The most common variable associated with cardiorespiratory fitness is VO2max. This is the maximal rate at which oxygen can be absorbed and used by the body during exercise. There is a strong association with endurance performance and VO2max, with high values (> 70 ml/min/kg) seen in elite endurance athletes.
In order for our muscles to use oxygen to generate force, the oxygen must first be delivered to and extracted by the working muscle. There are numerous processes involved in the delivery of oxygen to the working muscles, and each of these steps is potentially a limiting factor of an athlete’s VO2max.
1. Pulmonary diffusion capacity
Pulmonary diffusion is the process of moving oxygen from our lungs, which receive it from the external environment, and moving into the blood where it is then transported to the muscle. At sea level, the movement of oxygen from the lungs (external environment) to the blood, is not a limiting factor to oxygen consumption in healthy individuals.
2. Cardiac output
Cardiac output is a measure of the amount of blood leaving the heart per minute. Increases in exercise intensity will result in more muscle being recruited and this will, in turn, result in an increase in cardiac output to meet the demands. The heart is essentially a slave to the working muscles.
We typically see large differences in the VO2max values between trained and untrained athletes. The differences in maximal cardiac output between these two populations has been suggested as one of the main differences in VO2max. Endurance training results in an increase in plasma volume, the volume of the heart and the force of the heart’s contraction. All three of these will result in an increase in cardiac output.
3. Muscle’s capacity to extract oxygen
Increased cardiac output results in an increased blood flow within the body and as a result, an increase in oxygen delivery to the muscles. In order to make use of the increased oxygen delivery, the muscles must increase their capacity to extract the oxygen. Once the oxygen is extracted from the blood by the muscle, it is used in the mitochondria of the muscles to produce energy.
Endurance training results in an increase in the number of mitochondria in the muscles. The increase in mitochondria, means we have more power stations that can consume the oxygen and ultimately produce energy for the muscular work.
4. The blood’s capacity to transport oxygen
Increasing the oxygen carrying capacity of the blood is a target of both legal and illegal practices. Spending a prolonged period of time (~ 3 weeks) at a moderate altitude (2 000 – 2 500 m above sea level), is likely to result in an increased number of red blood cells. Increases in the oxygen carrying capacity of the blood will result in improved delivery of oxygen to the working muscles, and ultimately an improved performance.
If endurance performance was solely determined by VO2max, we would not need races to determine who the best cyclist in the World was. We could simply measure the VO2max of all the competitors and award the trophy to the cyclist with the highest value. Therefore, endurance performance is determined by more than just VO2max.
In Module 1 of our Cycling Science course we discuss, in detail, all of the factors associated with endurance performance. If you are interested in enrolling, you can find out more HERE.