Paradoxical motion of the rib cage is well recognized in patients with chronic obstructive pulmonary disease (COPD); most often this is seen in the lateral dimension (Hoover's sign), but paradoxical indrawing of the lower sternum during inspiration has also been described. We have examined the possible mechanisms of these abnormalities by relating rib cage motion (using magnetometers) to the changes in pleural (Ppl), abdominal (Pab), and transdiaphragmatic (Pdi) pressures in 13 patients with COPD and hyperinflation who had previously documented abnormal rib cage motion. During tidal breathing, Pab became more negative in early inspiration in 11 of the 13 patients. Paradox of the lateral rib cage was present in 11 patients, and in these, the abnormal movement related well to increasing Pdi or decreasing Ppl, but the peak distortion fitted more closely with Pdi. During attempted voluntary relaxation at full inflation, the lateral rib cage dimensions always increased as Pdi declined. Five of the 13 patients showed indrawing of the sternum in early inspiration, and in 4, the abnormal motion was confined to the lower sternum, with qualitatively normal motion at the angle of Louis. In these 4 patients, the peak distortion coincided with the nadir of Pab in early inspiration. In all 5 patients, an increase in Pab during expiration suggested expiratory muscle contraction, and this was associated with a paradoxical increase in the anteroposterior diameter of the rib cage.(ABSTRACT TRUNCATED AT 250 WORDS)