Distinct neural input strategies to motor units in the soleus and medial gastrocnemius during quiet standing

Abstract

Standing posture control critically depends on the activation of the soleus (SOL) and medial gastrocnemius (MG), which serve distinct functional roles. Identifying underlying neural mechanisms has been challenging, as conventional invasive techniques sample only limited motor units (MUs). Recent advances in high-density surface electromyography (HDsEMG) have enabled analysis of MU activity and estimation of common synaptic inputs to spinal motoneurons. Therefore, we aimed to elucidate the common synaptic inputs underlying the distinct MU behaviors of SOL and MG during standing. We recorded HDsEMG from the SOL and MG, alongside electroencephalography, from 20 male participants during standing and isometric voluntary contractions. EMG signals were decomposed into individual MU activity, with common synaptic inputs estimated through intramuscular and corticomuscular coherence analyses (IMC and CMC). Compared to SOL, the MG exhibited significantly higher delta-, alpha-, and beta-band IMC during standing. In task comparisons, alpha-band IMC increased during standing specifically in the MG. Furthermore, although beta-band CMC decreased in both muscles while standing, IMC was preserved in the MG but markedly reduced in SOL. This dissociation suggests that the common neural drive to the MG during standing is likely derived from subcortical rather than cortical pathways. These results demonstrate that the SOL and MG are governed by distinct neural control strategies, which likely underlie their functional roles. Given the low CMC, the MG relies on strong common synaptic input from subcortical pathways (e.g., vestibulospinal and reticulospinal) to produce rapid corrective torque, whereas the SOL functions with lower neural synchrony to ensure steady ankle stiffness. Key pointsO_LIThe soleus and medial gastrocnemius play distinct roles in standing control, however, due to technical limitations, it has been difficult to identify the underlying neural mechanisms responsible for these differences. C_LIO_LIUsing high-density surface electromyography, we examined motor unit activity and neural inputs to these muscles during standing. C_LIO_LIThe medial gastrocnemius shows greater common synaptic input, potentially facilitating rapid ankle plantarflexion torque generation to correct postural sway. C_LIO_LIThe soleus exhibits lower motor unit synchrony, enabling stable and continuous ankle plantarflexion torque generation for body weight support. C_LIO_LIThis study demonstrates that the soleus and medial gastrocnemius are governed by distinct neural control strategies, which likely underlie their distinct functional roles. C_LI Abstract figure legend O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=131 SRC="FIGDIR/small/698550v1_ufig1.gif" ALT="Figure 1"> View larger version (41K): org.highwire.dtl.DTLVardef@be53aorg.highwire.dtl.DTLVardef@f6793aorg.highwire.dtl.DTLVardef@190e8f9org.highwire.dtl.DTLVardef@af5e4b_HPS_FORMAT_FIGEXP M_FIG C_FIG Motor unit spike trains were decomposed from high-density surface electromyograms recorded from the medial gastrocnemius (MG; left; red) muscle and soleus (SOL; right; blue). To compare the neural input to the spinal motor neurons between them, we quantified the intramuscular coherence (IMC) of motor unit spike trains within the delta, alpha, and beta bands. MG exhibited greater IMC than SOL during standing, indicating stronger common synaptic input, likely mediated by vestibulospinal and reticulospinal pathways. In contrast, SOL showed lower IMC, suggesting a greater contribution of independent synaptic input. As a consequence, high motor-unit synchrony in the MG supports rapid, phasic torque generation for postural sway attenuation, whereas low synchrony in the SOL enables smooth, steady torque production for weight bearing during standing.