Control mechanism of pulmonary oxygen uptake kinetics during repeated bouts of heavy exercise in humans

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Prior heavy exercise which induces lactic acidosis speeds pulmonary oxygen uptake (VO_2) kinetics and reduces its slow component during the following heavy exercise. Two controversial mechanisms are hypothesized to explain this phenomenon : the improved perfusion to exercising limb (i.e. oxygen delivery) or the physiological cause(s) within exercising limb or musculatures (i.e. oxygen utilization). Firstly to clarify the former hypothesis, we investigated the temporal profile between VO_2 and femoral artery blood flow (BFf) during repeated bouts of heavy knee extension exercise. A Doppler ultrasound technique was utilized to measure continuously the BFf. The VO_2 kinetics for the 2nd bout was faster than that for the 1st bout. However, no significant difference was observed for the kinetics of BFf between both bouts. It was concluded that the speeding of VO_2 kinetics during 2nd heavy exercise was not associated with a similar speeding of BFf to the working muscles. Since the thigh muscles involved in leg cycling exercise are complex, especially during heavy exercise, the time- and intensity-dependent differences of muscle use pattern in the thigh may be postulated as one of the factors to explain the phenomenon. As a next step, we, therefore, examined whether the muscle use pattern evaluated by magnetic resonance imaging (MRI) transverse relaxation time (T2 ; i.e. index of muscle activation) in thigh influence the VO_2-kinetics during leg cycle exercise at three exercise intensities (i.e. moderate, heavy, and severe). The T2 in each thigh muscle was evaluated by MRI at rest, 3rd and 6th min of exercise. In heavy and severe exercises, the VO2 slow components were developed and the associated T2 were increases from 3rd to 6th min during exercise in a group of flexor muscles. It seems that the amount of VO2 slow component might be associated with the degree and/or pattern of muscle activation.