Central motor drive is increased during voluntary muscle contractions in multiple sclerosis

To test the hypothesis that central motor drive is increased during voluntary contractions in multiple sclerosis (MS) patients, we recorded the surface electromyogram (EMG) and force from the tibialis anterior muscle during isometric dorsiflexion in 14 MS and 18 control subjects. Measurements were obtained during contractions at 10-100% of maximal voluntary contraction (MVC), incremented by 10% MVC. Integrated EMG (% maximum) was elevated in MS compared to controls from 10 to 70% MVC (pairwise, P < 0.05; main effect, P=0.001). MS severity (Expanded Disability Status Scale) was highly correlated to the individual slopes of the EMG/force relationship (r=-0.87, P < 0.001). We conclude that central motor drive in MS is increased throughout a large range of submaximal contractions and that this increase is associated with overall disease severity.