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

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,P- tw) and transdiaphragmatic twitch pressure (P-di,P- 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,P- tw or P-di,P- 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,P- tw was significantly lower than baseline (34.3 +/- 9.6 cmH(2)O) at 3 (23.2 +/- 5.7 cmH(2)O, P < 0.001), 15 (24.2 +/- 5.1 cmH(2)O, P < 0.001) and 30 min post-IRL (26.3 +/- 6.0 cmH(2)O, P < 0.001). The P-ga,P- tw was not significantly different from baseline (37.6 +/- 17.1 cmH(2)O) at 3(36.5 +/- 14.6 cmH(2)O), 15 (33.7 +/- 12.4 cmH(2)O) and 30min post-IRL (32.9 +/- 11.3 cmH(2)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.