Human bipedal balance is controlled by feedback mechanism, which is appropriate activity of plantar flexors (PF) in response to center of mass (CoM) velocity (CoMvel). Based on the neurophysiological point of view, human equilibrium is able to be evaluated by the result obtained from cross-correlation function (CCF) from CoMvel to muscle activity of medial gastrocnemius (MG). The purpose of present study was to establish the assessment system for human balance on the basis of feedback control mechanism by using mechanomyogram (MMG). During the quiet stance in young (n = 20) subjects for 60-s, CoM, CoMvel, electromyogram (EMG) of MG and soleus (SOL), and MMG of PF were measured. Cross-spectral analysis from MMG to CoM, and EMGs indicated that MMG was coherent with CoM, EMG of MG, and EMG of SOL in frequency range from 0 to 1Hz, from 2 to 4Hz, and from 8 to 12Hz, respectively. Lowpass filtered MMG (cutoff frequency = 1Hz) was significantly related to the CoM, indicating that low-frequency component of MMG reflects the CoM fluctuation. Furthermore, time differentiation of filtered MMG (d MMG/dt) was strongly correlated with CoMvel. From cross-spectral analysis, variable from low-pass filtered MMG (cutoff frequency = 4Hz) to low-pass filtered MMG (cutoff frequency = 1Hz) would represent the muscle activities of MG. We compared the CCF results from two different assessments. As a result, the positive peak value from CCF between CoMvel and EMG of MG was significantly correlated with that from CCF between low-pass filtered MMG and variable from MMG signals. To evaluate the balance with aging by using MMG, the positive peak value of CCF from MMG analyses was compared between young and elderly subjects. The positive peak value was significantly higher in young than in elderly subjects, suggesting that the postural control based on the neurophysiological view in the elderly is inferior to that in the young adults. These results lead us to conclude that an appropriate analysis of MMG is able to assess the human balance controlled by feedback mechanism on the basis of neurophysiological point of view.

DESCENTE SPORTS SCIENCE Vol.28/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE