Centre of Precision Rehabilitation for Spinal Pain (CPR Spine), School of Sport, Exercise and Rehabilitation Sciences, College of Life and Environmental Sciences, University of Birmingham, Birmingham, United Kingdom.
Department of Clinical and Experimental Sciences, Università degli Studi di Brescia, Brescia, Italy.
J Neurophysiol. 2024 Oct 1;132(4):1198-1210. doi: 10.1152/jn.00391.2023. Epub 2024 Sep 4.
Recent studies combining high-density surface electromyography (HD-sEMG) and ultrasound imaging have yielded valuable insights into the relationship between motor unit activity and muscle contractile properties. However, limited evidence exists on the relationship between motor unit firing properties and tendon morpho-mechanical properties. This study aimed to determine the relationship between triceps surae motor unit firing properties and the morpho-mechanical properties of the Achilles tendon (AT). Motor unit firing properties [i.e. mean discharge rate (DR) and coefficient of variation of the interspike interval (COVisi)] and motor unit firing-torque relationships [cross-correlation between cumulative spike train (CST) and torque, and the delay between motor unit firing and torque production (neuromechanical delay)] of the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SO) muscles were assessed using HD-sEMG during isometric plantarflexion contractions at 10% and 40% of maximal voluntary contraction (MVC). The morpho-mechanical properties of the AT (i.e. length, thickness, cross-sectional area, and resting stiffness) were determined using B-mode ultrasonography and shear-wave elastography. Multiple linear regression analysis showed that at 10% MVC, the DR of the triceps surae muscles explained 41.7% of the variance in resting AT stiffness. In addition, at 10% MVC, COV SO predicted 30.4% of the variance in AT length. At 40% MVC, COV MG and COV SO explained 48.7% of the variance in AT length. Motor unit-torque relationships were not associated with any morpho-mechanical parameter. This study provides novel evidence of a contraction intensity-dependent relationship between motor unit firing parameters of the triceps surae muscle and the morpho-mechanical properties of the AT. By employing HD-sEMG, conventional B-mode ultrasonography, and shear-wave elastography, we showed that the resting stiffness of the Achilles tendon is related to mean discharge rate of triceps surae motor units during low-force isometric plantarflexion contractions, providing relevant information about the complex interaction between rate coding and the muscle-tendon unit.
最近的研究结合高密度表面肌电图（HD-sEMG）和超声成像，深入了解了运动单位活动与肌肉收缩特性之间的关系。然而，关于运动单位放电特性与肌腱形态力学特性之间的关系，证据有限。本研究旨在确定比目鱼肌运动单位放电特性与跟腱（AT）形态力学特性之间的关系。使用 HD-sEMG 在等长跖屈收缩时评估 10%和 40%最大随意收缩（MVC）下内侧腓肠肌（MG）、外侧腓肠肌（LG）和比目鱼肌（SO）的运动单位放电特性[即平均放电率（DR）和峰间间隔变异系数（COVisi）]和运动单位放电-扭矩关系[累积尖峰序列（CST）和扭矩之间的互相关，以及运动单位放电和扭矩产生之间的延迟（神经机械延迟）]。使用 B 型超声和剪切波弹性成像确定 AT 的形态力学特性（即长度、厚度、横截面积和静息刚度）。多元线性回归分析显示，在 10% MVC 时，比目鱼肌的 DR 解释了静息 AT 刚度变异的 41.7%。此外，在 10% MVC 时，SO 的 COV 预测了 AT 长度变异的 30.4%。在 40% MVC 时，MG 和 SO 的 COV 解释了 AT 长度变异的 48.7%。运动单位-扭矩关系与任何形态力学参数都没有关联。本研究提供了新的证据，证明比目鱼肌运动单位放电参数与 AT 形态力学特性之间存在收缩强度依赖性关系。通过使用 HD-sEMG、常规 B 型超声和剪切波弹性成像，我们表明，在低强度等长跖屈收缩时，跟腱的静息刚度与比目鱼肌运动单位的平均放电率有关，这为了解运动单位的速率编码与肌肉-肌腱单元之间的复杂相互作用提供了相关信息。
