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

先前的大强度运动可诱发乳酸酸中毒，在随后的大强度运动中，可加速肺氧摄取（VO_2）动力学并减少其慢成分。假设两种有争议的机制来解释这种现象：运动肢体的灌注改善（即氧气输送）或运动肢体或肌肉组织内的生理原因（即氧气利用）。首先，为了阐明前一假说，我们研究了在重复的大强度伸膝运动中VO_2和股动脉血流量（BFf）的时间曲线。利用多普勒超声技术连续测量BFf。第二次运动的VO_2动力学比第一次运动快。然而，两次发作之间的BFf动力学没有观察到显著差异。结果表明，第二次大负荷运动时VO_2动力学的加快与BFf对工作肌的加速无关。由于腿部自行车运动中涉及的大腿肌肉是复杂的，特别是在剧烈运动中，大腿肌肉使用模式的时间和强度依赖性差异可以被假定为解释该现象的因素之一。因此，下一步，我们研究了在三种运动强度（即中度、重度和重度）下，通过大腿的磁共振成像（MRI）横向弛豫时间（T2 ;即肌肉激活指数）评估的肌肉使用模式是否影响腿部周期运动中的VO_2动力学。在静息、运动第3分钟和第6分钟通过MRI评估每个大腿肌肉中的T2。在剧烈运动中，一组屈肌在运动过程中从第3分钟到第6分钟，VO 2慢成分发展，相关的T2增加。VO 2慢成分的量可能与肌肉激活的程度和/或模式有关。

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.