We can state that muscle fatigue is the inability of the muscle to express the same force produced in the previous instant despite the stimuli being equal. Factors and causes of muscle fatigue are numerous and depend on the type of performance and effort that the subject is making. It can be linked to a reduction of glycogen levels in the muscles (marathon runner, cyclist can incur muscle fatigue due to a reduction in glycogen levels in the muscle, since their activity is prolonged for a long time). But even boredom, motivation, mental fatigue are psychological factors that can determine muscle fatigue especially in endurance or ultra endurance races.
The lactic acid molecule is responsible for causing muscle fatigue. The lactic acid is divided into ion H⁺e lactate ion (La⁻): the increase of hydrogen ions inhibits the activity of the enzymes that break up glucose, so they will be able to produce a smaller quantity of ATP and therefore energy for the muscle. This especially concerns short-term maximal exercises.
Doing a low intensity exercise will have a constant concentration of lactic acid in the blood, which means that it does not accumulate. Lactic acid is produced but also consumed, it is not only negative because it has energy in itself and can be taken by some cells as an energy substrate (cells of the heart and muscle). In high intensity exercises, however, the concentration of lactic acid in the blood increases exponentially because there is an imbalance between its production and its consumption.
With a maximal exercise there is an accumulation of lactic acid very quickly, even in a few seconds, always depending on its intensity. The ability to accumulate lactic acid is not unlimited but has a certain value beyond which you can not go: the more the exercise is maximized the more early the production of lactic acid, but the speed with which it is formed does not change compared to less intense exercises.
After a maximal exercise in recovery the lactic acid returns to the resting values ​​in about 1 hour and a half. The half-life of its concentration is about 15-20 minutes. The peak of lactic acid in the blood is a few minutes after you have finished the exercise but not immediately: this is because it takes some time to get it from the muscles to the blood. With low-intensity fatigue, the lactic acid is disposed of first because the muscle cells use the lactic acid circulating in the blood to produce energy. A cool-down to 35% -65% intensity will make it dispose much faster.