Sex differences in the resistive and elastic work of breathing during exercise in endurance-trained athletes

It is not known whether the high total work of breathing (WOB) in exercising women is higher due to differences in the resistive or elastic WOB. Accordingly, the purpose of this study was to determine which factors contribute to the higher total WOB during exercise in women. We performed a comprehensive analysis of previous data from 16 endurance-trained subjects (8 men and 8 women) that underwent a progressive cycle exercise test to exhaustion. Esophageal pressure, lung volumes, and ventilatory parameters were continuously monitored throughout exercise. Modified Campbell diagrams were used to partition the esophageal-pressure volume data into inspiratory and expiratory resistive and elastic components at 50, 75, 100 l/min and maximal ventilations and also at three standardized submaximal work rates (3.0, 3.5, and 4.0 W/kg). The total WOB was also compared between sexes at relative submaximal ventilations (25, 50, and 75% of maximal ventilation). The inspiratory resistive WOB at 50, 75, and 100 l/min was 67, 89, and 109% higher in women, respectively (P < 0.05). The expiratory resistive WOB was 131% higher in women at 75 l/min (P < 0.05) with no differences at 50 or 100 l/min. There were no significant sex differences in the inspiratory or expiratory elastic WOB across any absolute minute ventilation. However, the total WOB was 120, 60, 50, and 45% higher in men at 25, 50, 75, and 100% of maximal exercise ventilation, respectively (P < 0.05). This was due in large part to their much higher tidal volumes and thus higher inspiratory elastic WOB. When standardized for a given work rate to body mass ratio, the total WOB was significantly higher in women at 3.5 W/kg (239 +/- 31 vs. 173 +/- 12 J/min, P < 0.05) and 4 W/kg (387 +/- 53 vs. 243 +/- 36 J/min, P < 0.05), and this was due exclusively to a significantly higher inspiratory and expiratory resistive WOB rather than differences in the elastic WOB. The higher total WOB in women at absolute ventilations and for a given work rate to body mass ratio is due to a substantially higher resistive WOB, and this is likely due to smaller female airways relative to males and a breathing pattern that favors a higher breathing frequency.