[Spectral Analysis of Sway of Human Body When Standing on Firm and Compliant Surfaces under Different Visual Conditions.].

The influence of different visual conditions on maintenance of vertical posture when standing on a firm and a compliant surface were compared. Motionless visual environment (MVE), closed eyes (CE) and virtual visual environment (VVE) were used as visual conditions. Displayed VVE included foreground and background. The foreground represented a room window with the walls adjoining to it, and the background - an aqueduct with the adjacent terrain. Destabilization of the VVE was produced by assignmentinphase or anti-phase relationship between the position of the foreground of visual scene and the body sway. We estimated changes in the amplitude spectra of two elementary variables calculated from trajectories of displacement of the center of pressure (CoP) in the anteroposterior and lateral directions, namely, trajectories of the center of gravity (CG variable) and differences between trajectories of CoP and CG (CoP-CG variable). The trajectory of CG was considered as a controlled variable, and a difference between CoP and CG trajectories - as the variable connected with acceleration of body and related to changes of resultant stiffness in ankle joints. In the condition of inphaserelation RMS of spectra of the CG variable were about the same, as when standing with CE, and RMS of spectra of the CoP-CG variable were significantly less, than at CE. When standing on acompliantsurface, body sway was increased significantly both in anteroposterior and lateral directions under all visual conditions. Moreover, during standing with CE RMS ofspectra of both variables were increased significantly more, than during standing with in phaserelation between the foreground of visual scene and the body sway. Furthermore, when standing on a compliant support RMS of spectra of CG variable, calculated from the body sway in the lateral direction, under the condition of antiphase relation were significantly greater than under the condition of inphaserelation, while RMS of spectra of CoP-CG variable had similar values underboth conditions. The results of the analysis of body sway showed that under somevisual conditions the transition from standing on a firm support to the standing on acompliant support have not causeda propor- tional change of amplitude characteristics of CG and CoP-CG variables. It is supposed that the revealed disproportion of changes of these two variables is probably connected with a contribution to posturalcontrol of one additional factor: the passive elastic component of muscular-articular stiffness generated by fascial-tendon tissue.