No effect of aerobic fitness on exercise-induced diaphragm fatigue in females

We tested the hypothesis that the incidence and magnitude of diaphragm fatigue following high-intensity exercise would be lower in females with a high aerobic capacity (Hi-Fit) compared with healthy females with an average aerobic fitness (Avg-Fit). Participants were assigned to groups based on their peak O₂ uptake (V̇o_2peak) obtained during cycle exercise: Hi-Fit <i>n</i> = 9, V̇o_2peak ≥ 56.1 ± 3.4 mL·kg^-1·min^-1 versus Avg-Fit <i>n</i> = 9, V̇o_2peak ≤ 35.7 ± 4.9 mL·kg^-1·min^-1. On the second day, diaphragm fatigue was assessed before and after constant load exercise test to exhaustion. Magnetic stimulation of the phrenic nerve roots was used to nonvolitionally assess diaphragm fatigue by measurement of transdiaphragmatic twitch pressure (P_di,tw). Both groups exercised at >90% of V̇o_2peak for a similar duration (Hi-Fit: 546.1 ± 177.8 vs. Avg-Fit: 559.3 ± 175.0 s, <i>P</i> = 0.9). Diaphragm fatigue was defined as a ≥15% reduction in P_di,tw, approximately two times greater than the coefficient of variation. The mean group average reduction in P_di,tw following exercise in the Hi-Fit (17.5%) and Avg-Fit groups (12.2%) was not different between groups (<i>P</i> = 0.2). The Hi-Fit group performed exercise at a higher absolute work rate that elicited significantly greater ventilatory work and inspiratory muscle force output. The Hi-Fit group did not experience greater fatigue compared with the Avg-Fit group, which we attribute to a greater reliance on accessory respiratory muscle recruitment, to training-induced increases in the aerobic capacity of the diaphragm, or a combination of the two. In summary, aerobic fitness is not predictive of exercise-induced diaphragm fatigue in healthy females.<b>NEW & NOTEWORTHY</b> We hypothesized that females with a high aerobic capacity would be resistant to exercise-induced diaphragm fatigue relative to healthy but untrained females. We assessed the pressure generated by the diaphragm following exhaustive exercise by stimulating the phrenic nerves. The magnitude and incidence of fatigue were similar between groups. Despite having higher ventilatory demands during exercise highly trained females were resistant to greater diaphragm fatigue. Aerobic fitness is not predictive of exercise-induced diaphragm fatigue in females.