Stiffness and postural stability in adults with Down syndrome.

The purpose of this study was to characterize postural sway in quiet standing under eyes-open and eyes-closed conditions, and to obtain a measure of postural stiffness during quiet standing in adults with Down syndrome (DS) versus control subjects. We obtained descriptive measures from centre-of-pressure (COP) data and analysed and compared COP trajectories and postural stiffness estimates from two stochastic models, the "pinned polymer" (PP) and "inverted pendulum" (IP) models. These estimates were correlated with clinical measures of muscle tone. Our results showed that overall, estimated values for postural stiffness from both models were larger for the DS group than for normal controls. In addition, average stiffness measures were greater under the eyes-closed condition than under the eyes-open condition for the DS group. The IP model detected significant trends over trials whereas the PP model did not. Clinical assessment of muscle tone for the DS group ranged from low to high-normal and there was no significant correlation with the postural stiffness measures obtained from either model. These results suggest that individuals with DS have the ability to modulate their underlying "stiffness" under conditions of quiet standing. Furthermore, there appears to be no strong relationship between clinical measures of muscle tone and postural stiffness measures under dynamic conditions.