Determinants and Physiological Consequences of Respiratory Muscle Fatigue: A Comprehensive Comparison of the Inspiratory and Expiratory Muscles

Exhaustive whole-body exercise elicits inspiratory and expiratory muscle fatigue, which may limit exercise tolerance through, at least in part, a reflexively-mediated reduction in leg blood flow (Q̇L). Despite phenotypical and functional differences between the inspiratory and expiratory muscles, an investigation of: 1) the cardiovascular consequences of; and 2) the factors that determine the development of inspiratory vs. expiratory muscle fatigue has not been conducted. Presently, the cardiovascular responses to fatiguing inspiratory (IRL) and expiratory resistive loading (ERL), including leg vascular resistance (LVR) and Q̇L, were assessed in otherwise resting adults (Chapters 5 and 6). Despite a cardiovascular response consistent with a metabolically-induced sympathoexcitation, neither IRL nor ERL elicited the expected increase in LVR and reduction in Q̇L. The potential mechanisms underpinning this observation, including ‘mechanical effects’ of large swings in intra-thoracic and/or intra-abdominal pressure, are considered. Next, the effect of exercise intensity and duration on the magnitude of exercise-induced respiratory muscle fatigue was assessed (Chapter 7). The magnitude of inspiratory muscle fatigue was a function of cumulative respiratory force output and exercise-intensity. Conversely, the magnitude of expiratory muscle fatigue was unaffected by exercise intensity or duration. This is likely due to a less fatigue-resistant phenotype and/or additional non-ventilatory roles of the expiratory versus inspiratory muscles. Finally, the ‘time-course’ over which inspiratory and expiratory muscle fatigue develops during severe-intensity exercise was assessed (Chapter 8). The magnitude of inspiratory and expiratory muscle fatigue increased progressively with exercise time (50% vs. 75% vs. 100% tolerable duration [TLIM]). However, expiratory muscle fatigue became evident earlier than inspiratory muscle fatigue (50% vs. 75% TLIM). The findings from this thesis suggest that the fatigue threshold of the expiratory muscles may be lower than that of the inspiratory muscles; whether a more abrupt fatigue-induced sympathoexcitatory response occurs during increased expiratory vs. inspiratory work remains to be determined.