Anticipatory postural adjustments prior to step initiation are hypometric in untreated Parkinson's disease: an accelerometer-based approach

Abstract

Background and purpose: Anticipatory postural adjustments (APAs), prior to step initiation, are bradykinetic in advanced Parkinson's disease (PD) and may be one of the factors associated with 'start hesitation'. However, little is known about APAs in the early stage of PD. In this study, we determined whether body-worn accelerometers could be used to characterize step initiation deficits in subjects with early-to-moderate, untreated PD.

Methods: Eleven PD and 12 healthy control subjects were asked to take two steps. Postural adjustments were compared from center of pressure (COP) and from acceleration of the trunk at the center of mass level (L5).

Results: Our findings show that APAs measured from the peak COP displacement toward the swing leg and the peak trunk acceleration toward the stance leg were smaller in untreated PD compared with control subjects. The magnitude of APAs measured from peak COP displacements and accelerations were correlated.

Conclusion: These results suggest that quantitative analysis of step initiation from one accelerometer on the trunk could provide useful information for the characterization of patients in early stages of PD, when clinical evidence of start hesitation may not be detectable. Ambulatory monitoring of step initiation is also promising for monitoring patient progression in the home environment, and eventually providing feedback for preventing freezing of gait episodes.

Figure 1

Experimental set-up and representative trunk acceleration and center of pressure (COP) trajectory during APA. A. Experimental setup: sensors placement on a subject and tape around feet for consistent foot placement on the force plate. B. The trajectories of trunk acceleration sensed at L5 level and of COP from the force plate during APAs in two representative subjects: control subject (black) and PD subject (grey). The stars represent the start of APAs. The total APA phase is represented by the bold COP and acceleration trajectories.

Figure 2

COP and trunk acceleration at L5 during APAs in two representative subjects: control subject (black) and PD subject (grey). Left panel: medio-lateral direction, A. COP trajectories and B. trunk acceleration trajectories. Right panel: antero-posterior direction, C. COP trajectories and D. trunk acceleration trajectories. Parameters computed from the trajectories (peak, time-to-peak, and duration of APAs) are shown. The onset and end of the APAs are represented by the dashed lines.

Figure 3

Comparison of group mean APAs in PD and control subjects. The mean values (±S.E.M) of Peak COP (A) and Peak Acc (B) are presented. Significant differences are showed with * p<0.05.

Figure 4

Linear correlation between Peak COP and Peak Acc presented for the medio-lateral (A) and antero-posterior (B) directions. Data are combined for PD (grey rhombus) and control subjects (white rhombus).