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 O 2 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.