Objective: To investigate respiratory muscle strength in mild to moderate Parkinson's disease in comparison with prediction equations and age-matched controls. To identify correlations between respiratory muscle strength and daily living function, activity levels, quality of life, and exercise capacity.

Design: A total of 66 participants with Parkinson's disease and 32 age-matched, healthy controls participated. Respiratory mouth pressures (representing respiratory muscle strength) were compared with predicted values. A comparison was also made between participants with Parkinson's disease and age-matched controls. Respiratory mouth pressures were correlated with results of the Barthel index (daily living function), a modified Baecke activity questionnaire (activity levels), the Parkinson's disease questionnaire (quality of life in Parkinson's disease), peak heart rate, peak oxygen consumption, blood lactate thresholds, and the number of stages completed during an incremental cycle ergometer test (representing exercise capacity).

Results: Respiratory mouth pressures were significantly lower (P < 0.05) in the Parkinson's disease group, but this did not influence the measures of daily living function, activity levels, and quality of life. Respiratory mouth pressures correlated with lactate thresholds (r = 0.308, P < 0.01) and the number of completed stages of the cycle ergometer test (r = 0.490, P < 0.01).

Conclusions: In mild to moderate Parkinson's disease, there is a significant weakness of the respiratory muscles. This can affect the individual during exercise but has no apparent effect on activities that do not require a large effort of the respiratory muscles.