Muscular fatigue is usually defined as the inability to maintain a given exercise intensity. As we will see, there is no one cause of fatigue. Fatigue is task - specific and its causes are multifocal and vary from occasion to occasion. Fatigue during muscular exercise is often due to impairment within the active muscles themselves, in which case the fatigue is peripheral to the CNS and is due to muscle fatigue. Muscular fatigue can also be due to more diffuse, or more central, factors. For example, for psychological reasons an athlete may be unable to bring his or her full muscle power to bear in performing an activity. Alternatively, environmental factors such as hot, humid conditions may precipitate a whole series of physiological responses that detract from performance. In such cases, the cause of the fatigue resides outside the muscles.
Not only does the cause of fatigue vary with the nature of the activity, but the training and physiological status of the individual, as well as environmental conditions, affect the progress of fatigue during exercise. Fatigue can be due specifically to depletion of key metabolites in muscle or to the accumulation of other metabolites, which can affect the intracellular environment and also spill out into the circulation and affect the general homeostasis. The failure of one enzyme system, cell, or muscle group is likely to affect numerous other cells, organs, and tissues. Therefore, the causes of fatigue are interactive.
The study of fatigue in exercise has occupied the attention of many of the best biological scientists. Identifying a cause of fatigue is not simple, as it is often difficult to separate causality from concurrent appearance.
