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-induced diaphragm fatigue between two groups of healthy subjects, a high-fit group [maximal O₂ consumption (V̇O₂(max)) = 69.0 ± 1.8 ml · kg^-1 · min^{- 1}, n = 11] and a fit group (V̇O₂(max) = 50.4 ± 1.7 ml · kg^-1 · min^{- 1}, n =13). Both groups exereised at 88-92% V̇O₂(max) for about the same duration (15.2 ± 1.7 and 17.9 ± 2.6 min for high-fit and fit subjects, respectively, P > 0.05). The supramaximal BPNS test showed a significant reduction (P < 0.01) in the BPNS transdiaphragmatic 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. Recovery of the BPNS Pdi took 60 min in both groups. The high-fit group exercised at a higher absolute workload, which resulted in a higher CO₂ production (+26%), a greater ventilatory demand (+16%) throughout the exercise, and an increased diaphragm force output (+28%) over the initial 60% of the exercise period. Thereafter, diaphragm force output declined, despite a rising minute ventilation, and it was not different between most of the high-fit and fit subjects. In summary, the high-fit subjects showed diaphragm fatigue as a result of heavy endurance exercise 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 diaphragm aerobic capacity was greater.