Characteristics of temporal fluctuation of the vertical ground reaction force during quiet stance in Parkinson's disease.

Stance instability is seen in late stage Parkinson's disease (PD). Stabilometer-based center-of-pressure (COP) evaluation is an easy, routine method for measuring postural control ability. Most of the stabilometer- and force plate-based studies on upright postural control have discussed horizontal COP component control. Previous studies on vertical component control have been few, and no fractal analysis-based study on the component has been reported. We aimed to show the influence of neurological changes and aging on the vertical component and the difference in fluctuation pattern behavior in healthy young and elderly subjects as well as Parkinsonian patients. Detrended fluctuation analysis was used to study characteristics of fluctuation of vertical ground reaction force. In the three groups, all scaling exponents (α, α1, α2), which are time-correlated data of vertical ground reaction force, had a value >0 and <0.5 (0<α<0.5). Additionally, α and α2 were significantly different between PD and the other groups. Significant differences were observed between PD and the other groups regarding RMS, maximum peak value, and coefficient of variance. We demonstrated statistically significant differences in vertical ground reaction force between Parkinsonian patients and the other groups, suggesting that a neurological influence with PD may be markedly reflected in the vertical ground reaction force.