The purpose of this study is to examine the muscle synergy during grip and pinch using recurrence networks (RNs). Twenty-four right-handed healthy young subjects participated in the experiment. The grip force and pinch force of the dominant hand were examined, during which the surface electromyographic (sEMG) signals were recorded from brachioradialis (BR), flexor carpi ulnaris (FCU), flexor carpi radialis (FCR), extensor digitorum communis (EDC), flexor digitorum superficialis (FDS), abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti minimi (ADM) of the dominant hand. The RNs were constructed based on the time series of sEMG signals. Two parameters-the average shortest path length ($mathcal{L}$) and the clustering coefficient ($mathcal{C}$) - were achieved from the RNs to analyze the sEMG. Results showed higher $mathcal{C}$ but lower $mathcal{L}$ in BR, FCU and FCR during grip than during pinch. In contrast, the FDI showed lower $mathcal{C}$ but higher $mathcal{L}$ during grip than during pinch. Significant differences of the two parameters were found among the three force levels in the BR, FCU, FCR. With increased force, the muscle networks of BR, FCU and FCR showed increases in $mathcal{C}$ or decreases in $mathcal{L}$. Our study suggests different muscle synergies between grip and pinch, and the extrinsic muscles play an important role in synergistic force production, the intrinsic muscles are performed well in fingers control for motor fine tasks. In addition, synergistic muscles will further coordinate in timing and strength with the force level increased. This finding might provide insights into the dynamical coordination across muscles with the force outputs and supply novel strategy for evaluating the neuromuscular function and making of the myoelectric prosthesis.
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