Impaired Firing Behavior of Individually Tracked Paretic Motor Units During Fatiguing Contractions of the Dorsiflexors and Functional Implications Post Stroke.

Francesco Negro,Claudio Orizio,Jennifer N Nguyen,Allison S Hyngstrom,Kathleen E Bathon,Sandra K Hunter,Cassidy G Bannon

doi:10.3389/fneur.2020.540893

Abstract

Introduction: This study quantified stroke-related changes in the following: (1) the averaged discharge rate of motor units (individually tracked and untracked) identified from high-density electromyography (HD-EMG) recordings, (2) global muscle EMG properties of the dorsiflexors during a fatiguing contraction, and the relationship between task endurance and measures of leg function.Methods: Ten individuals with chronic stroke performed a sustained sub-maximal, isometric, fatiguing dorsiflexion contraction in paretic and non-paretic legs. Motor-unit firing behavior, task duration, maximal voluntary contraction strength (MVC), and clinical measures of leg function were obtained.Results: Compared to the non-paretic leg, the paretic leg task duration was shorter, and there was a larger exercise-related reduction in motor unit global rates, individually tracked discharge rates, and overall magnitude of EMG. Task duration of the paretic leg was more predictive of walking speed and lower extremity Fugl-Meyer scores compared to the non-paretic leg.Discussion: Paretic leg muscle fatigability is increased post stroke. It is characterized by impaired rate coding and recruitment and relates to measures of motor function.

Highlights

This study quantified stroke-related changes in the following: [1] the averaged discharge rate of motor units identified from high-density electromyography (HD-EMG) recordings, [2] global muscle EMG properties of the dorsiflexors during a fatiguing contraction, and the relationship between task endurance and measures of leg function
These results suggest that during fatiguing contractions of the paretic leg, the nervous system is unable to modulate rate coding or recruitment subsequently leading to the inability to maintain a sub-maximal force
Our evidence of a strong central contribution to fatigue is consistent with previous investigations of mechanisms of neuromuscular fatigability post stroke

Summary

Introduction

This study quantified stroke-related changes in the following: [1] the averaged discharge rate of motor units (individually tracked and untracked) identified from high-density electromyography (HD-EMG) recordings, [2] global muscle EMG properties of the dorsiflexors during a fatiguing contraction, and the relationship between task endurance and measures of leg function. Impaired rate modulation and recruitment of motor units in chronic stroke survivors during sub-maximal fatiguing contractions of the leg muscles may limit functional endurance and interfere with mobility. Consistent with this notion, stroke survivors demonstrate altered kinematics after short bouts of walking [1,2,3] and decreased distance walked during the 6-min walk test compared with healthy controls [4]. If the central nervous system is unable to excite motor neuron pools to meet the force demands of the task, this would present as impaired rate coding and recruitment of motor units

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