This exploratory study aimed to identify which aspects of postural control are able to distinguish between subgroups of patients with Parkinson’s disease (PD) and controls. Balance was tested using static and dynamic posturography. Freezers ( ), nonfreezers ( ), and controls ( ) stood on a movable force platform and performed 3 randomly assigned tests: (1) sensory organization test (SOT) to evaluate the effective use of sensory information, (2) motor control test (MCT) to assess automatic postural reactions in response to platform perturbations, and (3) rhythmic weight shift test (RWS) to evaluate the ability to voluntarily move the center of gravity (COG) mediolaterally and anterior-posteriorly (AP). The respective outcome measures were equilibrium and postural strategy scores, response strength and amplitude of weight shift. Patients were in the “on” phase of the medication cycle. In general, freezers performed similarly on SOT and MCT compared to nonfreezers. Freezers showed an intact postural strategy during sensory manipulations and an appropriate response to external perturbations. However, during voluntary weight shifting, freezers showed poorer directional control compared to nonfreezers and controls. This suggests that freezers have adequate automatic postural control and sensory integration abilities in quiet stance, but show specific directional control deficits when weight shifting is voluntary. 1. Introduction Patients with Parkinson’s disease (PD) are prone to falling during daily activities. Recurrent falls are a frequent cause of injuries and hospital admissions for patients with PD and an important factor that negatively influences quality of life [1, 2]. The extent of this problem was shown in a meta-analysis of prospective studies that reported that 46% of the patient population with PD had one or more falls in a 3-month time frame [3]. In order to prevent recurrent falls it is important to gain more insight in the underlying deficits. Recently, a number of prediction studies have shown that postural control deficits and freezing of gait (FOG) are powerful determinants of recurrent falls [4, 5]. Although both signs were previously linked to falls, there are, to our knowledge, no conclusive reports on the relationship between postural control deficits and FOG. FOG is defined as an episodic inability to generate effective stepping while having the intention to walk [6]. It is most commonly experienced during turning and step initiation, but also when faced with spatial constraints, stress, and distraction [6]. A FOG episode can present
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