It is evident that residual strain, other things being equal, will be greater if the stimuli have been excessive. This is well seen in [fig. 23], where the set of first three curves A is for stimulus intensity of 45° vibration, and the second set B, with an augmented response, for stimulus intensity of 90° vibration. On reverting in C to stimulus intensity of 45°, the responses are seen to have undergone a great diminution as compared with the first set A. Here is seen marked fatigue, the result of overstrain from excessive stimulation.
Fig. 23.—Effect of Overstrain in Producing Fatigue
Successive stimuli applied at intervals of one minute. The intensity of stimulus in C is the same as that of A, but response is feebler owing to previous over-stimulation. Fatigue is to a great extent removed after fifteen minutes’ rest, and the responses in D are stronger than those in C. The vertical line between arrows represents ·05 volt. (Turnip leaf-stalk.)
If this fatigue be really due to residual strain effect, then, as strain disappears with time, we may expect the responses to regain their former height after a period of rest. In order to verify this, therefore, I renewed the stimulation (at intensity 45°) after fifteen minutes. It will at once be seen from record D how far the fatigue had been removed.
One peculiarity that will be noticed in these curves is that, owing to the presence of comparatively little residual strain, the first response of each set is relatively large. The succeeding responses are approximately equal where the residual strains are similar. The first response of A shows this because it had had long previous rest. The first of B shows it because we are there passing for the first time to increased stimulation. The first of C does not show it, because there is now a strong residual strain. D again shows it because the strain has been removed by fifteen minutes’ rest.
Fatigue under continuous stimulation.—The effect of fatigue is exhibited in marked degree when a tissue is subjected to continuous stimulation. In cases where there is marked fatigue, as for instance in certain muscles, the top of the tetanic curve undergoes rapid decline. A similar effect is obtained also with plants ([fig. 24]).
