Comparison of oxygen uptake kinetics during knee extension and cycle exercise

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 O-2 uptake (p(V)over dotO(2)) kinetics and 2) to characterize the muscle blood flow, muscle (V)over dotO(2) (m(V)over dotO(2)), and p(V)over dotO(2) kinetics during KE to investigate the rate-limiting factor(s) of p(V)over dotO(2) on kinetics and muscle energetics and their mechanistic bases after 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 p(V)over dotO(2) during CE and KE, simultaneous pulsed and echo Doppler methods, combined with blood sampling from the femoral vein, were used to quantify the precise temporal profiles of femoral artery blood flow (LBF) and m(V)over dotO(2) at the onset of KE. First, the gain (amplitude/work rate) of the primary component of p(V)over dotO(2) for both moderate and heavy exercise was higher during KE (similar to12 ml.W-1.min(-1)) compared with CE (similar to10), but the time constants for the primary component did not differ. Furthermore, 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. Second, the time constant for the primary component of m(V)over dotO(2) during heavy KE [25.8+/-9.0 s (SD)] was not significantly different from that of the phase II p(V)over dotO(2). Moreover, the slow component of p(V)over dotO(2) evident for the heavy KE reflected the gradual increase in m(V)over dotO(2). The initial LBF kinetics after onset of KE were significantly faster than the phase II p(V)over dotO(2) kinetics (moderate: time constant LBF=8.0+/-3.5 s, p(V)over dotO(2)=32.7+/-5.6 s, P<0.05; heavy: LBF=9.7 +/- 2.0 s, p(V)over dotO(2)=29.9 +/- 7.9 s, P<0.05). The MRT of LBF was also significantly faster than that of p(V)O-2. 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. Furthermore, the kinetics of muscle blood flow during KE are faster than those of p(V)over dotO(2), consistent with an intramuscular limitation to (V)over dotO(2) kinetics, i.e., a microvascular O-2 delivery-to-O-2 requirement mismatch or oxidative enzyme inertia.