Respiratory muscle dynamics and control during exercise with externally imposed expiratory flow limitation

To determine how decreasing velocity of shortening (U) of expiratory muscles affects breathing during exercise, six normal men performed incremental exercise with externally imposed expiratory flow limitation (EFLe) at ∼1 l/s. We measured volumes of chest wall, lung- and diaphragm-apposed rib cage (Vrc,p and Vrc,a, respectively), and abdomen (Vab) by optoelectronic plethysmography; esophageal, gastric, and transdiaphragmatic pressures (Pdi); and end-tidal CO2concentration. From these, we calculated velocity of shortening and power (W˙) of diaphragm, rib cage, and abdominal muscles (di, rcm, ab, respectively). EFLe forced a decrease in Uab, which increased Pab and which lasted well into inspiration. This imposed a load, overcome by preinspiratory diaphragm contraction. Udi and inspiratory Urcm increased, reducing their ability to generate pressure. Pdi, Prcm, andW˙ab increased, indicating an increased central drive to all muscle groups secondary to hypercapnia, which developed in all subjects. These result...