Title: The Effect of Aerobic Fitness on Exercise-Induced Diaphragmatic Fatigue in Females
Thesis Supervisor: Dr. William Sheel
Committee members: Dr. Don McKenzie, Dr. Jeremy Road
Defence Chair: Dr. Mike Koehle
Abstract: The magnitude of exercise-induced diaphragmatic fatigue (EIDF) is similar in males irrespective of aerobic fitness. When compared to aerobically fit males, aerobically fit females appear less susceptible to EIDF. We hypothesized that chronic endurance training and regular exposure to high levels of exercise hyperpnea may confer EIDF resistance in aerobically fit females when compared to healthy females with average aerobic fitness. Healthy females (25 ± 4 years) performed graded cycle exercise to exhaustion to characterize aerobic fitness and were assigned into a highly-aerobically fit (Hi-Fit) or averagely-aerobically fit (Avg-Fit) group (Hi-Fit: n = 9, V̇O2max = 56.1 ± 3.4 mL/kg/min; Avg-Fit: n = 9, V̇O2max = 35.7 ± 4.9, p < 0.001). Participants then completed high intensity (~90% V̇O2max) cycling exercise (Hi-Fit: 257 ± 29 W, Avg-Fit: 135 ± 10 W, p < 0.001) to exhaustion. Changes in transdiaphragmatic twitch pressure (Pdi,tw), evoked by cervical magnetic stimulation (CMS), were used to assess EIDF between baseline and 5-, 30-, and 60-minutes after exercise cessation. Time to exhaustion (TTE) was similar between groups (Hi-Fit: 546 ± 178 seconds; Avg-Fit: 559 ± 175 seconds, p = 0.876). The reduction in Pdi,tw relative to baseline at 5-, 30-, and 60-minutes after exercise cessation was 82.5 ± 12.0, 93.9 ± 9.8, 97.5 ± 5.6% and 87.8 ± 8.5, 97.7 ± 6.9, 101.8 ± 5.0%, in the Hi-Fit and Avg-Fit groups, respectively. There was no main effect of fitness on the exercise-induced fall in Pdi,tw or during the subsequent recovery (p = 0.186). We conclude that aerobic fitness does not affect the relative magnitude of EIDF in females. Similar to what has been observed in aerobically fit males, chronic endurance training and its concomitant hyperpnea do not appear to provide additional EIDF protection in females.