Sex differences in post‐inspiratory inspiratory activity and evolution of net respiratory muscle pressures during incremental exercise

Abstract

The respiratory muscle pressure (Pmus) waveform allows observation of net inspiratory andexpiratory muscle recruitment under a wide variety of conditions, including mechanical ventilation, spontaneous breathing, and during exercise. Observation of the Pmus waveform during incremental and constant work rate cycling reveals that inspiratory and expiratory muscle recruitment systematically increase with rising minute ventilations. Additionally, measuring Pmus allows one to assess the post-inspiratory inspiratory muscle activity (PIIA), whereby the inspiratory muscles “brake” some or all of expiration to allow for a smoother transition in muscle recruitment. In recent years, it has become increasingly evident that the work of breathing is systematically higher for women compared with men at a given minute ventilation during exercise. It is possible that a greater evolution of net respiratory pressures, as well as longer PIIA duration, may be a contributing factor to the greater work of breathing observed in women during exercise. However, to date, no study has examined the potential sex differences in Pmus, nor PIIA, during exercise. Thus, a comprehensive assessment of the Pmus and PIIA during exercise between sexes is warranted. PURPOSE To compare PIIA and inspiratory and expiratory Pmus between men and women during incremental cycling. METHODS Fifteen women (21 ± 2 yrs; 167 ± 7 cm; 62 ± 7 kg) and 19 men (24 ± 4 yrs; 182 ± 5 cm; 81 ± 10 kg) visited the laboratory on two separate occasions. The first occasion was used to obtain informed consent and to screen participants for normal pulmonary function (>85% age-predicted). On the second visit, participants were instrumented with an esophageal balloon catheter to estimate variations in pleural pressure (Ppl). Quasi-static relaxation curves were obtained to measure chest wall recoil pressure (Pcw). Participants then completed an incremental cycling protocol until volitional exhaustion. Pmus was calculated at given minute ventilations during the exercise protocol as follows: Pmus = Pcw – Ppl. RESULTS Total Pmus (sum of average inspiratory and expiratory Pmus) was greater in women than men (P < 0.05). Peak inspiratory Pmus was similar between sexes, but peak expiratory Pmus was greater in women. The average rates of change (slope) in Pmus during inspiration and expiration were significantly faster in women than men at a given minute ventilation (P < 0.05). Finally, the duration of PIIA (expressed as a fraction of expiratory time) gradually decreased with increasing minute ventilation during exercise in both sexes (P < 0.05), but the decrease was comparable between sexes (P > 0.05). CONCLUSION The primary finding of this study was that the pattern of respiratory muscle recruitment was different between sexes during incremental cycling, given that women displayed systematically faster rates, and greater magnitude, of net inspiratory and expiratory pressure-development compared with men at matched ventilations. Additionally, the duration of PIIA was similar in both women and men, which implies no difference in the pattern of transition from inspiration to expiration between sexes.