Impaired Scaling of Step Length in Parkinsonian Postural Instability

Abstract

Background: Postural stepping is an important strategy for recovery of balance in response to postural perturbations. It is disrupted by Parkinson's disease (PD) and other conditions. The nature of this disruption remains poorly understood. Understanding the motor control nature of this impairment can guide the development of novel interventions. Objectives: To identify the motor control abnormalities responsible for parkinsonian impairment of postural stepping. Methods: We studied four groups of participants: control, aged, PD, and normal-pressure hydrocephalus (NPH). We performed kinematic analysis of postural stepping by recording participants' body motion during a modified version of the clinical pull test, which was performed multiple times with different amounts of pulling forcefulness. Results: Successful postural stepping in the control group was accompanied by linear scaling of their first step's length and latency to the body's initial motion: more forceful pulls caused larger initial body acceleration, which resulted in longer steps that began earlier. PD patients exhibited reduced scaling of step length: they maintained normal reaction time but took steps that were inadequately short. Reduced step length scaling was present, but less severe, in aged individuals, and was more severe in NPH patients. Aged individuals and PD patients exhibited partial compensation for reduced step length scaling: their step length included a component that was independent of initial body acceleration, which was absent in control and NPH groups. Conclusions: the impairment of postural stepping caused by PD and related conditions is due to inadequate scaling of movement amplitude and is thus a form of hypokinesia.

Using kinematics to re-define the pull test as a quantitative biomarker of the postural response

“…Prior findings have demonstrated that the ability to scale step length with pull intensity declines with age and/or disease progression and that increased step length intercept values appear to be a compensatory measure to ensure postural stability in the face of this decline. 17 Studies examining the postural step response using standardized perturbations in PD patients have generally shown that PD patients take smaller steps than healthy subjects, [20][21][22][23] with slower reaction times that may be due to an increase in the number 22 and size 21 of anticipatory postural adjustments prior to stepping. This suggests that the kinematic deficits captured within these parameters are not necessarily disease-specific.…”

“…Plots were created using the R package "ggplot2." Supplemental Table 1 contains a description and interpretation of the relevant kinematic variables while Supplemental Figure 1 demonstrates the relevant points of interest for each individual trial 17 . Statistically significant differences in center of mass velocity (V COM ) plots were considered to be non-overlapping 95% confidence intervals of timeframes 50 ms or greater.…”