The effects of exercise duration on dynamics of respiratory gas exchange,ventilation, and heart rate

This study investigated the effect of exercise duration on the response dynamics of oxygen consumptionVO₂, carbon dioxide outputVCO₂, ventilation V_E), and cardiac frequency (f_c) following stepped changes in exercise intensity, by manipulating the duration of the pretransition exercise period. A group of 11 healthy men performed a stepped exercise intensity cycling protocol on three separate occasions, each consisting of a stepped increase from 55% to 65% peak oxygen consumptionVO_2,peak of 6-min duration, followed by a stepped decrease to 55%VO_2,peak of 10-min duration. This stepped protocol was preceded by either 5, 15, or 60 min of cycling at 55%VO_2,peak. The response times for each variable were calculated at 10% increments between the prestep baselines and poststep plateaux. Following the stepped increase, the response times forVO₂ at the 50%, 60%, 70%, 80%, and 90% relative increments were significantly reduced in the 60-min condition compared to the 15-min condition (P< 0.05); however, the response times forVCO₂ andf_c were not significantly altered across the three conditions. No significant differences were found in the response times forVO₂,VCO₂ andf_c, across the three conditions following the stepped decrease in exercise intensity. It was concluded that the faster response time of aerobic metabolism to a stepped increase in exercise intensity was mediated by increases in active muscle temperature, leading to improved oxygen utilisation.