Cycling Guide Part 1: Cycling Physiology 

Guide by:
Guido Vroemen (Coach Profile)
Author, sports physician, medical biologist, and certified triathlon trainer.
Contact Guido

Guido Vroemen

The heart of a cyclist is a superior and more efficient organ.

Cardiologist dr. J. Wolffe, MD 

In endurance sports like road cycling, triathlon, and mountain biking, the physiology of the human body is very important. In this guide, we will explore the basics of cycling physiology, while learning about the ‘human engine’.

In short, the human engine comprises of the (leg) muscles and the cardiovascular and respiratory (or heart-lung) system, which ensures the supply of oxygen to and the disposal of metabolites from the muscles.  

Which factors determine the capacity of the human engine? Which fuels are used by the muscles and how much power can be produced? And what is the impact of training?

Training leads to huge adaptations in our body as a result of which we become fitter. Below, I present a summary of the most important aspects of the human engine. 

Training Effects of Cycling on the Body

Consistent and balanced training leads to the following adaptations of the muscles and the cardiovascular system: 


The (leg)muscles become stronger. There is an increase of: 

– the number of mitochondria (the energy producers of the cells); 

– the number and size of the muscle fibers; 

– the number of capillaries and the blood flow through the capillaries; 

– the stockpile of ATP (adenosine triphosphate) and glycogen; 

– the number and activity of enzymes (improving the breakdown of glycogen and fatty acids). 

Research has also shown that training can even lead to a modification of the ratio of Fast Twitch (FT)-muscles to Slow Twitch (ST)-muscles. As a consequence, both speed and endurance can be improved by training. Such training should be continuous and focused. As a result of the training stress, initially, some muscles will be damaged. You can feel this, as your muscles may ache the first days after the workout.

However, in time your body will react by strengthening the muscles. Consequently, they can better cope with the training load. Training your leg muscles is a protracted process and you have to put many miles in the tank to get the best results. The majority of the training can be done at an easy pace, but in order to develop the FT muscles it is necessary to do some speed (high-intensity) work as well. 


The adaptation of the heart to the training is most remarkable. The number of heart muscle fibers increases and so do the number of the capillaries and the blood flow through the capillaries, in particular of the left heart chamber. As a result of this, the ‘sports heart’ is much more efficient than the heart of untrained, sedentary people.

We can illustrate this by considering the heart as a pump. The discharge of this pump (called cardiac output or heart minute volume) is the number of liters of blood pumped per minute. This equals the stroke volume (in milliliters) times the heart rate (HR, in beats per minute).

For example, in rest, the stroke volume can be 100 ml, and with a heart rate of 50/min the cardiac output is 50 x 100 = 5000 ml (5 liters). The stroke volume of a trained cyclist can be twice as large as that of an untrained person. Consequently, at rest, the heart of a trained cyclist has a large spare capacity and the HR can be quite low. It is quite common for well-trained cyclists to have a resting heart rate (RHR) of 40 or even lower! During exercise, the sports heart is capable to pump much more blood, leading to increased oxygen transport to the (leg)muscles. As the muscles need oxygen to produce energy, this oxygen transport capacity is the single most important factor to determine performance in sports in general and in cycling in particular.

The increase in stroke volume and the corresponding decrease of the RHR are important physiological adaptations of the heart. These adaptations increase the capacity of the heart. The sports heart is able to increase the blood flow during exercise from 5 l/min to 40 l/min (SV is 200 ml and HR 200/min), thus by a factor of 8. This is achieved by a combination of the increase in the stroke volume and (mainly) the HR. The adaptation of the sports heart depends mainly on the intensity of the training (a high HR and thus a high intensity of the training is required) and can occur relatively quickly. It is possible to achieve a significant reduction in the RHR in as little as 6 weeks.


The blood volume of a well-trained cyclist is some 10% larger than that of an untrained person. This is mainly caused by an increase in the plasma volume. Of course, this increase has a positive impact on the oxygen transport capacity. Another important adaptation is an increase in the flexibility of the blood vessels, leading to a decrease in blood pressure. The blood composition also changes: the cholesterol levels decrease, in particular those of the ‘bad’ LDL and the total cholesterol. The ‘good’ HDL increases.

The level of hemoglobin may increase as a result of altitude training. Hemoglobin is vital for the oxygen transport by the blood. 1 gram of hemoglobin can transport 1.34 ml oxygen (O2), so an average hemoglobin level of 15 g/100 ml blood, leads to an oxygen transport capacity of 15*1.34 = 20 ml O2/100ml blood or 20%.

A low level of hemoglobin may indicate an iron deficiency in nutrition or increased iron loss. An unnatural high level of hemoglobin may be the result of blood or EPO doping. Finally, the blood vessels dilate during exercise, leading to a reduction of the peripheral resistance and an automatic increase of the blood flow to the (leg)muscles. Less blood is diverted to non-essential body parts, such as the digestive system. 


As a result of training, your breathing muscles become stronger and the tidal volume (functional lung volume) increases. We illustrate this in the same way as we did for the heart: by considering the lungs as a pump. The capacity of this pump (called respiratory minute volume) is the tidal volume (in liters) times the breathing frequency (in breaths per minute).

