KINESTHETIC REFERENCE FOR HUMAN ORTHOGRADE POSTURE

Humans with occluded vision were subjected to superslow tilts of the s upporting platform, producing the inclination of the subject's body in the sagittal plane, but subthreshold for the most vestibular and prop rioceptive phasic reactions. Two types of perturbation were used: sinu soidal tilts (frequency 0.007 Hz, amplitude 1.5 degrees) and ramps (am plitude 1.0 and 0.25 degrees, angular velocity 0.04 degrees/s). During slow sinusoidal tilts of the platform, the ankle angle and body posit ion undergo periodical changes, but these changes have significant pha se lead relative to the platform movement: 119 +/- 26 degrees for ankl e angle and 55 +/- 19 degrees 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 til t produced an initial body deviation followed by a slow (seconds or te ns of seconds) approach of body position to a new steady level after t he termination of ramp. Large slow body movements were superimposed wi th small irregular oscillations (about 10% of the amplitude of large d isplacements) of higher frequency. These oscillations resembled normal stabilograms on a stationary support. Thus, the usual process of stab ilization of body gravity center was continued, though not around a fi xed set-point but relative to a slowly changing position. Data obtaine d support the hypothesis that, besides operative control assigned to c ompensate 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, musc ular torques and interaction with the support on the basis of criteria taking into account the energy cost of standing and demands for stabi lity and security.