Frequency analysis of center of pressure in anteroposterior direction sway. (A) Grand average waveforms of the frequency spectrum of center of pressure in anteroposterior direction (CoPap) sway. The frequency spectrum was averaged across right- and left-leg stances. No osc: No-oscillation condition. (B) Amplitude of the low-frequency peak of the frequency spectrum in no-oscillation and oscillation conditions. Asterisks and daggers indicate significant differences relative to no oscillation and 0.25 Hz oscillation, respectively. *P < 0.05, **P < 0.01, ***P < 0.001, †P < 0.05. (C) Ratio of the amplitude of frequency spectrum at oscillation frequency to that at 0.061 Hz. Asterisks indicate significant differences relative to 1.5 Hz. *P < 0.05, ***P < 0.001.