Distinct neural input strategies to motor units in the soleus and medial gastrocnemius during quiet standing
Hiroki Arakawa, Naotsugu Kaneko, Nadaka Hakariya, Hikaru Yokoyama, Kimitaka NakazawaAbstract
Standing posture control depends on the soleus (SOL) and medial gastrocnemius (MG), which serve distinct functional roles. Identifying neural mechanisms remains challenging because invasive techniques sample few motor units (MUs). High‐density surface electromyography (HDsEMG) enables MU analysis and estimation of common inputs to motoneurons. Leveraging this approach, and by comparing standing with an isometric voluntary contraction performed at an EMG level matched to standing, we aimed to elucidate standing‐specific common synaptic inputs underlying the distinct MU behaviors of the SOL and MG. We recorded HDsEMG and electroencephalographic signals from 20 males during standing and isometric contractions. EMG signals were decomposed into MU activity, with common inputs estimated through intramuscular and corticomuscular coherence (IMC and CMC). IMC was used as an index of common inputs to motoneurons, whereas CMC was used as an index of corticomuscular connectivity. During standing, the MG exhibited higher alpha‐ and beta‐band IMC than the SOL. Compared with isometric contraction, CMC decreased during standing in the delta‐, alpha‐ nd beta‐bands in both muscles, whereas beta‐band IMC remained unchanged only in the MG. These findings suggest that MG motoneurons receive more common synaptic input during standing than SOL motoneurons. Although CMC decreased similarly in both muscles, MG maintained higher IMC, suggesting a relatively greater role of non‐cortical common input in the MG. This pattern could reflect differences in the functional roles of the MG and SOL in postural control, with MG contributing to force adjustments that counteract body sway and SOL contributing to stable tonic force production.
Key points
Using high‐density surface electromyography, we examined motor unit activity and neural inputs to medial gastrocnemius and soleus during standing. The medial gastrocnemius shows greater common synaptic input than the soleus, potentially associated with its role in phasic ankle plantarflexion torque generation to correct postural sway. The soleus exhibits lower motor unit synchrony, potentially associated with its role in generating stable and continuous ankle plantarflexion torque for body weight support. Corticomuscular coherence was lower during standing than during isometric voluntary contraction in both muscles, suggesting that functional corticomuscular coupling was similarly reduced in the medial gastrocnemius and soleus during standing. This study suggests differences in common synaptic input to soleus and medial gastrocnemius motoneurons, potentially associated with their distinct functional roles during standing.