Cournand lecture: Mechanical action of the abdominal muscles.

The abdominal muscles, by causing a rise in abdominal pressure and a cranial displacement of the diaphragm, 1--deflate the lungs (minimize the increase in FRC which normally occurs when adopting the standing posture), 2--exert an expanding force on the lower rib cage, which may counterbalance or even dominate the insertional (expiratory) force they exert on the lower ribs by way of their insertions, 3--improve the pressure-generating ability of the diaphragm. In addition, because they prevent the diaphragmatic dome from descending during inspiration, they also improve the rib cage expanding action of the diaphragm.