Preparation for Compensatory Forward Stepping in Parkinson's Disease

Different postural preparation strategies were used in response to perturbation. (A) examples are shown with the weight under the stepping foot in black and the weight under the stance foot in gray. In (i), the weight increased more on the stance foot compared with the stepping foot, and in (ii), the weight of the stepping foot is weighted more than the stance foot, but because the weight on the opposite foot is also increasing, this is not considered an APA. A single APA from a subject with PD is shown in (iii), with a weight shift toward the stepping foot and away from the stance foot. A multiple APA from another PD subject is shown in (iv); the weight shifts diverge more than once before the step. The tracings shown in (i) and (ii) were common strategies for the control subjects, while (iii) and (iv) were common strategies for subjects with PD in both the off and on states. (B) The frequency of stepping strategies across groups. Out of the percentage of all trials, control subjects rarely exhibited an APA before stepping, whereas the people with PD in both the off and on states commonly used APA before lifting their foot off the floor. Abbreviation: multi APA, multiple APAs.

Postural preparatory strategy in people with PD related to (A) PIGD scores and (B) frequency of reported freezing; represented as group mean with SE. Abbreviation: multi APA, multiple APAs. *P<.05; ^†P<.001.

Stepping characteristics of control subjects, who did not use an APA (90%), and trials of subjects with PD, divided into 3 strategies (no APA, 1 APA, multiple APAs). (A) The latency to step initiation from onset of the perturbation. (B) The forward length of the first step. (C) The forward position of the CoM relative to quiet standing at the moment when the foot was lifted. (D) the mean velocity of the CoM during the first step. Significant differences (P<.05) between groups are shown. (E) Number of compensatory steps. All data represented as a mean with SE. Abbreviation: multi APA, multiple APAs. *P<.05; ^†P<.001.