Influence of inspiratory resistive loading on expiratory muscle fatigue in healthy humans

What is the central question of this study? This study is the first to measure objectively both inspiratory and expiratory muscle fatigue after inspiratory resistive loading to determine whether the expiratory muscles are activated to the point of fatigue when specifically loading the inspiratory muscles. What is the main finding and its importance? The absence of abdominal muscle fatigue suggests that future studies attempting to understand the neural and circulatory consequences of diaphragm fatigue can use inspiratory resistive loading without considering the confounding effects of abdominal muscle fatigue. Expiratory resistive loading elicits inspiratory as well as expiratory muscle fatigue, suggesting parallel coactivation of the inspiratory muscles during expiration. It is unknown whether the expiratory muscles are likewise coactivated to the point of fatigue during inspiratory resistive loading (IRL). The purpose of this study was to determine whether IRL elicits expiratory as well as inspiratory muscle fatigue. Healthy male subjects (n = 9) underwent isocapnic IRL (60% maximal inspiratory pressure, 15 breaths min^-1 , 0.7 inspiratory duty cycle) to task failure. Abdominal and diaphragm contractile function was assessed at baseline and at 3, 15 and 30 min post-IRL by measuring gastric twitch pressure (P_ga,tw ) and transdiaphragmatic twitch pressure (P_di,tw ) in response to potentiated magnetic stimulation of the thoracic and phrenic nerves, respectively. Fatigue was defined as a significant reduction from baseline in P_ga,tw or P_di,tw . Throughout IRL, there was a time-dependent increase in cardiac frequency and mean arterial blood pressure, suggesting activation of the respiratory muscle metaboreflex. The P_di,tw was significantly lower than baseline (34.3 ± 9.6 cmH₂ O) at 3 (23.2 ± 5.7 cmH₂ O, P < 0.001), 15 (24.2 ± 5.1 cmH₂ O, P < 0.001) and 30 min post-IRL (26.3 ± 6.0 cmH₂ O, P < 0.001). The P_ga,tw was not significantly different from baseline (37.6 ± 17.1 cmH₂ O) at 3 (36.5 ± 14.6 cmH₂ O), 15 (33.7 ± 12.4 cmH₂ O) and 30 min post-IRL (32.9 ± 11.3 cmH₂ O). Inspiratory resistive loading elicits objective evidence of diaphragm, but not abdominal, muscle fatigue. Agonist-antagonist interactions for the respiratory muscles appear to be more important during expiratory versus inspiratory loading.