The Long-Latency Reflex: A Biomarker for Functional Impairment Following Stroke

Walking is often impaired after a stroke, yet current motor rehabilitation strategies fail to produce meaningful improvements in walking function. Lack of functional improvement may be a consequence of providing rehabilitation to individuals that lack the neural substrate for the therapy to provide an effect. There is a critical need for non-invasive biomarkers that can predict potential for recovery of walking ability following stroke. This dissertation proposes the long-latency reflex (LLR) as a functional biomarker that relates to walking and other lower extremity function. To evaluate the potential for LLR presence as a biomarker, we utilize muscle stretch and electromyography to quantify the health of neural circuitry; probe motor and sensory contributions to the LLR response; and examine the relationship between LLRs and lower extremity function. We find that LLRs are absent in some individuals with chronic stroke, and across two independent samples these individuals are the lowest functioning. Data generated in this dissertation confirm a transcortical component in the tibialis anterior LLR and further suggest that many individuals retain the residual substrate necessary to elicit an LLR response in the context of concurrently timed transcranial magnetic stimulation. Altogether, the LLR shows promise as an unambiguous, clinically accessible biomarker of lower extremity dysfunction in chronic stroke. The specific mechanisms of LLR absence remain to be systematically investigated in future research