Respiratory muscle performance as a possible determinant of exercise capacity in patients with ankylosing spondylitis

Reduction of exercise capacity in patients with ankylosing spondylitis is associated with skeletal muscle performance. The contribution of respiratory muscle performance is questionable. This pilot study was designed to investigate the relationship between respiratory muscle performance and exercise capacity in ankylosing spondylitis. Subjects were 12 patients with ankylosing spondylitis. Measurements of maximal respiratory pressures and inspiratory muscle endurance were performed and correlated with maximal exercise capacity. Lung function and chest wall expansion were reduced on average. Maximal inspiratory and expiratory pressures were reduced to 82 +/- 20% of predicted values and 75 +/- 22% of predicted values respectively. On average there was no reduction in inspiratory muscle endurance which remained at 103 +/- 36% of predicted values. No overall reduction was found in maximal exercise capacity, either expressed as maximal workload or as peak oxygen uptake; however, a wide range was found. Maximal workload and peak oxygen uptake correlated significantly with maximal respiratory pressures and respiratory muscle endurance. The best regression model for explaining the total variation of maximal workload and peak oxygen uptake selected maximal inspiratory pressures as the independent variable (r(2) = 59.6%, p = 0.003 and r(2) = 62.5%, p = 0.05 respectively.) These data suggest respiratory pressure and respiratory muscle endurance, in particular maximal inspiratory pressure, may be determinants of exercise capacity in patients with ankylosing spondylitis.