Aim of this work was to evaluate parkinsonian patients (PARK) by static posturography tests in order to study the stability of posture system, the role of visual input and the influence of an acute administration of levodopa. The problem of characterizing postural control is approached from the perspective of nonlinear dynamics and chaos theory. The Largest Lyapunov Exponent (LLE) is a parameter that nonlinear analysis methods allow us to determine in a reliable manner. For a nonlinear deterministic system, a positive value of this parameter implies that this system is chaotic. In order to estimate LLE, as first step, it was necessary to confute the hypothesis of linear structure of data through a determinism test: in particular the surrogate data test has been applied. As second step, attention was paid on the choice of appropriate nonlinear filtering methods related to nonlinear analysis. Analysed subjects were 14 PARK and 20 control subjects (CTRL). Subjects were asked to maintain the orthostatic posture for 60 seconds on a dynamometric platform. Every trial was repeated twice and was performed in Eyes-Open and in Eyes-Closed conditions. PARK were analysed both before (PRE) and after (POST) levodopa administration. Results show positive LLE values that, in the case of PARK, tend to be higher than LLE estimated for CTRL. This is particularly true in the PRE condition. From these findings one can argue that PARK, if compared with CTRL, show less stability, mainly in PRE condition. This instability is not always evident looking at the centre of pressure geometric characteristics because these latter do not take into account the dynamical properties of the posture control system.
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