STOCHASTIC-PROCESSES IN POSTURAL CENTER-OF-PRESSURE PROFILES

The stochastic processes of postural center-of-pressure profiles were examined in 3- and 5-year-old children, young adult students (mean 20 years), and an elderly age group (mean 67 years). Subjects stood still in an upright bipedal stance on a force platform under vision and non vision conditions. The time evolutionary properties of the center-of-p ressure dynamic were examined using basic stochastic process models. T he amount of motion of the center of pressure decreased with increment s of age from 3 to 5 years to young adult but increased again in the e lderly age group. The availability of vision decreased the amount of m otion of the center of pressure in all groups except the 3-year-old gr oup, where there was less motion of the center of pressure with no vis ion. The stochastic properties of the center-of-pressure dynamic were assessed using both a two-process, random-walk model of Collins and De Luca and an Ornstein-Uhlenbeck model that is linear and has displacem ent governed only by a single stiffness term in the random walk. The t wo-process open- and closed-loop model accounted for about 96% and the Ornstein-Uhlenbeck model 92% of the variance of the diffusion term. D iffusion parameters in both models showed that the data were correlate d and that they varied with age in a fashion consistent with developme ntal accounts of the changing regulation of the degrees of freedom in action. The findings suggest that it is premature to consider the traj ectory of the center-of-pressure as a two-process, open- and closed-lo op random-walk model given that: (a) the linear Ornstein-Uhlenbeck dyn amic equation with only two parameters accommodates almost as much of the variance of the random walk; and (b) the linkage of a discontinuit y in the diffusion process with the transition of open- to closed-loop processes is poorly founded. It appears that the nature of the stocha stic properties of the random walk of the center-of-pressure trajector y in quiet, upright standing remains to be elucidated.