Fatigue of voluntary contractions in normal and myasthenic human subjects

The effect of fatigue developed during 96 repeated rapid maximal voluntary finger muscle contractions (MVC), 12 contractions per minute, was followed in 4 control subjects and 4 myasthenic patients. The tension-time 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 about 0.3-0.4 s separating two relative maxima and a larger electromyographic activity during the first half second of the contraction. The mechanograms from rapid MVC were divided by an arbitrary straight line connecting the starting pint of the contraction curve and the notch. The line separated an area (N X 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 more than 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 about one half second. The results suggest that human muscle contractions can be divided into two parts with a different sensitivity to fatigue in both control subjects and myasthenic patients.