Effects of Parkinson's disease and levodopa on functional limits of stability

Abstract

Background: The voluntary, maximum inclined posture reflects the self-perceived limits of stability. Parkinson's disease is associated with small, bradykinetic postural weight shifts while standing but it is unclear whether this is due to reduced limits of stability and/or to the selection of abnormal strategies for leaning. The aim of this study was to investigate the effects of Parkinson's disease and levodopa medication on voluntary limits of stability and strategies used to reach these limits.

Methods: Fourteen subjects with Parkinson's disease (OFF and ON levodopa) and 10 age-matched controls participated in the study. Functional limits of stability were quantified as the maximum center of pressure excursion during voluntary forward and backward leaning. Postural strategies to achieve functional limits of stability were assessed by (i) body segments alignment, (ii) the difference between center of pressure and center of mass in preparation for a lean, (iii) the timing and the velocity of the preparation phase.

Findings: Functional limits of stability were significantly smaller in subjects with Parkinson's disease compared to control subjects. Subjects with Parkinson's disease maintained their stooped posture while leaning, initiated leaning with a smaller difference between center of pressure and center of mass and had a slower leaning velocity compared to control subjects. Levodopa enlarged the limits of stability in subjects with Parkinson's disease because of an increase in maximum forward, but not backward leans, but did not significantly improve postural alignment, preparation for a leaning movement, or velocity of leaning.

Interpretation: Parkinson's disease reduces functional limits of stability as well as the magnitude and velocity of postural preparation during voluntary, forward and backward leaning while standing. Levodopa improves the limits of stability but not the postural strategies used to achieve the leaning.

Fig. 1

Signals collected from a representative control subject and main parameters considered in the data analysis. (A) Functional limits of stability and parameters that quantify the maximal leaning phase. (B) Parameters that characterize the motion phase (example for forward leaning).

Fig. 2

Functional limits of stability in control and parkinsonian subjects. (A) Position of antero-posterior center of pressure (mean and SD) during the maximal leaning tasks and in quiet stance. (B) Functional limits of stability (mean and SD) quantified as the difference between maximal forward and maximal backward lean position. * P < 0.05, ** P < 0.01.

Fig. 3

Postural strategies during maximal leaning tasks and quiet stance in control and parkinsonian subjects, represented by: (A) average stick diagrams. (B) Trunk, thigh, and shank inclinations (mean and SD). *_P < 0.05, **_P < 0.01.

Fig. 4

Peak of COP-COM time series during backward and forward leaning. (A) Example of COP-COM time-series for a representative control subject. (B) COP-COM peaks for control and parkinsonian subjects (mean and SD). *_P < 0.05.

Fig. 5

Spatio-temporal characterization of the motion phase (mean and SD) in control and parkinsonian subjects. (A) Motion duration. (B) Motion velocity quantified by the AP COP mean velocity.