Effect of changing visual condition and frequency of horizontal oscillations on postural balance of standing healthy subjects

The goal of this study was to describe the movement pattern of the body-segment rotations of healthy subjects in the horizontal plane while they were standing on a supporting platform that imposed steady sinusoidal horizontal rotations under three visual conditions: (a) eyes closed with no instructions (EC–NI), (b) eyes open with instructions to gaze at a stationary black dot located at eye level on a wall surface about four meters in front of them (EO–WI), and (c) eyes closed with instructions to imagine looking at the same target (EC–WI). The selected input signal was a sinusoid with an amplitude of ±45 deg at different frequencies equal to 0.25, 0.50 and 0.75 Hz, which were referred to as L, M and H. Bipedal balance measurements were taken in 10 adult subjects (mean age 30 ± 9 years; three men and seven women).Subjects’ kinematics were analyzed with an optoelectronic system. Under the three visual conditions, the movements of the pelvis, the trunk, and the head decreased and were inversely dependent on platform frequency; specifically, both the head and the trunk decreased their gain rotation of about 1.8–2.9 times from L to H, while the pelvis decreased its by about 1.3 times. However, the arm oscillations showed a gain and phase tendency opposite to that of the other body segments, with the gain rotation having increased of about 1.8–3.7 times from L to H.Comparing the three visual conditions, the finding suggests that the subjects were able to stabilize their head as a reference frame to maintain postural balance in a similar way under the EC–WI and EO–WI conditions. Instead, in the EC–NI trials, the subjects compensated less, in particular at the hip, the external perturbation producing higher absolute body rotations and lower relative body rotations. In fact, the head rotation was about four and three times the one showed in EC–WI and EO–WI, while for the trunk and the pelvis it was always equal to two and 1.5 times the correspondent rotation observed under the WI conditions. These results provide a quantitative assessment of compensatory balance reactions in healthy subjects to periodical horizontal perturbations.