Abstract

The effect of fatigue developed during 96 repeated rapid maximal voluntary finger muscle contractions (MVC) 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. 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 point 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. The results suggest that human muscle contractions can be divided into two parts with a different sensitivity to fatigue. Verf.-Referat