Intermuscular coupling characteristics based on variational mode decomposition-coherence

Intermuscular coupling is defined as the interaction, correlation and coordination between different muscles during the body movement, which could be revealed by the synchronization analysis of surface electromyogram (sEMG). The multiscaled coherence analysis of sEMG signals could describe the multiple spatial and temporal functional connection characteristics of intermuscular coupling, which could be helpful for understanding the multiple spatial and temporal coupling mechanism of neuromuscular system. Furthermore, the coupling characteristics in frequency band of sEMG generally reflect the functional connection between muscles which relate to motion control and coordinative mechanism of the central nervous system (CNS). In this paper, we combine variational mode decomposition (VMD) and intermuscular coherence (IMC) analysis to propose a new method named VMD-IMC to quantitatively describe the muscular coupling characteristics in the corresponding frequency bands. First, sEMG data of flexor digitorum superficialis (FDS), flexor carpi ulnaris (FCU) and extensor digitorum (ED) are recorded simultaneously from twenty healthy subjects (2 5 +/- 3 years) who perform the designed grip task at sustained 20% maximum voluntary contraction under the static load. Then, the VMD approach is employed to adaptively decompose sEMG into several intrinsic mode functions to describe the information about different time-frequency scales. Furthermore, the coherence on different time-frequency scales between different sEMG signals is analyzed, and the significant coherent area index is calculated to quantitatively describe the functional coupling characteristics of the feature bands. And combining VMD with Hilbert transform, we calculate root mean square and mean instantaneous frequency (MIF) to describe the variations of energy and frequency of each muscle. The results show that coupling strengths increase with time, respectively, in beta (15-30 Hz) and gamma (30-45 Hz) band between two muscles (FDS vs FCU, FDS vs ED) during the sustained static force with low load. In addition, compared with the coupling between FDS and ED, the couplings between FDS and FCU in beta and gamma band under the condition of fatigue present more significant changes and similar trend in MIF variation with time. The obtained results reveal that the congenerous muscle is coordinated by CNS in a more synchronous way during the sustained isometric fatiguing contraction.