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Endurance is erroneously associated with long-term efforts. However, we need stamina to perform any type of activity. It is much more correct to associate it with the ability to maintain an effort, without diminishing performance, for as long as possible. For example, in a 100-meter run, which lasts about 10 seconds, it is observed that the sprinters lose speed in the last meters. This is due to a decrease in their ability to deliver enough energy to their muscles in the final meters. Therefore, despite being a very short effort, we could say that its endurance decreases in the last meters.


ATP (Adenosine Triphosphate) is obtained from the food we eat, this is a molecule that is stored in the muscle and is responsible for providing energy to the muscle to contract. That ATP can be obtained through different processes:

  • Anaerobic alactic pathway: Related to maximum intensity efforts of a very short duration (10-15 seconds). It immediately provides energy to the muscle through the reserves stored in it. The amount of energy that can be provided is very little, so it is spent very quickly. Oxygen is not needed for energy, and lactic acid is not produced as waste.
  • Anaerobic lactic pathway: Related to high-intensity efforts for up to a maximum of 1 to 2 minutes. Energy is obtained from the breakdown of nutrients stored in the muscles and liver, but without oxygen, producing lactic acid as waste.
  • Aerobic way: It allows prolonged efforts (3′- hours) of low or moderate intensity. Energy is obtained from the breakdown of nutrients stored in the muscles, liver, and other areas of the body, along with oxygen. The waste substance that is generated is the carbon dioxide that we expel through breathing.


Oxygen consumption:

It is important to understand that, if a very prolonged aerobic effort is made or the intensity is raised a lot, the anaerobic lactic pathway is activated and lactic acid would begin to be produced, which would force us to have to stop the activity. The need for oxygen in an activity will therefore depend on the intensity, the duration, and the number of muscle groups involved. The more and larger the muscle groups involved, the greater the energy needs.

The oxygen debt

There is a linear relationship between the heart rate and the intensity of the effort, in such a way that the higher the intensity, the higher the heart rate. When an effort is very intense or it is immediate, the body does not have time to provide energy by the aerobic way, and then resorts to the energy supply of the anaerobic pathways. This causes what we call oxygen debt, which is the amount of oxygen that would have been needed to provide the same amount of energy by aerobic means. Up to this point, everything would be fine, since we are able to cover our energy needs, if it weren’t for the fact that every time we activate the anaerobic lactic pathway, we generate lactic acid that accumulates in the body. This oxygen debt is offset during the recovery periods.

The anaerobic threshold

The anaerobic threshold represents the intensity (speed, heart rate, etc…) from which the body is not capable of meeting its energy needs through the aerobic pathway and has to activate the anaerobic lactic pathway, starting to produce lactic acid. It is an individual value, different for each person. Increasing the anaerobic threshold is the fundamental goal of any endurance training plan. On a practical level, improving it means that if we initially begin to accumulate lactic acid when we reach 160 beats per minute (bpm), if we manage to raise our threshold to 175 bpm, we will begin to accumulate lactic acid much later, which will delay the onset of fatigue.


This is the transitory and reversible decrease in performance, due to the decrease in energy reserves and the accumulation of waste substances that are generated when producing energy: urea, lactic acid, carbon dioxide, water… that hinder different physiological functions and cause the appearance of the feeling of fatigue.


Endurance is of great importance in improving physical conditioning, and can be greatly improved with training. The main effects that your training causes in the body are:

  • Increase in the size of the heart: This allows it to receive and pump more blood and therefore more nutrients and oxygen through the blood.
  • Strengthens the heart: it increases the thickness of its walls, as well as the size of the atria and ventricles.
  • Slows the heart rate: By pumping more blood with each beat, the heart rate slows, making the heart more efficient.
  • Lung capacity increases, as well as the ability to take in oxygen with each breath, due to the increased number of pulmonary alveoli.
  • Increases the number of capillaries, which makes it easier for more oxygen and nutrients to reach all the cells of the body.
  • Improves the ability to eliminate waste substances generated by this energy production.


  • Between the ages of 8 and 12, the ability to resist moderate and continuous efforts increases.
  • Between the ages of 12 and 18, aerobic endurance should be developed.
  • Between the ages of 18 and 20, the maximum development of endurance is reached.
  • Between the ages of 23 and 30, maximum aerobic and anaerobic development is achieved.
  • From the age of 30 this ability decreases at a faster rate than others such as strength and speed.


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