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

Abstract

Introduction The purpose 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 the 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 functionally related 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.

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“…This claim rests on the premise that in order to fully compensate for any given load, weaker muscles require more activation than stronger muscles, and greater voluntary contractions give rise to greater levels of "signal dependent noise", which in healthy subjects manifests as a linear scaling of force variability with respect to the mean force level. 26 Indeed, a study of isometric force production in the paretic-spastic biceps brachii muscle 27 found that greater muscle weakness led to greater variability in the generation of muscle force (i.e., greater motor noise). Increased force production variability has similarly been observed at the wrist and fingers post-stroke.…”