Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, California, United States of America.
Chan Division of Occupational Science and Occupational Therapy, University of Southern California, Los Angeles, California, United States of America.
PLoS Comput Biol. 2021 Mar 8;17(3):e1008707. doi: 10.1371/journal.pcbi.1008707. eCollection 2021 Mar.
Variability in muscle force is a hallmark of healthy and pathological human behavior. Predominant theories of sensorimotor control assume 'motor noise' leads to force variability and its 'signal dependence' (variability in muscle force whose amplitude increases with intensity of neural drive). Here, we demonstrate that the two proposed mechanisms for motor noise (i.e. the stochastic nature of motor unit discharge and unfused tetanic contraction) cannot account for the majority of force variability nor for its signal dependence. We do so by considering three previously underappreciated but physiologically important features of a population of motor units: 1) fusion of motor unit twitches, 2) coupling among motoneuron discharge rate, cross-bridge dynamics, and muscle mechanics, and 3) a series-elastic element to account for the aponeurosis and tendon. These results argue strongly against the idea that force variability and the resulting kinematic variability are generated primarily by 'motor noise.' Rather, they underscore the importance of variability arising from properties of control strategies embodied through distributed sensorimotor systems. As such, our study provides a critical path toward developing theories and models of sensorimotor control that provide a physiologically valid and clinically useful understanding of healthy and pathologic force variability.
肌肉力量的可变性是健康和病理人类行为的一个显著特征。占主导地位的感觉运动控制理论假设“运动噪声”导致力的可变性及其“信号依赖性”（肌肉力量的可变性，其幅度随神经驱动强度的增加而增加）。在这里，我们证明了两种用于运动噪声的拟议机制（即运动单位放电的随机性和未融合的强直收缩）不能解释大部分力的可变性及其信号依赖性。我们通过考虑运动单位群体的三个以前未被充分认识但在生理上很重要的特征来做到这一点：1）运动单位抽搐的融合，2）运动神经元放电率、横桥动力学和肌肉力学之间的耦合，以及 3）用于跟腱和肌腱的串联弹性元件。这些结果强烈反对这样一种观点，即力的可变性和由此产生的运动学可变性主要是由“运动噪声”产生的。相反，它们强调了源自通过分布式感觉运动系统体现的控制策略的可变性的重要性。因此，我们的研究为开发感觉运动控制理论和模型提供了一条重要途径，这些理论和模型为健康和病理力量可变性提供了生理上有效和临床上有用的理解。
