ARE OXYGEN-UPTAKE KINETICS AT THE ONSET OF EXERCISE SPEEDED UP BY LOCAL METABOLIC STATUS IN ACTIVE MUSCLES

To assess the rate-limiting factor of oxygen uptake (VO2) 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 VO2 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 (Q(c)) was determined continuously by an impedance technique with a computer-based automated system. To determine the kinetics of heart rate (HR), Q(c), and VO2, 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, VO2 kinetics were significantly accelerated by repeated exercise using the same leg. On the other hand, when the exercise was changed to the other leg, VO2 kinetics were significantly slower, although Q(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 VO2 kinetics. These findings suggest that VO2 kinetics may be affected by metabolic conditions in the muscle, but not by blood flow (Q(c) and/or HR) kinetics.