Effect of expiratory resistive loading on inspiratory and expiratory muscle fatigue

Inspiratory and expiratory pressures are increased during expiratory resistive loading (ERL). We asked whether ERL elicits inspiratory as well as expiratory muscle fatigue. On four separate days, seven male subjects underwent ERL to task failure. Subjects maintained respiratory frequency at 15 breaths min⁻¹, expiratory gastric pressure at 40% or 60% of maximum, and expiratory duty cycle at 0.4 or 0.7 (ERL_40%0.4, ERL_40%0.7, ERL_60%0.4, ERL_60%0.7). Inspiratory and abdominal muscle contractility was assessed before and up to 30 min after ERL by measuring transdiaphragmatic twitch pressure (P_di,tw) and gastric twitch pressure (P_ga,tw) in response to magnetic nerve stimulation. After each trial, P_di,tw and P_ga,tw were reduced below baseline values (−9 to −15% for P_di,tw and −15 to −22% for P_ga,tw; P < 0.05). The severity of diaphragm fatigue was unaffected by expiratory pressure production or expiratory duty cycle, whereas extending the expiratory duty cycle increased the severity of abdominal muscle fatigue. In conclusion, ERL elicits contractile fatigue of the diaphragm and the abdominal muscles.