Humans with occluded vision were subjected to superslow tilts of the supporting platform, producing the inclination of the subject's body in the sagittal plane, but subthreshold for the most vestibular and proprioceptive phasic reactions. Two types of perturbation were used: sinusoidal tilts (frequency 0.007 Hz, amplitude 1.5°) and ramps (amplitude 1.0 and 0.25°, angular velocity 0.04°/s). During slow sinusoidal tilts of the platform, the ankle angle and body position undergo periodical changes, but these changes have significant phase lead relative to the platform movement: 119±26° for ankle angle and 55±19° for body sway. Gains were about 0.9 for both parameters. Large phase shift (tens of seconds) indicated a long delay in compensation of body inclination by ankle joint. The ramp tilt produced an initial body deviation followed by a slow (seconds or tens of seconds) approach of body position to a new steady level after the termination of ramp. Large slow body movements were superimposed with small irregular oscillations (about 10% of the amplitude of large displacements) of higher frequency. These oscillations resembled normal stabilograms on a stationary support. Thus, the usual process of stabilization of body gravity center was continued, though not around a fixed set-point but relative to a slowly changing position. Data obtained support the hypothesis that, besides operative control assigned to compensate deviations from a reference position, the system of postural control includes at least one additional level, which elaborates this reference using information about mutual position of body links, muscular torques and interaction with the support on the basis of criteria taking into account the energy cost of standing and demands for stability and security.
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