Oxygen cost of exercise hyperpnea: Implications for performance

We addressed two questions concerned with the metabolic cost and performance of respiratory muscles in healthy young subjects during exercise: 1) does exercise hyperpnea ever attain a 'critical useful level'? and 2) is the work of breathing (Ẇ(v)) at maximum O₂ uptake (V̇O(2 max)) fatiguing to the respiratory muscles? During progressive exercise to maximum, we measured tidal expiratory flow-volume and transpulmonary pressure- (Ptp) volume loops. At rest, subjects mimicked their maximum and moderate exercise Ptp-volume loops, and we measured the O₂ cost of the hyperpnea (V̇O(2 RM)) and the length of time subjects could maintain reproduction of their maximum exercise loop. At maximum exercise, the O₂ cost of ventilation (V̇E) averaged 10 ± 0.7% of the V̇O(2 max). In subjects who used most of their maximum reserve for expiratory flow and for inspiratory muscle pressure development during maximum exercise, the V̇O(2 RM) required 13-15% of V̇O(2 max). The O₂ cost of increasing V̇E from one work rate to the next rose from 8% of the increase in total body V̇O₂ (V̇O(2 T)) during moderate exercise to 39 ± 10% in the transition from heavy to maximum exercise; but in only one case of extreme hyperventilation, combined with a plateauing of the V̇O(2 T), did the increase in V̇O(2 RM) equal the increase in V̇O(2 T). All subjects were able to voluntarily mimic maximum exercise Ẇ(v) for 3-10 times longer than the duration of the maximum exercise. We conclude that the O₂ cost of exercise hyperpnea is a significant fraction of the total V̇O(2 max) but is not sufficient to cause a critical level of 'useful' hyperpnea to be achieved in healthy subjects. The V̇E and Ẇ(v) during maximum exercise are nonfatiguing and sustainable.