Author(s): MATHEWS, C., MILLS, D.E., SHIFFMAN, V.J., TAYLOR, J.L., FLEMING, E.C.M., BLOUIN, J.S., LAM, T., SHEEL, A.W., Institution: UNIVERSITY OF BRITISH COLUMBIA, Country: CANADA, Abstract-ID: 2416

INTRODUCTION:
The primary muscle of inspiration, the diaphragm, has been shown to fatigue following high levels of inspiratory work such as during whole-body endurance exercise or resistive breathing tasks. We have shown that following pressure threshold loading (PTL), females experience less diaphragm fatigue measured via cervical magnetic stimulation (CMS) of the phrenic nerves. A critique of CMS is that it may not supramaximally stimulate the diaphragm. The purpose of this study was twofold: (1) to evaluate sex-based differences in the contractility of the diaphragm by measuring transdiaphragmatic pressure (Pdi) in response to electrical bilateral phrenic nerve stimulation (BPNS), ensuring maximal recruitment, and (2) to determine low frequency fatigue (LFF) of the diaphragm.
METHODS:
Twenty-three (n=12F, 11M) young and healthy participants participated in the study. The phrenic nerves were located, underwent a recruitment protocol where current was increased by 10 mA until a plateau in Pdi was observed, then increased by 20-30%. Participants performed PTL to task failure at a duty cycle of 0.7 with a breathing frequency of 15 breaths/minute. Diaphragm contractility was assessed via Pdi in response to BPNS as single twitches and 10 Hz paired stimulations. BPNS was performed pre-PTL, post-PTL, and 30 and 60 minutes post PTL.
RESULTS:
All participants demonstrated a plateau in Pdi, defined as an equal to or less than 20% difference between the final 3 stimulation intensities during the ramp protocol. Immediately post-PTL single twitch Pdi was significantly lower in males than females (-34±18% vs.-23±11% respectively, p<0.05), however there was no difference in 10 Hz Pdi between males and females (-28±31% vs. -20.3±23.4 respectively, p>0.05). Females performed PTL for significantly longer (22.3±15.2 vs. 13.4±7.8 minutes, p<0.05) and accumulated greater diaphragmatic work compared to males (34 843 ± 21 558 vs. 21 867 ± 8 885 H2O·s, p<0.05). There was no difference in single twitch or 10 Hz Pdi between males and females at 30 or 60 minutes post PTL (p>0.05).
CONCLUSION:
We found that supramaximal stimulation of the phrenic nerves before and after high levels of inspiratory work revealed less diaphragm fatigue in females. These observations are consistent with our previous studies using CMS. These findings suggest that the difference in diaphragm fatigue between males and females is unlikely due to a difference in the calcium release from the sarcoplasmic reticulum during excitation-contraction coupling, as similar levels of LFF were observed between sexes. These results suggest that sex-based differences in diaphragm fatigue likely arise from mechanisms beyond excitation–contraction coupling and warrant further investigation.

Funding: Natural Sciences and Engineering Research Council of Canada