In resting state, we breathe around 10 – 15 times per minute and the tidal volume is around 0.5 liter, so the respiratory minute volume is 5 – 7.5 l/min. During exercise, the respiratory minute volume can increase dramatically to 180 – 200 l/min for well-trained athletes. This is the result of the increase in both the breathing frequency (to 60 breaths per minute) as well as the tidal volume (to 3 – 4 liter).

The increase in the capacity of the lungs is even larger than that of the heart, so the lungs are usually not the limiting factor. Consequently, we can conclude that normally the oxygen transport capacity of the cardiovascular system is the main factor that determines performance in endurance sports. However, we should remark that the breathing muscles themselves need a significant amount of oxygen. This can amount to some 10% of the maximum oxygen transport capacity or VO2 max. 

Energy systems 

In order to cycle (and/or run/swim) we need energy. This energy is produced in our muscle cells, to be precise in the mitochondria. The cells can do this, by using any (or a combination) of the 4 following energy systems:  

The box summarizes some important aspects of the 4 energy systems of the human engine. 
1. ATP
ATP -> ADP + Energy
Small stockpile, 10 seconds, sprint, maximum power and speed
2. Anaerobic Glycolysis
Glycogen -> Lactate +Energy
Limited time to exhaustion, few minutes, breakaways, High power and speed
3. Aerobic Breakdown of Glycogen
Glycogen + 6O2 -> 6CO2 + 6H2O + Energy
Large Stockpile, 1.5 hours, long distance, Endurance power and speed
4. Aerobic breakdown of fatty acids
Fatty acids + 23O2 -> 16CO2 +16H2O + Energy
Very large stockpile, many days, ultra distance, low power and speed, higher oxygen use



Adenosine triphosphate (ATP) is the primary fuel for sprinters. ATP can be transferred to ADP very quickly, releasing a large amount of energy and thus providing the muscles with the largest amount of power. Moreover, the process does not require oxygen. However, the stockpile of ATP in the muscles is extremely small, lasting only for a short sprint of some 10 seconds. During recovery, the muscle cells are able to regenerate the ATP from the ADP.

This process requires energy, which has to be supplied by the aerobic (using oxygen) breakdown of glycogen. The amount of oxygen needed to regenerate the ATP is called the oxygen debt. So, the energy debt is created during exercise and needs to be redeemed during recovery. As a result of training the efficiency of the stockpiling and the use and recovery of ATP can be increased. This requires many repetitions of short sprints at top speed. 

Anaerobic glycolysis 

The anaerobic breakdown of glycogen or glycolysis is the most important energy system for breakaways and prologues, lasting a few minutes. Glycogen is composed of large chains of glucose (sugar) units. Glycogen is stored in the muscles and the liver. The blood also contains a small amount of glucose. Glycogen can be broken down anaerobically (without the use of oxygen) into lactate.

This lactate may accumulate in the muscles to a certain amount and when it exceeds this amount it will increase also in the blood circulation. During exercise and recovery, the lactate can be combusted using oxygen, thus redeeming another oxygen debt. With training the efficiency of the anaerobic glycolysis can be improved. This requires training at a high intensity so that lactate is accumulating. This occurs only at a high HR, around 85 – 90 % of the maximum HR (MHR). This is called the anaerobic limit or threshold limit. The anaerobic breakdown of glycogen produces less power than the ATP system, but it is somewhat more durable. The time to exhaustion is a few minutes, depending on the anaerobic capacity and fitness. 

Aerobic breakdown of glycogen 

The aerobic breakdown of glycogen is the main energy system for endurance athletes, including cyclists. Glycogen is broken down into carbon dioxide and water, using oxygen. The carbon dioxide is removed from the muscles by the blood and the lungs (increases ventilation). The oxygen is supplied to the muscles by the lungs and the blood. This is a very durable process that can be maintained for a very long time when the oxygen transport capacity of the cardiovascular system is large enough.

This oxygen transport capacity can be increased by training at an intensity just below the anaerobic or threshold limit. Training at a lower intensity (e.g. 70% of MHR) is also useful as it stimulates the muscles themselves. The aerobic breakdown of glycogen produces less power than glycolysis, but the stockpile of glycogen lasts for at least 1.5 hours. With training and optimized nutrition (carbo-loading), this period can be increased to 2 – 3 hours. 

Aerobic breakdown of fatty acids 

The aerobic breakdown of fatty acids is the main energy system for cyclists and triathletes. Fatty acids are broken down into carbon dioxide and water, using oxygen. Consequently, this system is quite comparable to the previous one (the aerobic breakdown of glycogen). The main drawback is that it produces less power. This is the reason for the well-known phenomenon of ‘hitting the wall’. This happens when the stockpile of glycogen in your muscles is exhausted, so the muscles have to transfer to the breakdown of fatty acids. From that moment onwards, your power output is greatly reduced and your speed drops dramatically.

The main advantage of the breakdown of fatty acids is that the stockpile is extremely large and sufficient for many days. We use this system during rest and when exercising at low intensities. The efficiency of the fatty acid system can also be improved by training. This should be done by long rides at low intensity (less than 70% of your MHR).

Eating less carbohydrates may also help, as well as early morning training prior to breakfast. We should realize that the fatty acid system is used by all cyclists at low and moderate intensities. When we ride slowly, the amount of fatty acids in the fuel mix of our muscles may be up to 90%. At the threshold pace, this percentage may be only 25%.  

To learn more about cycling physiology, check the book The Secret of Cycling