Assessment of postural stability in young healthy subjects based on directional features of posturographic data: vision and gender effects.

Directional characteristics of postural sway and their impact on postural stability control (PSC) still need to be clarified. To assess the directional characteristics of PSC, center-of-pressure (COP) while standing quiet was recorded and analyzed in 46 young healthy subjects. The participants completed five 60-s trials with 'eyes open' and five with 'eyes closed'. The control asymmetry was characterized using three standardized COP measures: mean sway velocity (V), sway ratio (SR), and sway directional index (DI). The COP data were low-pass filtered at 10 Hz. In such conditions the V, SR and DI revealed consistent and unique differences in both the anteroposterior (AP) and the mediolateral (ML) controls. Each of the COP measures showed the involvement of different neuromuscular and biomechanical mechanisms in the stabilization of standing posture. The DI documented a very stable proportion between frontal and saggital controls, with roughly 60% of the swaying movements being performed in the AP direction. The AP DI was significantly higher in comparison with the ML value (0.71±0.04 vs. 0.55±0.06). This proportion remained at the same level in both experimental groups and it was only slightly affected by the visual conditions. Analysis of the SR confirmed that to maintain stable stance the neuromuscular system must allocate 50% more effort to control AP stability. The results also documented gender-related differences in postural stability. Generally the female subjects appeared to have lower postural stability as evidenced by higher COM and COP velocities. Consequently the higher SR values observed in the female group indicate a need for higher neuromuscular efforts to maintain stable posture. The introduced here set of COP measures proved to be a valuable standard for static posturography, creating the opportunity for improved and reliable assessment of postural stability.