Lack of importance of respiratory muscle load in ventilatory regulation during heavy exercise in humans

1. Seven active subjects (24 ± 1 years; maximal oxygen uptake (V(O₂,max), 3.77 ± 0.21 min^-1; mean ± S.E.M.) performed constant work rate heavy exercise (CWHE, ~80% of maximal incremental work rate) to exhaustion on 2 days, one with (unload) and one without (control) respiratory muscle unloading. 2. With unloading, a special device applied flow-proportional mouth pressure assist (positive with inspiratory (I), negative with expiratory (E) flows) throughout each breath. No pressure assist occurred during control CWHE. To confirm unloading, respiratory muscle pressures (P(mus)) were derived (n = 5) from measured pleural pressure and chest wall elastic and resistive pressures. 3. Other than minor differences in early exercise, the temporal course of minute ventilation (V(E)) was similar in both tests as exercise progressed. The fall in estimated mean alveolar CO₂ (P(Ā,CO₂)) throughout CWHE was identical in both tests. There were no significant differences (ANOVA) in V(E) tidal volume, frequency, oxygen consumption rate (V(O₂)), heart rate or P(Ā,CO₂) between unload and control CWHE, at matched times (at 50% of control duration and at the end of exercise). Unloading reduced P(mus) significantly throughout CWHE; at 50% control duration, peak P(mus,I) and P(mus,E) fell by 24 and 41%, respectively, with unloading, as did mean P(mus,I) and P(mus,E) (21 and 44%). 4. The lack of any significant changes in V(E) P(Ā,CO₂) or breathing pattern, despite a marked reduction in respiratory muscle load throughout CWHE, indicates that the load on the respiratory muscles has only a minor role in the regulation of ventilation during heavy exercise. 5. The absence of improvement in CWHE duration (control, 11.4 ± 1.2min; unload, 12.6 ± 2.1 min, n.s.) with unloading implies that respiratory muscle function does not limit endurance exercise performance during cycling in healthy humans.