Modulation of Common Synaptic Inputs and Motor Unit Recruitment Threshold in TricepsSurae Muscles: Effects of Ankle Position

Abstract

The objective of this study was to investigate how altering muscle length by changing ankle position affects force control, motor unit recruitment, 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{degrees} (PF20{degrees}), at the neural position (PF0{degrees}) and dorsiflexed at 20{degrees} (DF20{degrees}). 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. Changing the ankle position from a shortened to lengthened position resulted in increased GM-SM coherence in the delta and alpha bands and improved force steadiness, while intramuscular coherence and Force-MUCST coherence decreased in the alpha band. However, there were minimal changes in beta band coherence. Motor unit recruitment thresholds of GM and SL were reduced with muscle lengthening, while GL showed greater thresholds at lengthened positions, and SM was unaffected by ankle position. These findings highlight the role of inhibitory inputs associated with changes in muscle length and the modulation of these inputs by neighboring synergistic muscles. This study reveals a neuromuscular control strategy that modulates common synaptic inputs and motor unit recruitment of triceps surae to maintain force output during isometric plantarflexions at varying muscle lengths. Key pointsO_LIWhen comparing isometric contractions at different ankle joint angles, a lengthened position resulted in greater shared common inputs between the medial head of the gastrocnemius and the medial compartment of the soleus in delta and alpha frequency bands during submaximal force production. C_LIO_LIIncreased muscle length resulted in improved force steadiness, as well as reduced common synaptic inputs within each triceps surae muscle and coupling between force output and neural drive in the alpha band. C_LIO_LIShifting the ankle position from a shortened to a lengthened position resulted in a reduced motor unit recruitment threshold of GM and SL. However, GL exhibited an opposite pattern, showing higher recruitment thresholds at lengthened positions, while SM was not affected by changes in muscle length. C_LIO_LIThis study reveals a neuromuscular control strategy that integrates afferent feedback and muscle mechanical properties while modulating common descending inputs to synergistic plantarflexors. C_LI