Action Direction of Muscle Synergies in Three-Dimensional Force Space.

Shota Hagio,Motoki Kouzaki

doi:10.3389/fbioe.2015.00187

Abstract

Redundancy in the musculoskeletal system was supposed to be simplified by muscle synergies, which modularly organize muscles. To clarify the underlying mechanisms of motor control using muscle synergies, it is important to examine the spatiotemporal contribution of muscle synergies in the task space. In this study, we quantified the mechanical contribution of muscle synergies as considering spatiotemporal correlation between the activation of muscle synergies and endpoint force fluctuations. Subjects performed isometric force generation in the three-dimensional force space. The muscle-weighting vectors of muscle synergies and their activation traces across different trials were extracted from electromyogram data using decomposing technique. We then estimated mechanical contribution of muscle synergies across each trial based on cross-correlation analysis. The contributing vectors were averaged for all trials, and the averaging was defined as action direction (AD) of muscle synergies. As a result, we extracted approximately five muscle synergies. The ADs of muscle synergies mainly depended on the anatomical functions of their weighting muscles. Furthermore, the AD of each muscle indicated the synchronous activation of muscles, which composed of the same muscle synergy. These results provide the spatiotemporal characteristics of muscle synergies as neural basis.

Highlights

The fundamental problem in motor control is how the central nervous system (CNS) controls the immense number of variables in the musculoskeletal system (Bernstein, 1967)
An appropriate approach has been taken using EMG-weighted averaging (EWA) method (Kutch et al, 2010; Imagawa et al, 2013). This was formulated as a non-invasive technique instead of the spike-triggered averaging (STA), i.e., a well-established method to extract the force associated with single motor unit (SMU) contractions, based on the hypothesis that surface EMG is analogous to a superposition of SMU action potentials and its cross-correlation with endpoint force should produce the equivalent of an average spike-triggered force averaged across multiple motor units (Kutch et al, 2010)
We estimated the action direction (AD) of lower limb muscle synergies during isometric force-maintaining tasks on three-dimensional force space

Summary

Introduction

The fundamental problem in motor control is how the central nervous system (CNS) controls the immense number of variables in the musculoskeletal system (Bernstein, 1967). Previous research estimated the mechanical contribution of each muscle synergy (called as synergy-to-force mapping) by assuming the linear relationship between EMG (further linearly decomposed into muscle vectors of muscle synergies) and endpoint force in isometric condition (Berger and d’Avella, 2014). These techniques were advantageous to quantify the force vector produced by each muscle synergy in the force space. Our previous study directly compared the spatiotemporal correlation between the activation coefficients of muscle synergies and endpoint force fluctuations during voluntary isometric conditions, demonstrating the significant correlation between them (Hagio and Kouzaki, 2015), the mechanical contribution of muscle synergies in the task space was not estimated. It should be noted that we assumed the neural basis of muscle synergies: the estimated activation of muscle synergies represents the summation of the individual basis constructing muscle synergies, which might have been regarded as spinal interneuron in the previous studies (Hart and Giszter, 2010; Overduin et al, 2014)

Objectives

Methods

Results

Conclusion