Motor Unit Discharge Characteristics And Walking Performance Of Individuals With Multiple Sclerosis.

Abstract

PURPOSE: To examine the associations between strength, force steadiness, and motor unit discharge characteristics of lower leg muscle with assessments of walking performance and disability status in individuals with multiple sclerosis (MS). METHODS: Persons with MS (n=23, 9 men, 53 ± 7 yrs) attended 1 to 3 evaluation sessions that were performed before, immediately after, and 4 wks after a clinical trial of neuromuscular electrical stimulation. Outcomes included 3 questionnaires (Patient Determined Disease Steps [PDDS], Modified Fatigue Impact Scale [MFIS], and MS Walking Scale-12 [MSWS-12]), 25-ft walk test, 6-min walk test, and maximal torque and force steadiness (10 and 20% MVC) for lower leg muscles. High-density surface EMG recordings were obtained from the tibialis anterior (TA), soleus (Sol), and medial gastrocnemius (MG) during the steady contractions and decomposed into discharge times of motor unit action potentials. RESULTS: Greater levels of self-reported disability (PDDS, MFIS, MSWS-12) were associated with less distance walked in 6 min and longer times to walk 25 ft. Mean interspike intervals (ISI) were 110 ± 23 ms and 102 ± 22 ms (10% and 20% MVC force, respectively) for TA motor units (n= 1,634), 138 ± 36 ms and 135 ± 34 ms for MG motor units (n=696), and 150 ± 33 ms and 145 ± 34 ms for Sol motor units (n = 902). A multiple regression model explained 40% (P=0.001) of the variance in 6-min distance with two predictor variables: mean ISI of MG (partial r = -0.48) and MVC dorsiflexor torque of the more affected leg (partial r=0.37). Similarly, another multiple regression model explained 47% of the variance (P<0.001) in 25-ft time with three predictor variables: mean ISI of soleus (partial r=0.51), MVC dorsiflexor torque of the more affected leg (partial r = -0.43), and force steadiness of the plantar flexors (partial r = 0.39). CONCLUSION: Moderate amounts of the variance in two walking tests for persons with MS were explained by mean discharge times of action potentials by motor units in plantar flexor muscles during steady submaximal contractions, the strength of the dorsiflexor muscles in the more affected leg, and force steadiness during an isometric contraction. This research was supported by the Eunice Kennedy Shriver National Institute of Child Health & Human Development of the NIH (R03HD079508).