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Fitness level – a question about supply and demand

Economists are well familiar with the term “supply and demand”. We may use the same term about the conditions for our fitness level. The textbooks in sports are now separating between the terms endurance and fitness level. By fitness level we mean maximal oxygen intake – VO2 max.  Endurance is a more specific characteristic that depends on the sport you are doing. For example, a football player needs another type of endurance than a long distance runner.

Supply and demand for fitness level is about oxygen

The demand for oxygen is happening in the muscles. Our body has around 660 muscles that are controlled by our consciousness. These muscles constitute 35-45% of our body weight. Most of them are connected to the skeleton and are called the skeleton muscles. Sometimes this system of muscles is called the peripheral muscle system.  What is the source – the supply – that shall satisfy the mentioned demand for oxygen? The supply is coming from what is called the central system – the “engine”. This engine consists of the heart and the lungs. Normally it is the central system which is the bottleneck for fitness sports. By fitness sports we mean in this context sports with a high load on the cardiovascular system.

The peripheral muscle system and the central system depend on an infrastructure for transport of oxygen and nutrition, and for removal of waste substances such as CO2 and water vapor. This infrastructure mainly consists of the blood veins and the blood itself.



A person with a good fitness level has a good balance between supply and demand of oxygen. Efficient fitness training challenges the central system in such a way that it’s capacity is increased over time. The heart has a training potential (growth potential) up to 40-60%, from untrained condition. It is the heart’s displacement and stroke strength which are improved by effective training. The heart’s pumping ability is the displacement multiplied with the pulse, which will increase by fitness training.

Load on the heart can be done in many different ways.  If you activate many different muscles at the same time, it is not necessary to have a very high intensity before the heart gets enough challenges. Examples of this are swimming, rowing, cross country skiing, nordic walking and other activities where many parts of the body are active at the same time. This kind of activity feels more “comfortable”, since the intensity for each muscle is not so high. There are few nerves in the heart, so you don’t feel pain, stiffness or soreness in the heart itself.

Bicycling fitness levelActivities with higher intensity (load) on one or few muscles gives more pain and feeling of discomfort. Examples of this are running, bicycling or other activities where you primarily use the legs. These kind of sports are optimal for fitness training, since you will need to have a higher intensity level for the (few) involved muscles to challenge the central system. Consequently, this kind of training feels more heavy and provides a bigger portion of discomfort during high load.

The demand for oxygen is: (number of kilos active muscles) x (muscle/work intensity). If you activate smaller muscle groups, you need a very high intensity to challenge the heart. Doing this will quickly become anaerobic and very uncomfortable. For this kind of activity, a good fitness level is not required, and the heart will not be the bottleneck. Strength training is an example of this, since only small muscle groups are engaged at the same time. The training-/growth potential for oxygen turnover in the muscles is only around 15%. Fitness training can double the number of mithocondries, which are the active part of the muscle cells where oxygen turnover is executed. Fitness training can also triple the small veins – capillaries – that are going into the muscle cells. The burning process (metabolism) in the muscles is performed at 37 degrees Celsius, due to enzymes making this “cold burning” process possible. Fitness training increases the concentration of aerobic enzymes. The blood volume increases as an effect of fitness training, as much as up to 1-2 litres after training systematically over a long period.

Read more about the physical systems and physical basis here: More about running and More about the physical building blocks

Alternative training as a success factor

Cross country skiingMy “secret” as a runner was that I performed quite a lot of cross country training during the winter season. By this I could build the central system more efficient than by running alone. A higher amount of involved muscles on a moderate level of intensity gave the heart challenges enough, and at the same time not overloading muscles, joints and tendons. I did some running as well, in order to get the “specific” training that is needed for maintaining running capability and endurance. By this combination I achieved a better “engine”, which gave the working muscles a better supply of oxygen.

Author: Ingrid Kristiansen

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