Abstract
PurposeMuscle–tendon length can influence central and peripheral motor unit (MU) characteristics, but their interplay is unknown. This study aims to explain the effect of muscle length on MU firing and contractile properties by applying deconvolution of high-density surface EMG (HDEMG), and torque signals on the same MUs followed at different lengths during voluntary contractions.MethodsFourteen participants performed isometric ankle dorsiflexion at 10% and 20% of the maximal voluntary torque (MVC) at short, optimal, and long muscle lengths (90°, 110°, and 130° ankle angles, respectively). HDEMG signals were recorded from the tibialis anterior, and MUs were tracked by cross-correlation of MU action potentials across ankle angles and torques. Torque twitch profiles were estimated using model-based deconvolution of the torque signal based on composite MU spike trains.ResultsMean discharge rate of matched motor units was similar across all muscle lengths (P = 0.975). Interestingly, the increase in mean discharge rate of MUs matched from 10 to 20% MVC force levels at the same ankle angle was smaller at 110° compared with the other two ankle positions (P = 0.003), and the phenomenon was explained by a greater increase in twitch torque at 110° compared to the shortened and lengthened positions (P = 0.002). This result was confirmed by the deconvolution of electrically evoked contractions at different stimulation frequencies and muscle–tendon lengths.ConclusionHigher variations in MU twitch torque at optimal muscle lengths likely explain the greater force-generation capacity of muscles in this position.
Highlights
IntroductionAlessandro Cudicio and Eduardo Martinez-Valdes contributed to this work
Communicated by Toshio Moritani.Alessandro Cudicio and Eduardo Martinez-Valdes contributed to this work.Department of Clinical and Experimental Sciences, Università degli Studi di Brescia, Brescia, ItalyCentre of Precision Rehabilitation for Spinal Pain (CPR Spine), School of Sport, Exercise and Rehabilitation Sciences, University of Birmingham, Birmingham, UKThe amount of force that is generated by a muscle and its rate of change in force depend on both its length and shortening velocity (Askew and Marsh 1998; Hager et al 2020)
A total of 1848 motor units were identified across the different torque levels and ankle angles in the tibialis anterior (TA), with an average of 66 ± 27 motor units per subject
Summary
Alessandro Cudicio and Eduardo Martinez-Valdes contributed to this work. The amount of force that is generated by a muscle and its rate of change in force depend on both its length and shortening velocity (Askew and Marsh 1998; Hager et al 2020). When considering isometric voluntary contractions, a muscle can generate its highest force at its optimal length. A muscle’s maximal force output can be significantly reduced when shortened or stretched in relation to its optimal length (Haffajee et al 1972; Christova et al 1998; Del Valle and Thomas 2004). Previous literature suggested that this phenomenon is mainly related to changes in peripheral properties such as the amount of actin–myosin overlap and Ca2+ sensitivity (see Rassier et al 1999 for a review), other
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