Rate-limiting factors of oxygen uptake kinetics during exercise

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The knee extensor (KE) exercise model engenders different muscle and fiber recruitment patterns, blood flow and energetic responses compared with conventional cycle ergometry (CE). This investigation tested the hypotheses that: 1) upright two-leg KE and CE in the same subjects would yield fundamentally different pulmonary oxygen uptake (pVo_2) dynamics (H_1), and 2) pulmonary and muscle Vo_2 (mVo_2) kinetics for KE would be mechanistically linked (and therefore similar)(H_2). Six subjects performed KE and CE transitions from unloaded to moderate (<ventilatory threshold, VT) and heavy (>VT) exercise. In addition to pVo_2 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 mVo_2 at the onset of KE. H_1: The gain (amplitude/work rate) of the fast component of pVo_2 for both moderate and heavy exercise was higher during KE (〜12ml/W/min) compared with CE (〜10), but the time constants for the fast 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. H_2: The time constant for the component of mVo_2. Moreover, the slow component of pVo_2 evident for the heavy KE reflected the gradual increase in mVo_2. The initial LBF kinetics following onset of KE were significantly faster than the phase 2 pVo_2 kinetics (moderate: time constant LBF = 8.0 ^^+__- 3.5s, pVo_2 = 32.7 ^^+__- 5.6 s, P<0.05; heavy: LBF = 9.7 ^^+__- 2.0 s, pVo_2 = 29.9 ^^+__- 7.9 s, P<0.05). The MRT of LBF was also significantly faster than that of pVo_2. These data demonstrate similar time constants but a greater gain for pVo_2 dynamics of KE vs. CE. In addition, for KE there was a striking dissociation between the temporal profiles of muscle blood flow and pVo_2 during KE.