Aging and the complexity of center of pressure in static and dynamic postural tasks.

The experiment was set-up to investigate the hypothesis that there is an age-related bi-directional change in complexity of the center of pressure (COP) depending on postural task demands. Healthy young (19-28 year) and old (65-74 year) adults were instructed to match, in two 25s trials for each condition, the COP with constant and sine-wave targets at 2 levels (5 and 40%) of the maximal COP distance (MCD). The root mean square error of COP increased with aging in both MCD levels and tasks. Multi-scale entropy (MSE) and detrended fluctuation analysis showed that the complexity of COP in the old adults was lower compared to the young in the constant target, whereas it was higher in the sine-wave target. The task dependent age-related bi-directional change of COP complexity is counter to the hypothesis of a universal loss of complexity with aging but shows that there is loss of adaptive change in complexity driven by the COP dynamics.