Myogenic and central neurogenic factors in fatigue in multiple sclerosis.

Short episodes of electrical stimulation were applied to the right quadriceps muscle of patients with multiple sclerosis (MS) and healthy subjects at different times during 60 sec sustained voluntary muscle contractions at 0 to 100% levels of maximal voluntarily generated joint torque. 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. The dependence of delta T upon the time from the beginning of contraction has been assumed to reflect muscle fatigue. Patients with MS demonstrated an apparent involvement of central neurogenic mechanisms in fatigue manifested as a drop in muscle torque during sustained contractions at 75 and 100% levels when electrical stimulation was able to induce considerable increments in muscle torque. These patients also demonstrated a dependence of delta T upon the contraction level suggesting that they did not produce maximal voluntary contraction torque in the pre-trial. Fatigue in MS is due to central, neurogenic factors and does not seem to involve any myogenic factors such as might be related to secondary muscle changes due to the long-standing disorder. The subjective feeling of tiredness ('fatigue') may be related to a dissociation between central motor commands ('effort') and their mechanical consequences.

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.