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Does inspiratory resistive loading cause expiratory muscle fatigue? Peters, Carli Monica


Expiratory resistive loading (ERL) 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). The purpose of this study was to determine whether IRL elicits expiratory as well as inspiratory muscle fatigue. Male subjects (n=10) 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. During IRL (mean ± SE; 11.9 ± 2.5 min), mean arterial pressure and heart rate increased in a time-dependent manner (13 mmHg and 50 beats/min for the final min, respectively). Pdi,tw was significantly lower than baseline (34.1 ± 3.2 cmH₂O) at 3 min (23.2 ± 1.9 cmH₂O, p<0.05) and 15 min post-IRL (24.2 ± 1.7 cmH₂O, p<0.05). Pga,tw was not significantly different from baseline after IRL. These results suggest that IRL elicits objective evidence of diaphragm, but not abdominal, muscle fatigue. Agonist-antagonist interactions for the respiratory muscles appear to be more important during ERL than during IRL. Future studies attempting to characterize the physiological consequences of diaphragm fatigue, without the confounding effects of abdominal fatigue, can use IRL to induce diaphragm fatigue.

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