Effects of inspiratory loading on respiratory muscle activity during expiration.

We studied respiratory muscle activity during expiration in four normal subjects during inspiratory resistive and elastic loading, with and without positive and expiratory pressure. Net inspiratory muscle activity (Pmus) was measured by relating esophageal pressure to the chest wall relaxation pressure-volume (P-V) curve. Abdominal muscle activity (Pabd) was quantitated by relating gastric pressure to the relaxation P-V curve of the abdomen. The decay rates of Pmus and of transdiaphragmatic pressure (Pdi) were increased during loading, and end-expiratory lung volume decreased. The decay rates of Pmus and Pdi were inversely related to expiratory duration. Inspiratory intercostal/accessory activity during expiration was frequently observed together with Pabd, especially during elastic loading. We conclude that external mechanical loads, acting during inspiration, influence respiratory muscle behavior during expiration. Inspiratory loads diminish expiratory braking by the inspiratory muscles and recruit expiratory muscles. The rate of expiration is enhanced and its duration shortened. Simultaneous persistent action of the inspiratory intercostal and accessory muscles throughout expiration may serve to optimize diaphragmatic length.