Rib cage mechanics during quiet breathing and exercise in humans

During exercise, large pleural, abdominal, and transdiaphragmatic pressure swings might produce substantial rib cage (RC) distortions. We used a three-compartment chest wall model (J. Appl. Physiol. 72: 1338-1347, 1992) to measure distortions of lung- and diaphragm-apposed RC compartments (RCp and RCa) along with pleural and abdominal pressures in five normal men. RCp and RCa volumes were calculated from three-dimensional locations of 86 markers on the chest wall, and the undistorted (relaxation) RC configuration was measured. Compliances of RCp and RCa measured during phrenic stimulation against a closed airway were 20 and 0%, respectively, of their values during relaxation. There was marked RC distortion. Thus nonuniform distribution of pressures distorts the RC and markedly stiffens it. However, during steady-state ergometer exercise at 0, 30, 50, and 70% of maximum workload, RC distortions were small because of a coordinated action of respiratory muscles, so that net pressures acting on RCp and RCa were nearly the same throughout the respiratory cycle. This maximizes RC compliance and minimizes the work of RC displacement. During quiet breathing, plots of RCa volume vs. abdominal pressure were to the right of the relaxation curve, indicating an expiratory action on RCa. We attribute this to passive stretching of abdominal muscles, which more than counterbalances the insertional component of transdiaphragmatic pressure.