Impaired Voluntary Activation of an Ankle Dorsiflexor Muscle Predicts Walking Capacity in Multiple Sclerosis

Abstract

Background and Purpose Walking impairments are highly prevalent in people with Multiple Sclerosis (pwMS) and are related to factors including motor deficits and balance. Reductions in voluntary activation (VA) (i.e., reduced neural drive known as central fatigability) is a key contributor to motor deficits during fatiguing tasks in pwMS; however, the relationship between central fatigability and walking function is poorly understood. The purpose of this study was to determine associations between reductions in VA after a fatiguing task and walking capacity (WC) in pwMS and controls. Methods 19 pwMS (female=9) and 10 age-matched controls (female=6) participated. PwMS were sub-grouped for disease severity into those mildly affected (n=11, female=5, EDSS=2.5-4.0) and moderately affected (n=8, female=4, EDSS= 4.5-6.5). Twitch interpolation during a maximal voluntary contraction quantified VA of the tibialis anterior muscle before and immediately after a fatiguing time-to-task failure isometric task performed at 50% of maximal strength. The 6-minute walk test determined baseline WC. Values are mean ± SD. Results Moderately affected pwMS showed greater central fatigability after the fatiguing task compared with mildly affected pwMS (p=0.01), and controls (p=0.07) (moderately affected= 23.5% ± 24.9, mildly affected= 0.3% ± 5.4, controls= 6.4% ± 14.4). Both groups of pwMS showed lower WC than controls (moderately affected= 299.5m ± 75.2, mildly affected= 527.7m ± 68.9, controls= 613.7m ± 88.9, p<0.001). Further analyses revealed an association between central fatigability and WC, but only in the moderately affected pwMS (R2= 0.7, standardized estimate= -0.8, t=-3.8, p=0.009). Discussion Impaired neural drive of the ankle dorsiflexor muscles is a predictor of WC in pwMS who experience greater disease severity. Thus, mechanisms for limitation in walking function may be different based on disease severity in pwMS and this knowledge may assist in optimizing rehabilitation strategies to improve walking function in pwMS. Walking impairments are highly prevalent in people with Multiple Sclerosis (pwMS) and are related to factors including motor deficits and balance. Reductions in voluntary activation (VA) (i.e., reduced neural drive known as central fatigability) is a key contributor to motor deficits during fatiguing tasks in pwMS; however, the relationship between central fatigability and walking function is poorly understood. The purpose of this study was to determine associations between reductions in VA after a fatiguing task and walking capacity (WC) in pwMS and controls. 19 pwMS (female=9) and 10 age-matched controls (female=6) participated. PwMS were sub-grouped for disease severity into those mildly affected (n=11, female=5, EDSS=2.5-4.0) and moderately affected (n=8, female=4, EDSS= 4.5-6.5). Twitch interpolation during a maximal voluntary contraction quantified VA of the tibialis anterior muscle before and immediately after a fatiguing time-to-task failure isometric task performed at 50% of maximal strength. The 6-minute walk test determined baseline WC. Values are mean ± SD. Moderately affected pwMS showed greater central fatigability after the fatiguing task compared with mildly affected pwMS (p=0.01), and controls (p=0.07) (moderately affected= 23.5% ± 24.9, mildly affected= 0.3% ± 5.4, controls= 6.4% ± 14.4). Both groups of pwMS showed lower WC than controls (moderately affected= 299.5m ± 75.2, mildly affected= 527.7m ± 68.9, controls= 613.7m ± 88.9, p<0.001). Further analyses revealed an association between central fatigability and WC, but only in the moderately affected pwMS (R2= 0.7, standardized estimate= -0.8, t=-3.8, p=0.009). Impaired neural drive of the ankle dorsiflexor muscles is a predictor of WC in pwMS who experience greater disease severity. Thus, mechanisms for limitation in walking function may be different based on disease severity in pwMS and this knowledge may assist in optimizing rehabilitation strategies to improve walking function in pwMS.