Muscle synergies are associated with intermuscular coherence in an isometric upper limb task

Abstract

To elucidate the underlying physiological mechanism of muscle synergies, we investigated the functional corticomuscular and intermuscular binding during an isometric upper limb task in 14 healthy participants. Cortical activity was recorded using 32-channel encephalography (EEG) and muscle activity using 16-channel electromyography (EMG). Using non-negative matrix factorization (NMF), we calculated muscle synergies from two different tasks. A preliminary multidirectional task was used to identify synergy preferred directions. A subsequent coherence task, consisting of generating forces isometrically in the synergy PDs, was used to assess the functional connectivity properties of synergies. Functional connectivity was estimated using corticomuscular coherence (CMC) and intermuscular coherence (IMC). Overall, we were able to extract four different synergies from the multidirectional task. A significant alpha band IMC was present consistently in all extracted synergies. Moreover, alpha band IMC was higher between muscles with higher weights within a synergy. In contrast, no significant CMC was found between the motor cortex area and synergy muscles. In addition, there is a relationship between a synergy muscle weight and the level of IMC. Our findings suggest the existence of a consistent shared input between muscles of each synergy. Finally, the existence of a shared input onto synergistic muscles within a synergy supports the idea of neurally-derived muscle synergies that build human movement.

Generalizability of muscle synergies in isometric force generation versus point-to-point reaching in the human upper extremity workspace

“…Isometric tasks have been studied in motor control from a muscle synergy perspective, and these studies have demonstrated that different isometric motor behaviors can be accomplished by activating a small number of intermuscular patterns in the upper body ( Rashedi et al, 2010 ; Borzelli et al, 2012 , 2013 ; Berger and d’Avella, 2014 ; Budhota et al, 2017 ; Ortega-auriol et al, 2019 ; Roh et al, 2019 ). For instance, four to five synergies were required to predict upper extremity muscle activation across different spatial, load, and position protocols ( Roh et al, 2012 ), and during visuomotor adaptation ( Gentner et al, 2013 ).…”