Preparatory postural adjustments in parkinsonian patients with postural instability.

BACKGROUND Postural instability is a common problem in patients with Parkinson's disease. This paper reports results of a study undertaken to investigate some of the possible mechanisms responsible for this instability. METHODS Preparatory postural adjustments associated with a lateral leg raising task were studied in five parkinsonian patients and four age-matched controls. Recordings included ground reaction forces, kinematics, and surface EMG activity from multiple leg muscles. RESULTS In normal subjects there was a well-defined sequence of events preceding the onset of leg elevation, beginning with a transfer of centre of foot pressure (CP), initially toward the moving leg and then back to the support side, followed by displacement of the trunk toward the support side. In the more severely affected parkinsonian patients, the amplitude of the initial displacement of CP was markedly reduced. The interval between the earliest force changes and the onset of leg elevation was prolonged and the relative timing of the kenematic adjustments during this interval was disrupted. In addition the alternating burst and periods of inhibition observed in the EMG recordings from the normal subjects were replaced by continuous tonic EMG activity. CONCLUSIONS These observations suggest that abnormalities in programming preparatory postural adjustments may contribute to postural instability in some patients with advanced Parkinson's disease.