We studied the dynamic mechanical properties of the chest wall in 7 patients with severe chronic air-flow obstruction (CAO). Measurements were made during quiet breathing at rest and during exercise on a bicycle ergometer at work rates equivalent to 50 and 100% of their maximal work rate (Wmax). The peak inspiratory pleural pressure relative to the chest wall relaxation curve (Pmus) increased from 13.5 +/- 1.5 cm H2O at rest to 22.4 +/- 1.7 cm H2O at Wmax, while the coincident transdiaphragmatic pressure increased from 9.7 +/- 2.1 cm H2O at rest to 16.5 +/- 2.3 cm H2O at Wmax. Consequently, the coincident gastric pressure relative to its value during relaxation (Pab) was negative at rest (-4.5 +/- 1.7 cm H2O) and became even more negative (-6.3 +/- 2.3 cm H2O) at Wmax. Yet the increase in ventilation with increasing exercise was associated with an increase in the passive outward displacement of the abdomen (delta Vab) relative to the total volume change (delta Vab + delta Vrc), such that the ration delta Vab/(delta Vab + delta Vrc) increased from 0.37 +/- 0.08 at rest to 0.52 +/- 0.05 at Wmax. There was no respiratory paradox. From the analysis of volume-pressure tracings of the chest wall compartments we inferred that expiratory intercostal and abdominal muscles contracted forcefully during expiration on exercise, resulting in a marked increase in pleural pressure and a change in thoraco-abdominal configuration. This represented the storage of elastic and gravitational energy, which was released during inspiration, contributing to inspiratory pleural pressures and the enhanced inspiratory flow.(ABSTRACT TRUNCATED AT 250 WORDS)