EXPIRATORY MUSCLE RECRUITMENT DURING INSPIRATORY FLOW-RESISTIVE LOADING AND EXERCISE

Both exercise and inspiratory flow-resistive loading may cause recruitment of expiratory muscles. To evaluate the extent of recruitment in combined exercise and flow-resistive loading, and to estimate the effect on inspiratory muscle work, we studied five men, 26 to 39 yr of age, during mild exercise with different degrees of inspiratory flow-resistive loading. Each subject performed four 1-h exercise runs at 30% of their maximal oxygen consumption on different days while inspiring through an external resistor of either 1.4, 14.5, 19.9, or 30.6 cm H2O/s/L. Mouth and esophageal pressure, inspiratory flow rate, and abdominal and rib cage motion were recorded continuously. Abdominal expansion tended to lead and rib cage expansion tended to lag the start of inspiration as judged from the beginning of negative pressure development at the mouth. These time differences increased as resistive load increased. Plots of abdominal versus rib cage motion also showed increase in phase shift, with the abdomen leading the rib cage on inspiration. For all subjects, the esophageal pressure at the end of expiration became less negative as the resistive load increased, indicating that the end-expiratory volume decreased with increasing resistive load. We conclude that there was increasing use of expiratory muscles as the resistive load increased, and that the initial expansion of the abdomen at high resistive loads represented elastic recoll of structures that had been compressed below the volume at FRC by the expiratory muscles. Calculations suggest that, at different resistive loads, from 10 to 20% of the work of inspiration was transferred to the expiratory muscles, and that the power of the inspiratory muscles approached a limiting value at high resistive loads.