Smaller sway size during quiet standing is associated with longer preceding time of motor command to body sway

In previous studies, it was found using cross-correlation analysis that the modulation of the motor command to the calf muscles largely precedes body sway during quiet standing. The purpose of this study was to investigate whether this preceding time is correlated with an improved stabilization of the body. 26 young and 23 elderly healthy subjects were asked to stand quietly. Body sway was measured using a laser displacement sensor, and the electromyogram of the right soleus was measured as a representative of the motor command. The correlation and the time shift between motor command and body sway were estimated by means of cross-correlation analysis. We found that sway size was correlated with the identified time shift: that is, a smaller sway size was associated with a longer preceding time. The obtained results suggest that a control strategy generating a larger preceding time can stabilize the body more effectively. This result was found in both the young and elderly, suggesting that the particular control aspect associated with the time shift is a common feature in both age groups.