Frequency features of mechanomyographic signals of human soleus muscle during quiet standing

The purpose of the present study was to determine whether the frequency features of the MMG signals during quiet standing reflect body sway as well as recurring muscle activity. Twenty healthy men maintained quiet standing in a barefoot position with their eyes open or closed. During quiet standing, MMG detected using uniaxial piezoresistive accelerometer and surface electromyogram (EMG) signals were recorded from the soleus (SOL) muscle, and the center of mass (CoM) displacement (CoMdis) in the anteroposterior direction was measured by a high-resolution laser displacement sensor. In addition, CoMdis was time-differentiated to yield CoM velocity (CoMvel). Cross-spectral analysis revealed that significant coherency spectra from MMG to CoMdis and from MMG to rectified EMG of SOL were observed below 2 Hz and 8-12 Hz frequency band, respectively. Furthermore, we revealed that the trajectories of MMG and the calculated dMMG/dt were significantly correlated to CoMdis and CoMvel, respectively. These results suggest that kinematic and physiological parameters of postural control during quiet standing can be quantified by frequency features of the MMG.