Intermuscular coherences of plantarflexors during walking suggest distinct neural origin and function for alpha and beta/low-gamma bands after stroke

ABSTRACT Plantarflexors provide propulsion during walking (late stance) and receive input from both corticospinal tract (CST) and corticoreticulospinal tract (CReST). Both descending motor tracts exhibit some frequency-specificity, which allows potential differentiation of neural drive from each tract using intermuscular coherence (IMC). Stroke may differentially affect each tract, thus impair the function of plantarflexors. However, the evidence concerning this frequency-specificity and its relation to plantarflexors’ neuromechanics post-stroke remains very limited. Here, we investigated the intermuscular coherences of alpha, beta, and low-gamma bands between the Soleus (SOL), Lateral Gastrocnemius (LG), and Medial Gastrocnemius (MG) muscles and their relationships with walking-specific measures (propulsive impulse; speed). Fourteen individuals with chronic stroke walked on a treadmill at self-selected and fast walking speed (SSWS and FWS, respectively). Inter-limb IMC comparisons revealed that beta LG-MG (SSWS) and low-gamma SOL-LG (FWS) IMCs were degraded on the paretic side. At the same time, within each limb, the IMCs, which were significantly different to a surrogate dataset denoting random coherence, were in the alpha band (both speeds). Further, alpha LG-MG IMC was positively correlated with propulsive impulse in the paretic limb (SSWS). Findings suggest differential functional role of alpha and beta/low-gamma, which may be related to the frequency-specificity of the underlying descending drives. The persistence of alpha in plantarflexors and its strong positive relationship with propulsive impulse suggests relative preservation and/or upregulation of CReST. Future research should address whether entraining motor system at alpha frequencies via neuromodulation can improve the neuromechanical function of paretic plantarflexors and subsequently promote post-stroke walking recovery. Key Points Summary Cortical and subcortical motor drives may be frequency-specific, have a role in walking, and be degraded after stroke. Whether this frequency-specificity exists and how it is related to neuromechanical function of ankle plantarflexors post-stroke remains to be determined. Here, we investigated bilaterally the intermuscular coherences of alpha, beta, and low-gamma bands for the Soleus (SOL), Lateral Gastrocnemius (LG), and Medial Gastrocnemius (MG) muscles and their relationships with walking-specific measures (propulsive impulse; self-selected and fast speed) during treadmill walking in individuals post-stroke. The beta LG-MG (self-selected speed) and low-gamma SOL-LG (fast speed) were degraded on the paretic side. Alpha coherence was significantly present across plantarflexors mainly on the non-paretic side (both speeds). Paretic alpha LG-MG was positively correlated with paretic propulsive impulse (self-selected speed). Given that paretic propulsive impulse is impaired post-stroke, entraining the motor system at alpha frequency via neuromodulation may improve propulsive impulse and subsequently promote post-stroke walking recovery.