Although there are multiple factors and mechanisms that can produce the dreaded fatigue, classically it is classified as central fatigue and peripheral fatigue. In the first case, we would be talking about when the mechanisms that produce it occur mainly at the level of the central nervous system, while in the case of peripheral fatigue, we would be putting the focus mainly on the muscle. This, added to the fact that fatigue is dependent on the task, makes it essential to study the mechanisms that produce it in any type of exercise specifically. For example, the mechanisms that produced fatigue in exercises as widespread as the incremental test to exhaustion until recently were unknown, and their study has opened a new possibility to take into account both in mechanistic research and in its possible application to performance sports. The concept of "functional reserve" is undoubtedly a variable that will receive great attention in the coming years.