Combining electrical muscle stimulation with voluntary contraction for studying muscle fatigue

Short episodes of electrical stimulation were applied to the right quadriceps muscle of healthy subjects at different times during 60 second sustained voluntary muscle contractions at different levels ranging from 0% to 100% of maximal voluntary contraction force. The amplitude of electrically-induced increments of torque (delta T) has been shown to depend upon both the level of muscular contraction and time from the beginning of the contraction. We have considered that the dependence of delta T upon the time from the beginning of contraction reflects muscle fatigue. A group of subjects was asked to simulate fatigue and to decrease the level of contraction to 50% of the required level. They demonstrated strikingly different dependencies of delta T upon time from the beginning of contraction. Two kinds of malingering have been analyzed with the help of a simple mathematical representation of the dependence of delta T upon actual voluntary contraction force. The described method is promising for studying muscular fatigue of central and peripheral origin and for detecting malingerers.