Postural sway variability during precision aiming in Parkinson's disease patients

Previous research has shown that postural fluctuations can be adaptively manipulated independently in the anteroposterior (AP) and mediolateral (ML) directions when performing a precision aiming task. It has been identified that the sway in the AP and ML directions are reciprocally related. The present experiment looked at the postural fluctuations of patients with Parkinson?s disease (PD) during the performance of such precision aiming tasks. We analyzed the variability of the centre of pressure (CoP) under both feet during a 30 sec. precision aiming task. A laser pointer was affixed to the hip of participants, and they were instructed to maintain the laser beam inside a target placed at varying distances (1 or 2m), in either a parallel or perpendicular orientation to the body. Eleven young healthy subjects, PD patients both on and off medication and age-matched elderly subjects participated in the experiment. PD patients both on and off medication showed greater postural sway variability than healthy controls across all conditions in both AP and ML planes. While young, healthy subjects and to a lesser extent older adults showed modulation in the AP and ML sway patterns due to task demands, PD patients did not. Sway variability was greater in patients off medication when the target was at a perpendicular orientation to the body compared to the parallel orientation condition. PD patients on medication showed an increase in ML sway variability and a decrease in AP sway variability across all conditions. ML sway variability in PD patients on medication was greater in parallel orientation, while PD patients off medication showed decreased ML sway variability as compared to the perpendicular orientation. PD patients on medication showed greater sway variability when the target was placed at a 2m distance than when the target was at a 1m distance. The converse was true for patients off medication. These findings suggest that the experimental manipulation of a precision aiming task during upright standing induced the recruitment of different postural control strategies in PD patients when compared to healthy control subjects.