidative (aerobic) system.

The reality is that you produce energy using all three metabolic pathways simultaneously – it is all one big system working together all the time.

The proportion of energy produced by each system is different depending on the intensity and duration of exercise.

When I perform fitness tests for riders, I create a physiological profile that highlights how well your body produces energy using the three different mechanisms above. This is useful to help understand your strengths and weaknesses as a rider and to plan your training program.

ATP-PCr (energy from stored ATP & phosphagen system), Glycolytic & Oxidative energy production. After Gastin (2001);

The Phosphagen System

The phosphagen system is all about sprinting (and lifting weights). It produces ATP from phosphocreatine (PCr) molecules stored in the muscle cells. It creates a small amount of ATP very quickly since it only involves a single chemical reaction. Whilst it produces energy quickly, which is what you need for sprinting or at the start of exercise, the downside is that it can only create enough ATP to last another 5-8 seconds (in addition to the 1-3 seconds of stored ATP) because muscles only store a small amount of PCr.

In this article, we’ll focus on the other 2 methods of producing energy since this is how the body generates the majority of power required for training or racing.

The Glycolytic System

The glycolytic system also produces energy quickly by breaking down glucose (a carbohydrate or sugar molecule). In simple terms it converts glucose into lactate and hydrogen ions (H+): 1 glucose molecule produces 2 ATP molecules and 2 lactate molecules.

When you start exercising glycolytic energy production ramps up, until the oxidative system has really “spun up” and taken over. Your muscles need glycolytic power whenever the intensity level gets high. It is a very significant method of energy production for higher intensity efforts lasting anywhere from 20 seconds up to around 2-3 minutes. It is required for any effort above your “FTP” or “threshold power”.

There is a downside to glycolytic energy production – it creates hydrogen ions (acid!) which, once the acid level get too high, causes muscles to stop contracting as strongly and causes pain. It is the rising acidity level that stops you pedalling at hard intensities for too long (for information about lactate read my post here).

Glycolytic power is a key part of your physiological profile – it highlights how powerful your top-end efforts are. These are the type of high intensity efforts needed to win races, typically repeated multiple times at the pointy end of a race.

The Oxidative System

The oxidative (aerobic) system uses oxygen (O2) to burn both carbohydrates and fat to create ATP. Oxidative energy production creates by far the largest amount of ATP (around 36 ATP molecules from 1 glucose molecule, and around 130 ATP molecules from 1 fatty acid molecule) but does so significantly more slowly than glycolytic or phosphate metabolism as the process involves many chemical reactions. In addition, oxidative combustion of carbohydrates is faster than combustion of fatty acids. It is the primary energy source for all efforts lasting more than a about one minute – which covers pretty much all cycling events lasting longer than a track sprint.

Energy production via oxidation of carbohydrates and fatty acids occurs within the mitochondria of your cells (hence why I call mitochondria the “energy factory” for your muscles). The higher the intensity of the endurance event the more important carbohydrate becomes as a fuel (for example XCM mountain bike racing). Longer events performed at low intensity (e.g., a 600km Audax ride) rely primarily oxidative combustion of fatty acids.

When we start exercising from rest, the phosphagen and glycolytic pathways of energy production provide most of our energy for the first few seconds, and although the oxidative system begins to produce energy straight away it is much slower to get going. Depending on the duration and intensity of exercise, our muscles will replenish ATP using a combination of the 3 methods listed above. Each method has its advantages and disadvantages, and helping your body develop and balance these methods of energy production is a key part of training.

Each system takes a different time frame to adapt and improve, so in a training program the length of training blocks to improve the oxidative, glycolytic and phosphagen components of the system will be different.

If you want to improve and start winning, then you need to identify the demands of your event and establish what type of energy production is required and the resultant power output. A training plan to develop each of the three energy production methods to match what is required on race day can then be created.


Gastin P (2001): Energy System Interaction and Relative Contribution During Maximal Exercise. Sports Med 31: 725-741.

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