Stationarity distributions of mechanomyogram signals from isometric contractions of extrinsic hand muscles during functional grasping

This study investigates the stationarity of steady state mechanomyogram signals for the purpose of determining appropriate features for signal classification. Mechanomyography is the superficial recording of low frequency vibrations detected over contracting muscles. Steady state mechanomyogram signals, recorded at the belly of the extensor digitorum, flexor digitorum superficialis and flexor pollicis longus muscles during functional grasps were tested for weak stationarity. Twenty percent of the contractions were found to be non-stationary, indicating that time frequency methods may be appropriate for automatic pattern recognition of functional grasp from the mechanomyogram. The distribution of the stationary test statistic was dependent on the type of muscle contractions, suggesting that the test statistic itself might be a discriminating feature for mechanomyogram pattern recognition in applications such as multifunction prosthetic control. Since the major known source of non-stationarity was decreasing variance, it is suggested that shifts in the distribution of the test statistic may indicate the time course of relative muscle contributions to functional grasp.