Overestimation of stability limits leads to a high frequency of falls in patients with Parkinson's disease.

Objectives: To test a hypothesis that patients with Parkinson's disease may not notice discrepancies between their perceived and actual stability limits and cannot keep their centre of gravity within the stability region.

Settings: Outpatients with neurological disorders in rehabilitation service.

Subjects: Twenty-one patients with Parkinson's disease (11 men, 10 women; mean duration 5.9 +/- 3.9 years) and age- and sex-matched healthy volunteers were recruited.

Methods: Each subject's right arm length was subtracted from the distance between the right acromion and the perceived reachable boundary. This was called 'perceived reach'. The figure given by subtracting the right arm length from the maximum forward reach length measured by the Functional Reach Test was named 'actual reach', and is an index of actual stability limits in each subject. The difference between actual and perceived stability limits (DAP) is given by actual reach minus perceived reach. The motor score of the Unified Parkinson's Disease Rating Scale were used to evaluate disease severity.

Results: The mean DAP for the Parkinson's disease group was negative (-1.8 +/- 5.7 cm) and significantly different from that of controls (3.3 +/- 9.2 cm) (P < 0.05). In Parkinson's disease, DAP was significantly correlated with the Unified Parkinson's Disease Rating Scale score (correlation coefficient = -0.39, P < 0.05).

Conclusions: These results indicated that patients with Parkinson's disease overestimated their stability limits, which may result in falls. In addition, the results demonstrate that patients with Parkinson's disease develop overestimation of stability limits in parallel with their disease progression.