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Respiratory mechanics and diaphragmatic fatigue during exercise in men and women Guenette, Jordan Ali


Purpose: The purpose of this thesis was to determine the underlying mechanisms associated with a higher total WOB in women (Study 1) and to determine if women experience greater levels of diaphragmatic fatigue relative to men (Study 2). Methods: Study 1: Sixteen endurance-trained subjects (8M:8F) underwent a progressive cycling test to exhaustion while esophageal pressure and lung volumes were measured. Modified Campbell diagrams were used to calculate the inspiratory and expiratory resistive and elastic components at 50, 75, 100 l·min⁻¹ and maximal ventilations and also at standardized mass-corrected work-rates. Study 2: Thirty-eight endurance-trained subjects (19M:19F) underwent a constant-load cycling test at 90% of peak work-rate until exhaustion. Pressure-time product of the diaphragm (PTPdi) was calculated during exercise. Trans-diaphragmatic pressure twitches (Pdi,tw) were assessed using cervical magnetic stimulation before and 10, 30 and 60 minutes after exercise. Diaphragm fatigue was defined as a ≥ 15% reduction in Pdi,tw post-exercise. Results: Study 1: The inspiratory resistive WOB was higher in women at all absolute ventilations (P<0.05). The expiratory resistive WOB was higher in women at 75 l·min⁻¹ (P<0.05). There were no sex-differences in the elastic WOB. However, the total WOB was significantly higher in men at relative percentages of maximal ventilation (P<0.05) but this sex-difference was reversed when the WOB was standardized for a given work-rate to body mass ratio. Study 2: Diaphragm fatigue was present in 11 males and 8 females. The reduction in Pdi,tw at 10 and 30 min following exercise was significantly greater in men relative to women (P<0.05). Men consistently had higher absolute values for PTPdi during exercise but this sex-difference was reversed when body mass was taken into account. Over time, men continued to have a reduced contribution of the diaphragm to total inspiratory force output whereas diaphragmatic contribution in women remained relatively constant over time. Conclusions: The higher total WOB in women is due to an increased resistive WOB which is likely attributable to their smaller airways. Despite a respiratory system that may have a higher mechanical cost of breathing, women appear to be more resistant to exercise-induced diaphragmatic fatigue.

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