To maintain an upright standing posture, voluntary control is indispensable together with regulation by the postural reflexes and cooperative movements controlled by the cerebellum and basal ganglia. We developed a new method for frequency analysis of the sway of the center of gravity in a standing posture (i.e. peak-area frequency spectrum analysis) and previously reported on the properties of the spectrum related to the posture regulation mechanism. In this study, we attempted to examine the characteristics of voluntary control using this method. The experimental conditions and the results are as follows.
A Natural quiet standing.
The subjects comprised 8 healthy adults ranging in age from 25 to 57. The spectrum showed power-law distribution of different slopes (value of scaling exponent) in the low frequency band (0.01～1Hz) and the high frequency band (1～5Hz). The difference in the slope shows that the system which acts on postural control is different in both bands. Based on previous studies, it is conceivable that sway in the low frequency depends on the regulation of the visual and vestibular system, and sway in the high frequency depends on regulation of the spinal proprioceptive system.
B Standing straight while actively shaking the body.
The subjects comprised 9 healthy adults ranging in age from 22 to 78. 1) The locus of center of gravity of the body (stabilogram) tended to show a circular pattern. 2) The area-frequency spectrum showed that distribution of the low frequency band indicated the same slope as distribution of the high frequency band. In voluntary body sway, regulation by the spinal proprioceptive system plays the leading role even in the low frequency band. 3) In the elderly, the spectrum showed an age-related abnormality. These findings suggest the failure of posture control by the basal ganglia precedes voluntary movement.
