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The influence of age and posture on compliance of the rib cage (Crc) and diaphragm-abdomen (Cab) compartments of the chest wall was studied in 61 healthy adults (33 men, 28 women) aged 24–75 yr. Chest wall compliance (Cw) was measured by the weighted spirometer technique; Crc and Cab were derived from the slope of the relaxation line of the thoracoabdominal system obtained with two pairs of linearized magnetometers. While Cw was being measured, we monitored electrical activity of the abdominal external oblique muscle with a concentric needle electrode and thoracoabdominal configuration. In 52 subjects, the electromyogram did not show any abdominal muscle activity and the end-expiratory level never departed from the relaxed thoracoabdominal configuration, thus suggesting adequate respiratory muscle relaxation. Aging was associated with significant decreases in Crc and Cab. In the upright …