Evidence for Increased High Alpha Intermuscular Coherence as a Measure of Reticulospinal Motor Drive

The reticulospinal (RS) system is a key descending motor pathway involved in human movement control. However, it remains poorly characterized, largely due to methodological challenges in assessing its anatomy and physiology. Current approaches to gauge RS drive rely on intense stimuli (auditory cues, transcranial magnetic stimulation), limiting their feasibility. Here, we examined intermuscular coherence as a potential biomarker of common neural drive related to the RS system. Intermuscular coherence was evaluated during the startle reflex and StartReact paradigm, both known to be primarily mediated by the RS system. In 10 healthy participants, intermuscular coherence was analyzed across the sternocleidomastoid, lateral deltoid, and biceps brachii muscles bilaterally during the startle reflex. In 16 participants, intermuscular coherence was examined in the tibialis anterior (TA) during StartReact, as well as during postural perturbations, a postural balance task, and non-postural ankle movements. Intermuscular coherence was enhanced across all muscles in the high alpha band during both the startle reflex and StartReact. A similar, albeit smaller peak in coherence was found at the same frequency during postural perturbations. TA intermuscular coherence was significantly enhanced at the high alpha and low beta bands during balance control vs. non-postural ankle movements. The findings from the startle reflex and StartReact suggest that intermuscular coherence in the high alpha band reflects RS drive. RS contributions appear to be critical for postural control, particularly during balance-related tasks, but not for non-postural tasks involving TA. Results suggest that intermuscular coherence is a promising approach for assessing RS motor drive during movements.