AEROBIC FITNESS EFFECTS ON EXERCISE-INDUCED LOW-FREQUENCY DIAPHRAGM FATIGUE

We used bilateral phrenic nerve stimulation (BPNS; at 1, 10, and 20 Hz at functional residual capacity) to compare the amount of exercise-in duced diaphragm fatigue between two groups of healthy subjects, a high -fit group [maximal O-2 consumption (Vo(2max)) = 69.0 +/- 1.8 ml . kg( -1). min(-1), n = 11] and a fit group (Vo(2max) = 50.4 +/- 1.7 ml . kg (-1). min(-1), n = 13). Both groups exercised at 88-92% Vo(2max) for a bout the same duration (15.2 +/- 1.7 and 17.9 +/- 2.6 min for high-Bt and fit subjects, respectively, P > 0.05). The supramaximal BPNS test showed a significant reduction (P < 0.01) in the BPNS transdiaphragmat ic pressure (Pdi) immediately after exercise of -23.1 +/- 3.1% for the high-fit group and -23.1 +/- 3.8% (P > 0.05) for the fit group. Recov ery of the BPNS Pdi took 60 min in both groups. The high-fit group exe rcised at a higher absolute workload, which resulted in a higher CO2 p roduction (+26%), a greater ventilatory demand (+16%) throughout the e xercise, and an increased diaphragm force output (+28%) over the initi al 60% of the exercise period. Thereafter, diaphragm force output decl ined, despite a rising minute ventilation, and it was not different be tween most of the high-fit and fit subjects. In summary, the high-fit subjects showed diaphragm fatigue as a result of heavy endurance exerc ise but were also partially protected from excessive fatigue, despite high ventilatory requirements, because their hyperventilatory response to endurance exercise was reduced, their diaphragm was utilized less in providing the total ventilatory response, and possibly their diaphr agm aerobic capacity was greater.