• The muscles get bigger and stronger.
  • There is an increase of the mitochondria. These are the energy factories in muscle cells.
  • The number and the size of the muscle fibers increase.
  • The number of capillaries increases causing a better blood supply to the muscles.
  • An increase of ATP (adenosine triphosphate) and glycogen.
  • Increase of enzymes allowing the burning of fat – and glycogen conversion proceed efficiently.

The number of heart muscle fibers increases as well as the number of capillaries so that the blood flow, especially to the important left ventricle, improves.

Sports heart works much more effectively than the hearts of untrained people.

The heart is a pump in which the cardiac output, which is the volume of blood pumped around by the heart in a minute and that is equal to the stroke volume in liters multiplied by heart rate.  In formula: Cardiac output = stroke volume x HR.

Stroke volume = the amount of blood the heart pumps per beat, in liters per stroke.

Heart rate is the number of heart beats per minute.

The stroke volume of the heart of an athlete can be twice as large as the stroke volume of an untrained person. That is the reason the HR in well-trained endurance athletes is much lower than in untrained people.

Well-trained endurance athletes often have a low heart rate. Sometimes, even lower than 40 beats per minute. The individual differences between, as regards the heart rate at rest, at well-trained endurance athletes are particularly great.

A sports heart is able to pump more blood during exertion thus increasing the transport of oxygen to the working muscles. This oxygen transport is one of the most important performance factors for endurance athletes like cyclists.

The low heart rate at rest and relatively lower heart rate during exercise is an important physiological adaptation of the body to training and is a sign of a stronger and better functioning heart after a period of training.

During exercise a sports heart is capable of pumping a cardiac output of 40 liters per minute. While cardiac output at rest is only 5 liters per minute. The increase in the cardiac output is the result of an increase of the stroke volume and of the increase in HR during exercise. Training the sports heart takes place at a high level of intensity at a high heart rate. After only a short training period of 6 weeks, a marked decrease in HR at rest may be observed.


The blood volume of well-trained endurance athletes can be 10 percent larger than in untrained state. This volume increase is caused by an increase in the plasma volume. The blood is thus thinner, less viscous. This change can be regarded as a physiological adaptation of the body to regular endurance exercise.

Due to the increase in plasma volume Hb can fall substantially. This decline is often wrongly mistaken for anemia and is known as sports anemia. However, there is no anemia at all.  The increase in the plasma volume in endurance athletes is not followed by an increase in the number of red blood cells. During the Tour de France the average haematocrit of the riders during the race, reduces by two to three percentage points. If the mean Ht value, at the beginning of the Tour de France is 45 it may result in 42 in the end.  A sports anemia does not require treatment because it is not a real anemia.  Many riders with a sports anemia but still get iron per tablet or per injection administered. This treatment is futile and not free from risks.

The blood vessels become more flexible so that the blood pressure is lower.

Blood composition changes for the better:

  • decreasing the total cholesterol.
  • the harmful LDL cholesterol also decreases.
  • the beneficial HDL cholesterol rises.

For example 1 gram of hemoglobin can carry 1.34 ml of oxygen, for example,  to the working muscles.

When there is a Hb of 15 g / 100 ml, the blood carries 15 x 1.34 = 20 ml oxygen per 100 ml of blood.

A low Hb content may be a symptom of anemia caused by iron deficiency. In order to distinguish with a sports anemia also the ferritin content of the blood is determined. A low Hb in combination with a low-ferritin level is proof of an anemia. A low Hb with normal ferritin level means a sports anemia. Ferritin is the most sensitive laboratory test for the detection of iron deficiency. Ferritin is a good measure of iron stores in the body.

During exercise the blood vessels of the muscles dilate  so that more blood flows to the muscles. At that time, there is less blood flow to the gastrointestinal tract, where the digestion takes place.


By training the respiratory muscles become stronger and increase the functional lung volume. As well as the heart, the lungs can be also considered as a pump. The respiratory minute ventilation = RMV = functional lung volume x respiration rate. At rest, we breathe 10 to 15 times per minute, with a functional lung volume of 500 ml.

RMV rest = 10/15 x 500 = 5 to 7.5 liters / minute
RMV effort = 180-200 l / minute
Respiratory Rate = 60 / minute.
Functional lung volume = 3-4 liters.
The increase in the RMV is greater than the increase of cardiac output. On this basis, it is clear that the oxygen-transport capacity of the cardiovascular system and not that of the lungs is the limiting factor for sports performance.


The energy for muscular work is produced in the energy factories in the muscles. They are officially called the mitochondria. The muscle cells get their energy from four energy-supplying systems.


ATP is adenosine triphosphate or readymade energy that can be used directly. It is the energy source for the sprinters. ATP can be rapidly converted to ADP, whereby much energy is released which we can use for cycling or running maximum for a short time.  This type of energy does not require oxygen. The ATP stock is already exhausted after 8 to 12 seconds, making it suitable only for short (final) sprints. After the sprint muscle cells can build up the ATP again from ADP. To do this requires energy, which is supplied from the conversion of glycogen with the aid of oxygen.

