Neuromuscular responses of patients with multiple sclerosis

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 greater than 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 approximately 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 greater than 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 1/2-relaxation time (75 to 115 ms, cf. approximately 60 ms for normals), and high twitch/tetanus ratio (0.4, cf. 0.2).