Reduced muscle strength is the major determinant of reduced leg muscle power in Parkinson's disease

Background: Reduced muscle power (speed x strength) is associated with increased fall risk and reduced walking speed in people with Parkinson's disease (PD) as well as in the general older population. This study aimed to determine the relative contribution of motor impairments (bradykinesia, tremor, rigidity and weakness) to reduced leg muscle power in people with PD. Methods: Eighty-two people with PD were tested while on medication. Leg extensor muscle strength and muscle power were measured using pneumatic variable resistance equipment. Lower limb bradykinesia, rigidity and tremor were measured using the Movement Disorders Society-sponsored Unified Parkinson's Disease Rating Scale. Associations between motor impairments and leg muscle power were examined using linear regression. Results: Univariate models revealed that muscle strength (R-2 = 0.84), bradykinesia (R-2 = 0.05) and rigidity (R-2 = 0.05) were significantly associated with leg muscle power, while tremor was not. A multivariate model including bradykinesia, tremor, rigidity, muscle strength, age and gender explained 89% of the variance in leg muscle power. This model revealed reduced muscle strength to be the major determinant of reduced muscle power (beta = 0.7), while bradykinesia was a minor contributor to reduced muscle power (beta = -0.1), even when accounting for age and gender. Conclusions: The findings that reduced strength and bradykinesia contribute to reduced muscle power in people with PD tested on medication suggest that these impairments are potential targets for physical interventions. (C) 2012 Published by Elsevier Ltd.