Analyzing cross-talk in Mechanomyographic signals of forearm muscles during grip force task and different wrist postures
Md. Anamul Islam
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In mechanomyography (MMG), cross-talk refers to the contamination of the signal from the muscle of interest by a signal from another muscle or muscle group in close proximity. This study analyzed the cross-talk in MMG signals generated by the extensor digitorum (ED), extensor carpi ulnaris (ECU), and flexor carpi ulnaris (FCU) muscles during submaximal to maximal isometric contractions of the grip force (MVIC) and wrist postures of flexion (WF) and extension (WE), and radial (RD) and ulnar (UD) deviations. Twenty, healthy right-handed men (mean ± SD: age = 26.25 ± 3.13 y) participated in this study. During each muscle action, three microelectromechanical systems (MEMS)-based tri-axial accelerometers (full-scale range = ± 3g, typical frequency response = 0.5-500 Hz, sensitivity = 330 mV/g) were used to obtain the MMG signals from the longitudinal (Lo,), lateral (La) and transverse (Tr) directions with respect to muscle fibres. Peak cross-correlation coefficients at zero time lags were used for quantification of the cross-talk. Repeated-measures analysis of variance (ANOVA) followed by least significant difference (LSD) post hoc tests at a significant level = 0.05 were performed to analyze the cross-talk. The level of cross-talk in the MMG signals generated by the three axes of the muscles ranged from R2 xy = 27-70% for the Lo-axis, 14-53% for the La-axis, and 9-26% for the Tr-axis providing significantly lower crosstalk in the Tr-axis MMG signals for all the wrist postures (p < 0.05). Additionally, the wrist postures, except the RD, did not significantly influence the level of cross-talk between the muscles (p > 0.05). There were strong positive correlations between the level of cross-talk and the grip forces for the muscles (r2 ≥ 0.857). The cross-talk also occurred among the MMG signals due to force tremor (MMGTF), slow-firing (MMGSF) and fast-firing (MMGFF) motor unit fibres for the muscles with significantly greater and smaller cross-talk values for the MMGTF and MMGFF signals, respectively (p < 0.05). There were weak positive correlations between the level of cross-talk and circumference of the forearm during maximally activated MMG signals (r2 ≤ 0.216). However, there were weak negative correlations between the cross-talk and the length of forearm (r2 ≤ 0.082) and the cross-talk and muscles’ skin-fold thickness (r2 ≤ 0.30). The results may be used to improve our understanding on mechanics of the forearm muscles during the wrist postures and gripping task for using the MMG technique.