This study was designed to investigate the change of postural control while repeatedly imposing the horizontal floor vibration in upright stance. A vibration table mounted with a force platform was vibrated sinusoidaly in anteroposterior direction under the condition of 2.5 cm amplitude and 0.5 Hz frequency. Ten female subjects, aged from 18 to 21 years, were equally divided into O-group and C-group. The subjects maintained the standing posture on the vibration table, for one minute with open and closed eyes in both groups in the first trial, for two minutes with open eyes in O-group and with closed eyes in C-group in the 2 nd to 11 th trials, and for one minute with closed eyes in O-group and open eyes in C-group in the 12th trial. The fluctuation of the center of foot pressure (CFP) in anteroposterior direction and EMGs of m. tibialis anterior and m. gastrocnemius were analyzed. The controllability of standing posture was evaluated by the mean speed of the CFP fluctuation. The muscle activity was examined with EMGs. The results were summarized as follows :
1) In a great number of subjects, the controllability of standing posture rapidly improved till the 3rd trial in each eye condition, while after the 3 rd trial changes of controllability were relatively small. Accordingly, it was suggested that by investigating the change of controllability for 5 trials, we could sufficiently detected an individual difference of the learning ability of postural control.
2) At the beginning of the practice, the controllability with open eyes was greatly superior to that with closed eyes. Although the difference of controllability between open and closed eyes decreased with advance of practice, in the 11 th trial that difference was obviously observed.
3) In a great number of subjects, the phase lag of the CFP fluctuation to the floor vibration increased till the 3rd trial according to improving controllability. In some subjects, the change of controllability was relatively small, the change of phase lag was small and also, correlations between the postural controllability and the phase lag weren't significant.
4) M. gastrocnemius was active when the CFP fluctuated forward and m. tibialis anterior when the CFP fluctuated backward greatly. In each eye condition, the magnitude of muscle activity decreased with practice and m. tibialis anterior was inactive in a great number of subjects in the last stage of the practice.
5) The controllability with open eyes didn't show a significant change after practice with closed eyes. By contrast, the controllability with closed eyes improved greatly after practice with open eyes and was approximately equal to that in the last stage of practice with closed eyes.