Fatigue of voluntary contractions in normal and myasthenic human subjects

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Abstract
The effect of fatigue developed during 96 repeated rapid maximal voluntary finger muscle contractions (MVC), 12 contractions/min, was followed in 4 control subjects and 4 myasthenic patients. The tension-timne integral (T-TI) was determined during either the first 1 or 1.25 s of each contraction. Fatigue decreased the T-TI by 21% in the control subjects and by 65% in the patients. While ordinary MVC showed a slow rate of rise of tension and gradually increasing electromyographic activity, rapid MVC from both groups showed a steep rate of rise of tension, a notch after .apprx. 0.3-0.4 s separating 2 relative maxima and a larger electromyographic activity during the 1st half second of the contraction. The mechanograms from rapid MVC were divided by an arbitrary straight line connecting the starting point of the contraction curve and the notch. The line separated an area (N .times. s) above and to the left (.alpha.-component) from an area below and to the right (.beta.-component) of the line. Fatigue affected the .beta.-component selectively in both control subjects and myasthenic patients. Only when the .beta.-component was reduced by > 80%, was a reduction of the .alpha.-component also seen. At any time during a sustained contraction, the patients could briefly increase their instantaneous strength, but for only .apprx. 1/2 s. Human muscle contractions can evidently be divided into 2 parts with a different sensitivity to fatigue in both control subjects and myasthenic patients.