Nadel, E. R. (1985). Physiological adaptations to aerobic training. American Scientist, 73, 334-343.

Nadel (1985, p 334) described the symptoms of fatigue as being related to the progressive inability to supply sufficient energy to the active muscles. Three types of fatigue that correspond to each of the three energy systems involved in exercise were defined.

  1. Fatigue resulting from maximum contractions. When a muscle contracts maximally, the frequency of neural stimulation is high so that maximum tension is developed through the recruitment of as many muscle fibers as possible. In this type of work, fatigue sets in rapidly (the time span usually being in the range of two to six seconds) and is generally thought to be due to the inability of fast-twitch fibers to maintain tension, perhaps because of an impairment of neuromuscular transmission particularly at the nerve endings in the muscle. This impairment may be the result of an accumulation of extracellular potassium ions, which alter the electrical potential across the muscle membrane and at the nerve endings. This form of fatigue is of relatively short duration when it is localized to a specific muscle group. When it involves the total body as in an all-out sprint, recovery is somewhat longer but still several repetitions at training are usually possible if adequate between-repetitions recovery is provided.
  2. Fatigue that results from exercise requiring moderate intensities of muscular contraction. When the major proportion of the exercise work is performed through the activation of slow-twitch fibers, fatigue develops more slowly than in higher-intensity work (it could take from several minutes to a few hours). The resulting fatigue is thought to be a consequence of impaired excitation-contraction coupling between the electrical activity in the muscle and the chemical changes that induce muscle shortening. The efficiency of muscular contraction and the ability to sustain contractions is reduced. This is possibly a result of a critical depletion of intracellular calcium ions that play an essential role in the contraction process. Recovery from this type of fatigue usually is not possible within a training session and is directly affected by post-exercise diet and recovery activities.
  3. Low-frequency fatigue. During long-duration activities that last several hours or more, the major source of fatigue that develops is probably attributable to the distinctive mechanics of force generation. This type of fatigue develops as the slow-twitch fibers become depleted of glycogen reserves and fast-twitch fibers are increasingly activated until maximum recruitment occurs. At that point, power output begins to decline because no greater recruitment is possible to compensate for the activity decline in the slow-twitch fibers. Recovery from this form of fatigue is not possible within a training session and is more likely to take several to a considerable number of days depending upon the type of activity and the severity of the muscle cell damage that has occurred (Noakes, 1986, p. 233).

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