Are oxygen uptake kinetics at the onset of exercise speeded up by local metabolic status in active muscles?

Abstract

To assess the rate-limiting factor of oxygen uptake ($\dot V{\text{O}}_{\text{2}}$) kinetics at the onset of exercise, six healthy male sedentary subjects performed repeated one-legged constant-load cycle exercise. The one-legged constant-load exercise test consisted of two 5-min periods of pedalling at an exercise intensity of 50 W, with a 5-min rest between periods (these exercise periods, i.e. first and second exercises, were performed by the same leg). The exercise was then repeated using the other leg. In addition, two-legged incremental exercise was investigated to establish whether$\dot V{\text{O}}_{\text{2}}$ kinetics were affected by slower heart rate kinetics. The incremental exercise test consisted of two-legged pedalling first with the cycle unloaded as a warm-up for 5 min followed by 50-W exercise for 5 min. The exercise intensity was then increased to 100 W for 5 min. During exercise, gas exchange parameters were determined by the breath-by-breath method and cardiac output ($\dot Q_{\text{c}}$) was determined continuously by an impedance technique with a computer-based automated system. To determine the kinetics of heart rate (HR),$\dot Q_{\text{c}}$ and$\dot V{\text{O}}_{\text{2}}$, a best fit procedure was employed using least-squares criteria with a time delay, except during the initial increase. During the one-legged constant-load exercise test,$\dot V{\text{O}}_{\text{2}}$ kinetics were significantly accelerated by repeated exercise using the same leg. On the other hand, when the exercise was changed to the other leg,$\dot V{\text{O}}_{\text{2}}$ kinetics were significantly slower, although$\dot Q_{\text{c}}$ kinetics continued to be faster. During the incremental exercise test, although the HR response was slower at the transition from 50-W to 100-W exercise than at the transition from warm-up to 50-W exercise, there were no significant changes in$\dot V{\text{O}}_{\text{2}}$ kinetics. These findings suggest that$\dot V{\text{O}}_{\text{2}}$ kinetics may be affected by metabolic conditions in the muscle, but not by blood flow ($\dot Q_{\text{c}}$ and/or HR) kinetics.