An intensive sprint creates therefore an oxygen debt. During the recovery period extra oxygen is consumed in order to build up the ATP again. Sprint training may increase the amount of ATP and the breakdown and the reconstruction of ATP can be accelerated. For this repetition of short sprints at maximum speed is the most appropriate training. Essential here is that the recovery time between sprints is long enough because the reconstruction of ATP requires time. The recovery period is often longer than five minutes.


During breakaways, short time trials, escapes from the peloton and closing the gap to the leading group energy is supplied by the anaerobic breakdown of glycogen. In the anaerobic breakdown glycogen is converted to lactic acid or lactate. The result is an acidosis of the legs so that the intensity of the effort can only be sustained for a limited period of time. At the end of the effort the formed lactic acid is broken down with oxygen, so that there is once again an oxygen debt that has to be paid off.

Glycogen is composed of glucose molecules that form long chains. Glycogen is stored in the liver and muscles. In addition, the blood also contains a small amount of glucose. Using specific training, teaches the body to break down lactic acid faster. These are workouts with high intensity, usually in the form of interval training. The intensity is above the deflection point of the anaerobic threshold. The anaerobic threshold of highly trained cyclists is at 93% HR max.

The breakdown of glycogen produces less power than the breakdown of ATP, but we can make use of it for a longer time.  Textbooks often say that the anaerobic breakdown of glycogen is good  for only a short period of time (several minutes). But that is not always true. That depends on the intensity of the anaerobe effort. Intensity which lies only a few percent above the anaerobic threshold then trained That depends on the intensity of the anaerobe effort. If the intensity is only a few percent above the anaerobic threshold, well trained cyclists are able to   sustain this intensity, in the red zone, for more than 30 minutes.  Is the intensity is many percents above  the anaerobic threshold, then the assertion will be correct, and well trained cyclists are forced within a few minutes to stop the effort because of the skyrocketing levels of lactic acid that are created.


During the aerobic breakdown of glycogen with enough oxygen carbon dioxide (CO2) and water (H2O) are formed. Carbon dioxide is exhaled through the blood and the lungs. Oxygen is transported to the muscles through the lungs and the blood. If the oxygen transport capacity of the cardio-respiratory system is functioning properly, this system can supply energy for a long time ie the effort can be sustained long.  The average carbohydrate stock supplies energy, at the level of a marathon run, for about 90 minutes. This is an intensive aerobic effort just below the intensity of the deflection point. This corresponds to intensity at 85 to 90% of the HR max. With less intense aerobic exercise, the glycogen reserves are sufficient for 1,5 – 2,5 hours. This corresponds to intensity at 70 to 85% of the HR max.


In case of the aerobic breakdown of fats, with sufficient oxygen, fats are converted into carbon dioxide and water. Also, this system can supply energy for a very long time.  The breakdown of fat produces less power than the breakdown of glycogen and also fat needs more oxygen for its oxidation.

When the glycogen in the muscles is exhausted the body must switch to burning fats. That is the moment of ‘hitting the wall’ in which the cycling speed or running speed decreases dramatically.

The stock of fat is very large and delivers enough energy for many days of cycling. The fat burning is mainly used during low intense exercise.  With an increase in the intensity of the effort the body switches over to other energy systems. First to the aerobic breakdown of glycogen next to the anaerobic breakdown of glycogen and finally to the breakdown of ATP.

Fat burning is trainable by prolonged training sessions with low intensity, lower than 70% HR max. For losing weight  one must train at a low intensity and also with a large extent. The so called  ‘long slow distance’ (LSD) training. Also care should be taken to a diet with fewer carbohydrates.   Low intense training  while fasting can  also  improve fat burning.  These work outs are useful for cyclists because fat burning always plays a major role during exercise. During slowly cycling  fat  burning is  an important energy provider, sometimes more than 50%. Increasing the speed and the intensity  of the effort  also the burning of glycogen increases, and even 90% at top speed.


ATP → ADP + energy
Creatinephosphaat + ADP→ Creatine + energy
Low stock. Sprint, 10 – 12 seconds.

Glycogen → lactic acid + energy
Limited stock. Minutes. High speed. Acidosis.

Glycogen + 6 O2 → 6 CO2 + 6 H2O + energy
Large stock, 2 hours, long distance, tempo endurance.

Fats + 23 O2 →16  CO2 + 16 H2O +  energy
Very large stock, many days, very long distances, low speed, higher oxygen consumption  than for glycogen breakdown.


The average glycogen amount in the body supplies energy for about 90 minutes marathon running. At an intensity of 85-90% HR max. The fat supply is consumed after about 120 hours. However, the burning of fats takes more oxygen. Per unit of time glycogen produces more ATP than fats can produce. Glycogen is the main source of energy during prolonged intense physical activity. Depletion of the carbohydrate stock then perforce increases fat burning. This moment is called ‘hitting the wall’ and is accompanied by a lowering of the level of effort.

The proportion of fat oxidation and carbohydrate burning depends on the intensity of the effort.
The proportion of fat oxidation and carbohydrate burning depends on the intensity of the effort.

ART: active recovery training
EET: easy endurance training
IET: intensive endurance training
TET: tempo endurance training