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Abstract

The rhythmic contraction of the main inspiratory muscle, the diaphragm, is essential for maintaining ventilation. During exercise, the greater blood flow demands of the diaphragm are supported by an inspiratory muscle metaboreflex (IMM). How diaphragm blood flow (Q̇dia) is related to IMM sensitivity and development of diaphragm fatigue is not well understood, as minimally invasive measures of Q̇dia have only recently become available. The IMM is attenuated in females compared with males during pressure-matched diaphragmatic loading. This suggests that females develop less diaphragm ischaemia and metaboreceptor activation than males. I hypothesized that post-exercise hyperaemia of the diaphragm would be less in females than males during matched diaphragmatic work. Healthy males (n=11, 28 ± 6 years, mean ± standard deviation) and females (n=11, 26 ± 4 years) completed a 6-minute loading task targeting a transdiaphragmatic pressure (Pdi) of 80 cmH₂O with a duty cycle of 0.7. Contrast-enhanced ultrasound was used to quantify Q̇dia during an end-expiratory apnoea at baseline, and 50% and 100% of the time-to-task completion. The change in mean arterial pressure (ΔMAP) during loading, and the pre- to post-task change in transdiaphragmatic twitch pressure (ΔPdi,tw) in response to bilateral electrical stimulation of phrenic nerves, assessed metaboreflex activation and fatigue, respectively. Throughout loading, the accumulated pressure-time-product for Pdia (males: 14,351±919 cmH₂O s, females 14,003 ± 1,947 cmH₂O; P=0.600) and percentage of maximal Pdia (%PDImax; males: 51 ± 8%, females: 51 ± 9%; P=0.870) was not different between sexes. Inspiratory loading resulted in IMM activation (ΔMAP, +21 [95% Confidence Interval: 18, 24] mmHg; P<0.0001), diaphragm fatigue (ΔPdi,tw, -18.1 [-13.3, -7.6] %; P<0.0001) and a post-exercise hyperaemia response (ΔQ̇dia, +15.8 [12.5, 19.1] AU/s; P<0.0001) at 100% of task completion. The ΔMAP, ΔPdi,tw, and ΔQ̇dia did not differ between sexes ( P>0.352 ). There was a significant positive relationship between ΔQ̇DIA and %PDIMAX (r²=0.211; P=0.032), and no relationship between the ΔQ̇DIA and ΔMAP (r²= 0.011; P= 0.638) or ΔPdi,tw (r²=0.001, P=0.914). These data demonstrate that Q̇DIA relates to the degree of exercise intensity and that the metaboreflex response may depend more on inter-individual differences MAP responses (i.e., genetics, fitness levels) or central command.