Spasticity, weakness, force variability, and sustained spontaneous motor unit discharges of resting spastic-paretic biceps brachii muscles in chronic stroke

Introduction: The purpose of our study was to examine relations among spasticity, weakness, force variability, and sustained spontaneous motor unit discharges in spastic-paretic biceps brachii muscles in chronic stroke. Methods: Ten chronic stroke subjects produced submaximal isometric elbow flexion force on impaired and non-impaired sides. Intramuscular EMG (iEMG) was recorded from biceps and triceps brachii muscles. Results: We observed sustained spontaneous motor unit discharges in resting biceps on iEMG. Spontaneous discharges increased after voluntary activation only on the impaired side. The impaired side had greater matching errors and greater fluctuations in isometric force. Spontaneous discharges were not related functionally to spasticity, force variability, or weakness. However, greater strength on the impaired side correlated with less force variability. Conclusion: Weakness rather than spasticity is a main factor interfering with voluntary force control in paretic-spastic biceps brachii muscles in chronic stroke.