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 to 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 n=9, V̇O_2peak > 56.1±3.4 mL·kg^-1·min^-1 vs. Avg-Fit n=9, V̇O_2peak < 35.7±4.9 mL·kg^-1·min^-1. On a 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 non-volitionally 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 seconds, p = 0.9). Diaphragm fatigue was defined as a > 15% reduction in P_di,tw, approximately 2 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 (p = 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 to 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.