Effect of Inspiratory Resistive Loading on Expiratory Muscle Fatigue

Abstract

Expiratory resistive loading elicits inspiratory as well as expiratory muscle fatigue, suggesting parallel co-activation of the inspiratory muscles during expiration. It is unknown whether the expiratory muscles are similarly co-activated to the point of fatigue during inspiratory resistive loading (IRL). PURPOSE: To determine whether IRL elicits expiratory as well as inspiratory muscle fatigue. METHODS: Healthy male subjects (n=9) underwent isocapnic IRL to task failure (60% maximal inspiratory pressure, 15 breaths/min, 0.7 inspiratory duty cycle). Abdominal and diaphragm contractile function was assessed at baseline and at 3, 15 and 30 min post-IRL by measuring gastric twitch pressure (Pga,tw) and transdiaphragmatic twitch pressure (Pdi,tw) in response to potentiated magnetic stimulation of the thoracic and phrenic nerves, respectively. Electromyographic activity of the diaphragm, rectus abdominis, and external oblique was monitored to ensure consistency of stimulation. Fatigue was defined as >15% reduction from baseline in Pga,tw or Pdi,tw. RESULTS: During IRL (mean±SE;15.0±2.5 min), mean arterial pressure (MAP) and heart rate (HR) increased in a time-dependent manner (13 mmHg and 50 beats/min for the final min, respectively). Peak Pdi was significantly lower during the final min of IRL than during the first min (p<0.05). Peak Pga did not change significantly throughout IRL. Pdi,tw was significantly lower than baseline (34.3±3.2 cmH2O) at 3 min (23.2±1.9 cmH2O, p<0.001), 15 min (24.2±1.7 cmH2O, p<0.001) and 30 min post-IRL (26.3±2.0 cmH2O, p<0.001). Pga,tw was not significantly different from baseline (37.6±5.7 cmH2O) at 3 min (36.5±4.9 cmH2O), 15 min (33.7±4.13 cmH2O) and 30 min post-IRL (32.9±3.8 cmH2O). CONCLUSION: IRL 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. Funding: NSERC