Neuromuscular responses of patients with multiple sclerosis

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Abstract
Muscle weakness, studied in 4 patients with multiple sclerosis (MS), was compared with values from normal subjects. Twitch occlusion showed that normal subjects could activate their muscles maximally, but patients rarely achieved >60% activation. In both groups, motoneuron firing rates increased linearly with force. Consistent with the reduced level of activation, MCV firing rates in MS muscles rarely exceeded 17 Hz (compared with ∼24 Hz for normals). However, for right and left muscles of one patient, mean maximum firing rates were 14.2 ± 2 Hz and 8.0 ± 2 Hz, but her muscles could be activated to levels >92% and 60%, respectively. This patient's ability to achieve higher than expected forces at low firing rates was probably due to her slow muscle contractile speeds, especially ½-relaxation time (75 to 115 ms, cf. ∼60 ms for normals), and high twitch/tetanus ratio (0.4, cf. 0.2). © 1992 John Wiley & Sons, Inc.