An upper limit of training intensity has been defined, because it has been observed that when a certain threshold is reached, no additional results are obtained. This upper limit coincides with the anaerobic threshold. Once the intensity of the training becomes so high that the body can’t produce energy aerobically, lactic acid starts to accumulate and aerobic fitness doesn’t improve anymore.
If we train closer to the lower limit of this interval, we will benefit from an improvement of fat metabolism and modifications at slow contracting muscle fibers level, while training at the upper limits will induce the development of fast contracting muscle fibers and benefits at the cardiovascular level.

Like in the case of strength training, the training intensity is the most important parameter we have to take into account when we create our exercise program.
Intensity reflects the energy requirements for exercising, the quantity of oxygen consumed and calories burned.
In the case of aerobic training, we have to gradually increase physical effort, to obtain an adaptive response from our body. This means we have to train with a higher intensity that the one used in every day activities. For a sedentary person, at the beginning even walking in a sustained rhythm can be a sufficient aerobic exercise.
Heart rate increases proportionally with exercise intensity. This is why it is frequently used to estimate this parameter.

Cardio training improves the functioning and capacity of the respiratory and circulatory systems. It also increased the blood volume, increases the muscle’s capacity to produce energy aerobically and to metabolize fat.
This way muscles are able to use more efficiently lipids as fuel. In this way the fat deposits are reduced, the level of lipids in the blood is decreased and the risk of developing cardiovascular diseases is reduced.
Also the insulin sensitivity is significantly increased and the risk of diabetes decreases.
Some studies brought up the idea that aerobic training can decrease the risk of certain cancers to appear, by influencing the metabolism of lipids.

Aerobic fitness, defined as the maximum capacity to assimilate oxygen in the body, transport it and use it, can be measured in the lab through a test called maximum oxygen consumption test. This test shows the biggest effort intensity which can be sustained and is usually done on a treadmill.
Practically the maximum quantity that enters the body is measured, gradually increasing the speed and incline of the treadmill, until a plateau is reached (meaning that no matter how much the treadmill incline is increased the same oxygen values are registered) or until the person can no longer sustain the effort.
The maximum oxygen volume is measured in ml/kg x min, so the oxygen volume (milliliters) for each kilogram of body weight in each minute of effort. The maximal values vary and depend on sex, age , but mostly on training level.

You can do the following test: walk slowly, then try to gradually increase your pace, until at one moment, you will have to start running (about 7-8 km/h). If you continue to increase speed until the effort begins to be hard to bear, you will notice that breathing is more difficult and you will start doubting that you can continue to run. Until this point you can say your effort was aerobic, meaning in the presence of oxygen. The energy was produced from carbohydrates and lipids. If you continue your effort, your body will switch to anaerobic systems, which means the energy must be generated faster than it can be produced aerobically. In this situation lactic acid forms in the muscles which together with an increased level of carbon dioxide lead to more difficult breathing, tiredness and discomfort.
In other words, we can talk bout aerobic exercising until the moment when the level of lactic acid in the blood begins to rise.
Aerobic metabolism is more efficient than the anaerobic one. Aerobic exercises can be sustained from several minutes to several hours.