Modulation of motor unit recruitment threshold and common synaptic inputs in triceps surae muscles: effects of ankle position

The objective of this study was to investigate how altering muscle length by changing ankle position affects force control, motor unit recruitment threshold, and motor unit coherence within and across triceps surae muscles. Sixteen healthy young adults performed isometric plantarflexion (PF) at three ankle positions with the ankle plantarflexed at 20° (PF20°), at the neural position (PF0°), and dorsiflexed at 20° (DF20°). High-density surface electromyography was used to record the medial and lateral heads of the gastrocnemius muscle (GM and GL) and the medial and lateral portions of the soleus muscle (SL and SM). Motor unit cumulative spike trains (MUCST) were used to calculate intramuscular, intermuscular, and Force-MUCST coherence in the delta (0-5 Hz), alpha (5-15 Hz), and beta (15-35 Hz) frequency bands. Recruitment thresholds of motor units matched across positions decreased when shifting the ankle from a shortened to lengthened position in GM, SL, and SM, while remaining unchanged and highest in GL. This change in ankle position also led to increased GM-SM coherence and intramuscular coherence in SM and SL in the delta band and improvements in force steadiness. In contrast, alpha band intramuscular coherence and Force-MUCST coherence decreased. Minimal changes were observed in beta band coherence across positions. This study reveals a neuromuscular control strategy that modulates motor unit recruitment and common synaptic inputs of triceps surae to maintain force output during isometric plantarflexions at varying muscle lengths.<b>NEW & NOTEWORTHY</b> This study demonstrates that changes in ankle position alter muscle length, impact force steadiness, motor unit recruitment thresholds, and oscillatory common inputs in the triceps surae. It highlights a neuromuscular control strategy that integrates afferent feedback, muscle mechanical properties, and descending inputs to synergistic plantarflexors. These findings provide a foundation for further investigation into muscle coordination during multijoint, dynamic movements.