Contributions of motoneuron hyperexcitability to clinical spasticity in hemispheric stroke survivors

Objective: Muscle spasticity is one of the major impairments that limits recovery in hemispheric stroke survivors. One potential contributing mechanism is hyperexcitability of motoneurons. Previously, the response latency of the surface electromyogram (EMG) record evoked by joint rotation has been used to characterize motoneuron excitability. Given the limitations of this method, the objective of the current study was to reexamine the excitability of motoneurons in chronic stroke survivors by estimating reflex latency using single motor unit discharge. Methods: We quantified the excitability of spastic motoneurons using the response latency of a single motor unit discharge elicited by a position controlled tap on the biceps brachii tendon. We applied tendon taps of different amplitudes on the biceps tendons of both arms of the stroke survivors. Unitary reflex responses were recorded using intramuscular EMG recordings. Results: Our results showed that the latency of unitary discharge was systematically shorter in the spastic muscle compared with the contralateral muscle, and this effect was consistent across multiple tap amplitudes. Conclusions: This method allowed us to quantify latencies more accurately, potentially enabling a more rigorous analysis of contributing mechanisms. Significance: The findings provide evidence supporting a contribution of hyperexcitable motoneurons to muscle spasticity.