Sex Differences in Diaphragmatic Fatigue

Abstract

Inspiratory muscle fatigue (IMF) is associated with a sympathetically mediated metaboreflex, resulting in time-dependent sympathoexcitation, including increased heart rate, mean arterial pressure and limb vascular resistance that impairs tolerance to exercise. Women may be more resistant to IMF relative to men and therefore, may experience an attenuated inspiratory muscle metaboreflex. PURPOSE: To examine sex-based differences in a) the cardiovascular response to inspiratory pressure-threshold loading (PTL) and b) the effect of prior-induced IMF on subsequent exercise performance. METHODS: Healthy men (n = 9) and women (n = 9) completed a maximal incremental cycle test on day 1. On day 2, subjects performed PTL to task failure followed by a constant load submaximal time-to-exhaustion (TTE) exercise test (85% peak work rate). On day 3, subjects performed the same exercise test without prior-induced IMF. Diaphragmatic fatigue was assessed by measuring transdiaphragmatic twitch pressure (Pdi,tw) in response to cervical magnetic stimulation. Heart rate (HR) and mean arterial pressure (MAP) were measured beat-by-beat throughout PTL via photoplethysmography, and low-frequency systolic pressure (LFSBP, surrogate for sympathetic vasomotor tone) calculated from arterial waveforms using power spectrum analysis. RESULTS: At task failure of PTL, the severity of IMF was similar between sexes (W = 23 ± 6%, M = 24 ± 8% reduction in Pdi,tw, p = 0.33). However, time to task failure was significantly longer in women than men (27 ± 11 vs. 16 ± 11 min, p = 0.02) and consequently, women tended to produce greater cumulative diaphragmatic pressure (46,805 ± 16,723 vs. 32,288 ± 20,752 cmH O⋅s-1; p = 0.06). Furthermore, women exhibited less of an increase in HR (13 ± 8 vs. 19 ± 12 bpm, p = 0.01) and MAP (10 ± 8 vs. 14 ± 9 mmHg, p = 0.02) during PTL, and significantly lower LFSBP (27 ± 12 vs. 38 ± 7 mmHg2, p = 0.04) compared to men. Prior-induced IMF resulted in the premature termination of exercise that did not differ on the basis of sex (W = 15 ± 19%, M = 16 ± 19% decrease in TTE, p = 0.89). CONCLUSIONS: Inspiratory muscle endurance time is significantly longer in women than men. Fatiguing contractions of the diaphragm elicit a blunted cardiovascular response in women, whilst IMF contributes to exercise impairment independent of sex.