Influence of passive tactile contact of arms on the maintenance of upright posture in humans

The influence of light passive contact of the forearm with a pliable external object (flexible plate) on the maintenance of upright posture was studied in healthy subjects in several conditions, with the eyes closed and on immersion in a virtual visual environment (VVE). The visual environment was either stable or unstable as a result of a synphase (SP) or antiphase (AP) association between the environment and body sway. The posture maintenance analysis focused on estimating the amplitude and frequency characteristics of two elementary variables, which were calculated from the foot center of pressure (CoP) trajectories in the mediolateral and anteroposterior directions. The variables were trajectory of the vertical projection of the center of gravity (variable CG) and difference between the CoP and CG trajectories (variable CoP–CG). In both the absence and presence of passive tactile contact, the root mean square (RMS) values of the oscillation spectra of the two variables were the lowest in the stable visual environment and in the case of the antiphase association of the environment with body sways and the highest in the cases of the synphase association and standing with the eyes closed. Passive contact decreased body sways in both directions, and the RMSs of the spectra of the two variables decreased in all visual conditions. A greater decrease in RMS was observed for the CG variable. Body sways changed not only in amplitude, but also in frequency. Tactile contact increased the median frequencies (MFs) of the CG variable spectra calculated from the anteroposterior and mediolateral body sways. In contrast, a significant increase in MFs calculated for the CoP–CG variable was observed only for anteroposterior body sways. The results showed that passive contact of a forearm with a pliable external object, which does not provide a mechanical support for the subject, significantly improves the maintenance of the upright posture even in an unstable visual environment.