Abnormalities of rib cage-abdominal motion are common in patients with chronic obstructive pulmonary disease (COPD), but the basis of the abnormal motion has not been completely determined. Although airway obstruction has been shown to be a major factor in causing abnormal chest wall motion, the effect of hyperinflation (which has numerous adverse effects on respiratory muscle function) has not been systematically examined. We induced graded levels of hyperinflation in six healthy volunteers using continuous positive airway pressure (CPAP) levels of 10, 20, and 30 cm H2O. Chest wall motion was measured by a calibrated inductive plethysmograph. Rib cage-abdominal asynchrony and paradox were quantitated by the Konno-Mead method of analysis. CPAP levels of 10, 20, and 20 cm H2O produced increases in end-expiratory lung volume of 0.98 +/- 0.14 (SE), 1.90 +/- 0.31, and 2.42 +/- 0.37 L, respectively (p < 0.0001). This corresponded to an increase in the ratio of functional residual capacity to predicted total lung capacity from 0.38 +/- 0.08 at baseline to 0.74 +/- 0.14 at 30 cm H2O CPAP-comparable to that seen in patients with COPD. Hyperinflation induced an increase in inspiratory abdominal paradox, 1.0 +/- 0.7% at baseline versus 3.6 +/- 1.7% at 30 cm H2O (p < 0.05), but this is unlikely to be clinically significant. A significant increase in asynchrony or rib cage paradox did not develop with hyperinflation. In conclusion, the primary factor contributing to abnormal chest wall motion in patients with COPD is likely to be increased airway resistance, and hyperinflation makes only a minor contribution.