Though human erect standing posture is very unstable, constant incline of the body due to gravity, etc., will always be restored holding a dynamic equillibrium ("homeostasis") of the posture to exhibit only invisible sway. Hence, the sway can be regarded as the response of the "posture control (holding) system" in the body elicited by various stimulations (gravity, ect.).
Random time series of an appropriately magnified sway was able to describe as a higher-order autoregressive (AR-) process given as a dynamic extension of the basic static "stimulation-system (activity)-response" relation of a bio-system in the body. The AR-power spectral density of the postural sway, therefore, elucidated an average frequency-pattern of the higher-order posture control activity, which was decomposed into one or two first- and some secondorder component activities. One of the former exhibited more than 90% power displaying a damped exponential average time-pattern and a peak at OHz in its power spectrum, while each of the latter did less than about 5% power displaying a damped oscillatory average time-pattern with a damping frequency and a peak at the respective resonance frequency in its spectrum. In addition to the power, the smoothness and/or regularity of the posture controlv activity was given as the "bio-informing activity amount".
Applying bivariate AR-processes to the anteroposterior sways of the center of gravity, head and sway of the dorsi-plantar flexion of the ankle joint, the "interactive control activities" between two of them were elucidated, which displayed also the average time-and frequency-patterns as the "impulse" and "frequency responses", respectively. No first-order component responses, but only the second-order ones were observed in the interactive posture control activities.
