Objectives. The primary objective of this study was to examine the effect of environmental predictability on postural control after stroke. A reaching task for seated subjects was used as the postural perturbation. Trajectory stability (the pathway followed by the subject’s body center of pressure with respect to time during the reaching task) was used as the index of postural control. It was hypothesized that trajectory stability would be greater under predictable conditions.

Method. A specially designed electromechanical system was used to measure the trajectory stability ratios for 100 subjects, 50 with poststroke hemiplegia and 50 who had not had stroke. All subjects completed a task that required reaching to the left versus reaching to the right, under predictable versus unpredictable conditions. Postural control was measured via a trajectory instability ratio in both the anterior-posterior and medial-lateral planes.

Results. Although the effect of predictability on postural control was significant, it was not as hypothesized for both groups. There was greater trajectory stability under unpredictable conditions when reaching to the right as measured in both the anterior-posterior and medial-lateral planes and when reaching to the left as measured in the medial-lateral plane.

Conclusion. These findings refute the assumption of the hierarchical, predictable-to-unpredictable-environment model for postural control evaluation and treatment. The relationship between information processing demands and postural skill is probably more complex than the simple linear association implied. Perhaps the two conditions, predictable and unpredictable, should be worked on concurrently, not sequentially.

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