The relationship between muscle deoxygenation and activation in different muscles of the quadriceps during cycle ramp exercise.

The relationship between muscle deoxygenation and activation was examined in three different muscles of the quadriceps during cycling ramp exercise. Seven young male adults (24 ± 3 yr; mean ± SD) pedaled at 60 rpm to exhaustion, with a work rate (WR) increase of 20 W/min. Pulmonary oxygen uptake was measured breath-by-breath, while muscle deoxygenation (HHb) and activity were measured by time-resolved near-infrared spectroscopy (NIRS) and surface electromyography (EMG), respectively, at the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM). Muscle deoxygenation was corrected for adipose tissue thickness and normalized to the amplitude of the HHb response, while EMG signals were integrated (iEMG) and normalized to the maximum iEMG determined from maximal voluntary contractions. Muscle deoxygenation and activation were then plotted as a percentage of maximal work rate (%WR(max)). The HHb response for all three muscle groups was fitted by a sigmoid function, which was determined as the best fitting model. The c/d parameter for the sigmoid fit (representing the %WR(max) at 50% of the total amplitude of the HHb response) was similar between VL (47 ± 12% WR(max)) and VM (43 ± 11% WR(max)), yet greater (P < 0.05) for RF (65 ± 13% WR(max)), demonstrating a "right shift" of the HHb response compared with VL and VM. The iEMG also showed that muscle activation of the RF muscle was lower (P < 0.05) compared with VL and VM throughout the majority of the ramp exercise, which may explain the different HHb response in RF. Therefore, these data suggest that the sigmoid function can be used to model the HHb response in different muscles of the quadriceps; however, simultaneous measures of muscle activation are also needed for the HHb response to be properly interpreted during cycle ramp exercise.

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 (pVo(2)) kinetics and 2) to characterize the muscle blood flow, muscle Vo(2) (mVo(2)), and pVo(2) kinetics during KE to investigate the rate-limiting factor(s) of pVo(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 pVo(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 mVo(2) at the onset of KE. First, the gain (amplitude/work rate) of the primary component of pVo(2) for both moderate and heavy exercise was higher during KE ( approximately 12 ml.W(-1).min(-1)) compared with CE ( approximately 10), 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 mVo(2) during heavy KE [25.8 +/- 9.0 s (SD)] was not significantly different from that of the phase II pVo(2). Moreover, the slow component of pVo(2) evident for the heavy KE reflected the gradual increase in mVo(2). The initial LBF kinetics after onset of KE were significantly faster than the phase II pVo(2) kinetics (moderate: time constant LBF = 8.0 +/- 3.5 s, 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 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 pVo(2), consistent with an intramuscular limitation to Vo(2) kinetics, i.e., a microvascular O(2) delivery-to-O(2) requirement mismatch or oxidative enzyme inertia.

Dynamics of the heart rate response to sinusoidal work in humans: influence of physical activity and age.

The purpose of the present study was to define the influence of age and exercise training on the heart rate (HR) dynamic response (i.e. kinetics) to sinusoidal work. A total of 63 healthy subjects (31 men and 32 women; age range 19-69 years) underwent a three-step incremental work test, during which peak oxygen uptake (.V(O)(2peak)) was estimated by the YMCA method. Sinusoidal work varying between 20% and 60% of HR(reserve) was employed for periods of 1, 3, 6, 9 and 12 min. HR was monitored in a beat-by-beat manner with a cardiotachometer. The kinetics of the HR response were analysed by frequency analysis and estimated by a first-order transfer function with time constant (tau) and time delay (TD). Physical training status was estimated as stepping frequency, as measured with a pedometer during the daytime, and averaged over seven consecutive days. The mean response time of HR kinetics (HR(MRT): tau pulse TD) tended to increase gradually with age (0.36 s.year(-1)), and linear regression analysis revealed that the correlation between HR(MRT) and age was significant (r=0.31, P<0.05), although not as highly significant as that between HR(MRT) and physical activity (r=-0.48, P<0.0001). HR(MRT) was not related to the S.D. of HR variation (an indicator of parasympathetic mediation) at rest. In addition, .V(O)(2peak) showed a significantly greater correlation with age (r=-0.60, P<0.0001) than with physical activity (r=-0.14, not significant). In conclusion, these findings suggest that HR dynamics, which may depend on sympathetic nervous activity, are more sensitive to physical activity than to age, but that .V(O)(2peak), as estimated by the age-associated decline in maximum HR, is unrelated to physical training status.