INFLUENCE OF GLOBAL INSPIRATORY MUSCLE FATIGUE ON BREATHING DURING EXERCISE

We evaluated the effect of global inspiratory muscle fatigue (GF) on r espiratory muscle control during exercise at 30, 60, and 90% of maxima l power output in normal subjects. Fatigue was induced by breathing ag ainst a high inspiratory resistance until exhaustion. Esophageal and g astric pressures, anteroposterior displacement of the rib cage and abd omen, breathing pattern, and perceived breathlessness were measured. I nduction of GF had no effect on the ventilatory parameters during mild and moderate exercise. It altered, however, ventilatory response to h eavy exercise by increasing breathing frequency and minute ventilation , with minor changes in tidal volume. This was accompanied by an incre ase in perceived breathlessness. GF significantly increased both the t onic and phasic activities of abdominal muscles that allowed 1) the di aphragm to maintain its function while developing less pressure, 2) th e same tidal volume with lesser shortening of the rib cage inspiratory muscles, and 3) relaxation of the abdominal muscles to contribute to lung inflation. The increased work performed by the abdominal muscles may, however, lead to a reduction in their strength. GF may impair exe rcise performance in some healthy subjects that is probably not relate d to excessive breathlessness or other ventilatory factors. We conclud e that the respiratory system is remarkably adaptable in maintaining v entilation during exercise even with impaired inspiratory muscle contr actility.