The Slow Restoration of Fatigued Muscle to Normal Irritability by Rest
That rest decreases the fatigue threshold of both nerve-muscle and muscle can be seen in [Fig. 18]. The time taken for total recovery, however, is dependent upon the amount of work done, but this change, like that of fatigue, varies widely with different individuals. In some animals the threshold returned to normal in 15 minutes; in others, in which the same amount of work was done, it was still above normal even after 2 hours of rest. This may be due to the condition of the animals—in some the metabolites are probably eliminated more rapidly than in others. There were also variations in the rate of restoration of the normal threshold when tested on the nerve and when tested on the muscle in the same animal. In [Fig. 18] (at 3) the nerve-muscle returned to normal in 30 minutes, whereas the muscle (at 4) after an hour’s rest had not returned to normal by a few β units. This, however, is not typical of all nerve-muscles and muscles. The opposite condition—that in which the muscle returned to normal before the nerve-muscle—occurred in as many cases as did the condition just cited. The failure of the two tissues to alter uniformly in the same direction may be explained as due to variations in the location of the electrodes when thrust into the muscle at different times (e. g., whether near nerve filaments or not). The results from observations made on the nerve are more likely to be uniform and reliable than are those from the muscle.
The time required for the restoration of the threshold from fatigue to normal, in denervated muscles, is approximately the same as that for the normal muscle.