Dynamic Analysis of Muscle Coordination at Different Force Levels during Grip and Pinch with Multiplex Recurrence Network.

Muscle synergistic contraction to produce force has been recognized as an important neurophysiological mechanism in neuromuscular system. Despite a range of approaches, such as nonnegative matrix factorization or principal component analysis that have been widely used, limitations still exist in analysis of dynamic coordination of multiple muscles. In addition, it is still less studied about the potential difference of muscle dynamic coordination at different force levels during grip and pinch within the same framework. With this aim, this study analyzed the dynamic coordination of multiple upper-limb muscles at low, medium and high force levels during pinch and grip with multiplex recurrence network (MRN). Twenty-four healthy subjects participated in the experiment. Subjects were instructed to grip an apparatus to match the target force as stably as they could for 10 s. Surface electromyographic (sEMG) signals were recorded from 8 upper-limb muscles and analyzed by the MRN. The interlayer mutual information (I) and the average edge overlap (ω) of MRNs were calculated to quantify muscle correlation and muscle synchronization, respectively. Results showed that I and ω values of extrinsic muscles' MRNs during grip were significantly higher than that during grip at medium and high force. Furthermore, the I and ω values of extrinsic muscle networks during grip increased with augmented force levels. No significant changes were found for the intrinsic muscles with force output levels. These findings indicate that the muscles coordination patterns between grip and pinch were different and higher co-contraction of extrinsic muscles is favorable to synergistic force production. With the force output increased, muscles' coordination was augmented in extrinsic muscles, but no change in intrinsic muscles because of independent and complicated control of fingers. This study provides an analytical tool for dynamic muscles coordination and provides insights into the mechanisms of synergistic control of muscle contractions for force production.Clinical Relevance-This study provides a novel analytical tool for muscle coordination during force production, which may facilitate the evaluation of neuromuscular function or serve as indicators for neuromuscular disorders.