Elsevier

Gait & Posture

Volume 84, February 2021, Pages 205-208
Gait & Posture

Comparison of forward and backward postural perturbations in mild-to-moderate Parkinson's disease

https://doi.org/10.1016/j.gaitpost.2020.12.012Get rights and content

Highlights

  • Forward falls required fewer steps to recover balance than backward.

  • However, PD differed from controls more with forward than with backward stepping.

  • PCA of the COP trajectory is a sensitive, objective measure of reactive stepping.

Abstract

Background

Assessing postural stability in Parkinson’s disease (PD) often relies on measuring the stepping response to an imposed postural perturbation. The standard clinical technique relies on a brisk backwards pull at the shoulders by the examiner and judgement by a trained rater. In research settings, various quantitative measures and perturbation directions have been tested, but it is unclear which metrics and perturbation direction differ most between people with PD and controls.

Objectives

(1) Use standardized forward vs. backward perturbations of a support surface to evaluate reactive stepping performance between PD and control participants. (2) Evaluate the utility of using principal components analysis to capture the dynamics of the reactive response and differences between groups.

Methods

Sixty-two individuals participated (40 mild-to-moderate PD, off medication). Standardized rapid translations of the support surface were applied, requiring at least one step, backward or forward, to restore balance. The number of steps taken and the projection of the first principal component (PC1) of the center of pressure (COP) time series were entered in linear repeated-measures mixed effect models.

Results

Forward falls required significantly fewer steps to recover than backward falls. PC1 captured more than half of the variance in the COP trajectory. Analysis of the PC1 projection revealed a significant interaction effect of group (PD vs. controls) by direction, such that there was a group difference in forward stepping, but not backward.

Significance

Forward reactive stepping in PD differed from controls more than backward-stepping. PC1 projections of the COP trajectory capture the dynamics of the postural response and differ between PD and controls.

Introduction

Postural instability in Parkinson’s disease (PD) is clinically evaluated with a sudden backward pull on the shoulders (the “pull test”). The response is rated on an ordinal scale (0–4), heavily weighting the number of steps required to recover balance [1], often with imperfect standardization [2] and a poor ability to discriminate fallers from non-fallers [3]. Laboratory balance studies in PD are more standardized and quantitative, but methods are heterogeneous: some have tested reactive backward steps, others forward [[4], [5], [6], [7], [8]]. The best direction to detect Parkinsonism remains unclear. In this study we: (1) evaluated backward versus forward reactive stepping to compare performance between PD and controls and (2) evaluated the utility of using principal components analysis to capture the dynamics of the reactive response and differences between groups.

Section snippets

Materials and methods

We studied 62 individuals (40 mild-to-moderately severe PD and 22 controls, Table S1). All gave written informed consent. PD subjects were tested after overnight withdrawal from immediate release PD medications (24 h for sustained release formulations). No PD participant had been treated surgically. The protocol was approved by the University of Minnesota Institutional Review Board.

Reactive steps were induced by rapid translations of the support surface using a servocontrolled treadmill (Cmill,

Results

Forward-stepping perturbations required significantly fewer steps than backward (p < 0.001). The main effect of group was nonsignificant (p = 0.84). The group by direction interaction for number of steps was significant (p < 0.01) reflecting that the magnitude of the difference between PD and controls was greater in the forward than the backward direction (Fig S1; mean ± SD: Forward: PD: 1.9 ± 0.8; control: 1.7 ± 0.5) (Backward: PD: 2.4 ± 0.9; control: 2.5 ± 1.0). Equivalently, the differences

Discussion

For both PC1 projections and number of steps, the magnitude of the group difference was greater in the forward than the backward direction, as reflected in a significant group x direction interaction. The difference between PD and controls in the forward direction was statistically significant for PC1 projections, but not for number of steps, which we attribute to the coarse integer step-count.

PC1 projections were inversely related to the number of steps (Fig. 2B); thus, a higher projection

CRediT authorship contribution statement

Chiahao Lu: Methodology, Software, Data curation, Visualization, Writing - original draft, Writing - review & editing. Sommer L. Amundsen-Huffmaster: Methodology, Investigation, Writing - review & editing. Kenneth H. Louie: Investigation, Writing - review & editing. Robert Lowe: Data curation, Validation, Writing - review & editing. Reme Abulu: Data curation, Validation, Writing - review & editing. Robert A. McGovern: Data curation, Validation, Writing - review & editing. Jerrold L. Vitek:

Declaration of Competing Interest

None.

Acknowledgement

This work was supported by the National Institutes of Health [grant numbers: P50NS098573, R01NS088679, R01NS081118, R01NS094206] and the State of Minnesota’s MnDRIVE Brain Conditions Initiative.

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