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1 Applied Physiology Laboratory, Kobe Design University, Kobe, Japan
2 Department of Kinesiology, Kansas State University, Manhattan, Kansas, Japan
3 Exercise Science, Yokohama City University, Yokohama, Japan
4 Applied Human Physiology, Kobe University, Kobe, Japan
5 Exercise Science, Hiroshima Women's University, Hiroshima, Japan
* To whom correspondence should be addressed. E-mail: s-koga{at}kobe-du.ac.jp.
The knee extension exercise (KE) model engenders different muscle and fiber recruitment patterns, blood flow and energetic responses compared with conventional cycle ergometry (CE). This investigation had two aims: 1) to test the hypothesis that upright two-leg KE and CE in the same subjects would yield fundamentally different pulmonary oxygen uptake (pVO2) kinetics, and 2) to characterize the muscle blood flow, muscle VO2 (mVO2) and pVO2 kinetics during KE to investigate the rate-limiting factor(s) of pVO2 on-kinetics and muscle energetics and their mechanistic bases following the onset of heavy exercise. Six subjects performed KE and CE transitions from unloaded to moderate (<ventilatory threshold, VT) and heavy (>VT) exercise. In addition to pVO2 during CE and KE, simultaneous pulsed and echo Doppler methods, combined with blood sampling from the femoral vein, were utilized to quantify the precise temporal profiles of femoral artery blood flow (LBF) and mVO2 at the onset of KE. 1) The gain (amplitude/work rate) of the primary component of pVO2 for both moderate and heavy exercise was higher during KE (~12 ml/W/min) compared with CE (~10), but the time constants for the primary component did not differ. Further, the mean response time (MRT) and the contribution of the slow component to the overall response for heavy KE were significantly greater than for CE. 2) The time constant for the primary component of mVO2 during heavy KE (25.8 ± 9.0 s (SD)) was not significantly different from that of the phase II pVO2. Moreover, the slow component of pVO2 evident for the heavy KE reflected the gradual increase in mVO2. The initial LBF kinetics following onset of KE were significantly faster than the phase II pVO2 kinetics (moderate: time constant LBF = 8.0 ± 3.5 s, pVO2 = 32.7 ± 5.6 s, P<0.05; heavy: LBF = 9.7 ± 2.0 s, pVO2 = 29.9 ± 7.9 s, P<0.05). The MRT of LBF was also significantly faster than that of pVO2. These data demonstrate that the energetics (as gain) for KE are greater than for CE, but the kinetics of adjustment (as time constant for the primary component) are similar. Further, the kinetics of muscle blood flow during KE are faster than those of pVO2, consistent with an intramuscular limitation to VO2 kinetics, i.e., a microvascular O2 delivery-to-O2 requirement mismatch or oxidative enzyme inertia.
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