Direct parameterization of postural stability during quiet upright stance: effects of age and altered sensory conditions.

The purpose of this study was to determine the relationship between measures of local dynamic stability (LDS) during upright stance and both descriptive measures of postural sway and a scaling index (alpha) derived from detrended fluctuation analysis. Center of pressure (COP) time series were obtained from healthy participants (16 young and 16 older) during upright quiet stance. Vision and somatosensation were altered by eye closure and standing on a compliant surface, respectively. A non-linear time-series analysis method was used to compute three LDS parameters from the COP data: A which was defined as the COP excursion range in state space, and tau(S) and tau(L) which were defined as the divergence rates over short- and long-term timescales, respectively. LDS parameters, descriptive COP measures, and alpha had generally consistent sensitivities to age and/or altered sensory conditions. Age x sensory condition interactions, however, had distinct effects on LDS parameters compared to the other COP-based measures. Older individuals exhibited faster divergence rates while having similar magnitudes of A, compared to young individuals. These results suggest that older individuals stiffen the musculoskeletal system via increased muscle activity, perhaps as an age-related postural adaptation. In addition, correlations between LDS parameters and other COP measures were relatively small (r2<<0.29). Hence, LDS parameters (A, tau(S) and tau(L)) provide distinct information on postural control and stability, supplementing other COP-based measures.