Dynamic properties of body oscillation with visual tracking movement on upright posture

The purpose of this study was to determine the characteristic frequency range of body oscillation in the anterior and posterior directions during upright standing. The 24 subjects were asked to track the motion of a target with sinusoidal waveform in the frequency range of 0.05 to 1.5 Hz, by displacing their body centre of pressure (COP). The gain of the COP responses to the target showed a broad peak in the stimulus frequency range between 0.3 and 0.6 Hz. The peak accelerations of the body parts were greatest at 0.6 Hz. The resonance frequency for motion around the ankle joint was at 0.6 Hz. The resonance frequency is suggested to reflect mostly the spontaneous body sway and to be involved in the control mechanism of standing posture. In the control system of tracking movement we speculate that there are three separate systems each corresponding to one of the three frequency bands of 0.05-0.2 Hz (position control system), 0.4-0.8 Hz (optimal resonant control system), and 1.0–1.5 Hz (non-linear bang-bang control system). The locus of postural control is transferred from the ankle to the hip joint in the frequency range of 1.0–1.5 Hz.