Abstract
The central nervous system (CNS) controls the limb movement by modulating multiple skeletal muscles with synergistic modules and neural oscillations with different frequencies between the activated muscles. Several researchers have found intermuscular coherence existing within the synergistic muscle pairs, and pointed out that the intermuscular synchronization existed when functional forces were generated. However, few studies involved the time-varying characteristics of the intermuscular coherence in each synergy module though all activated muscles keep in a dynamic and varying process. Therefore, this study aims to explore the time-varying coherence amongst synergistic muscles during movements based on the combination of the non-negative matrix factorization (NMF) method and the time-frequency coherence (TFC) method. We applied these methods into the electromyogram (EMG) signals recorded from eight muscles involved in the sequence of the wrist movements [wrist flexion (WF), wrist flexion transmission to wrist extension (MC) and wrist extension (WE)] in 12 healthy people. The results showed three synergistic flexor pairs (FCR-PL, FCR-FDS, and PL-FDS) in the WF stage and three extensor pairs (ECU-ECR, ECU-B, and ECR-B) in both MC and WE stages. Further analysis showed intermuscular coherence between each pairwise synergistic muscles. The intermuscular coherence between the flexor muscle pairs was mainly observed in the beta band (15–35 Hz) during the WF stage, and that amongst the extensor muscle pairs was also observed in the beta band during the WE stage. However, the intermuscular coherence between the extensor muscle pairs mainly on gamma band during the MC stage. Additionally, compared to the flexor muscle pairs, the intermuscular coherence of the extensor muscle pairs were lower in the WF stage, and higher in both MC and WE stages. These results demonstrated the time-varying mechanisms of the synergistic modulation and synchronous oscillation in motor-control system. This study contributes to expanded researches for motor control.
Highlights
The central nervous system (CNS) regulates the movement by modulating multiple skeletal muscles (Gottlieb, 1998)
The EMG signals of the palmaris longus (PL), flexor digitorum superficialis (FDS), and flexor carpi radialis (FCR) muscles showed a high peak during the wrist flexion (WF) stage, while the EMG signals of the B and extensor carpi radialis (ECR) muscles showed a high peak during the wrist extension (WE) stage
The AZ value in the beta band is the largest compared with the other bands during both WF and WE stages for flexor and extensor muscle pairs, and it is largest in the gamma band during MC stage for extensor muscle pairs
Summary
The central nervous system (CNS) regulates the movement by modulating multiple skeletal muscles (Gottlieb, 1998). In this process, modular structures are often used to organize and coordinate multiple degree-of-freedom change among muscles (Carpenter, 1968; d’Avella et al, 2003), and different movement behaviors are formed by arranging certain synergistic modules (Ting and Mckay, 2007). Considering that the NMF method can break the data into a target matrix without any negative, large studies have applied this method to analyze the muscle activation during pedaling (De et al, 2015), walking (Haghpanah et al, 2017), and elbow movement (Tang et al, 2017) and so on. Muscle synergy can reflect the relationship of the combination and coordination among the multiple muscles and provide insight into the overall muscular activity (d’Avella et al, 2013), it has limitation in exploring the synchronous oscillations among muscles that play an important role in motor-control system (Li et al, 2016)
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