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We tested the hypothesis that baseline cardiac autonomic function and its acute response to all-out interval exercise explains individual fitness responses to high-intensity interval training (HIT). Healthy middle-aged sedentary men performed HIT (n = 12, 4-6><30s of all-out cycling efforts with 4-min recovery) or aerobic training (AET, n = 9,40-60 min at 60% of peak workload in exercise test [Loadpeak]), comprising 6 sessions within 2 weeks. Low (LF) and high frequency (HF) power of R-R interval oscillation were analyzed from data recorded at supine and standing position (5+ 5 min) every morning during the intervention. A significant training effect (p< 0.001), without a training*group interaction, was observed in Loadpeak and peak oxygen consumption (VO2peak). Pre-training supine LF/HF ratio, an estimate of sympathovagal balance, correlated with training outcome in Loadpeak (Spearman's rho [rs]=-0.74, p = 0.006) and VO2peak (rs = -0.59, p = 0.042) in the HIT but not the AET group. Also, the mean change in the standing LF/ HF ratio in the morning after an acute HIT exercise during the 1st week of intervention correlated with training response in Loadpeak ((rs = -0.68,p = 0.014) and VO2peak (r5=-0.60, p = 0.039) with HIT but not with AET. In conclusion, pre-training cardiac sympathovagal balance and its initial alterations in response to acute HIT exercise were related to fitness responses to short-term HIT. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the effects of endurance training on skeletal muscle hemodynamics and oxygen consumption. Seven healthy endurance-trained and seven untrained subjects were studied. Oxygen uptake, blood flow, and blood volume were measured in the quadriceps femoris muscle group by use of positron emission tomography and [15O]O2, [15O]H2O, and [15O]CO during rest and one-legged submaximal intermittent isometric exercise. The oxygen extraction fraction was higher (0.49 +/- 0.14 vs. 0.29 +/- 0.12; P = 0.017) and blood transit time longer (0.6 +/- 0.1 vs. 0.4 +/- 0.1 min; P = 0.04) in the exercising muscle of the trained compared with the untrained subjects. The flow heterogeneity by means of relative dispersion was lower for the exercising muscle in the trained (50 +/- 9%) compared with the untrained subjects (65 +/- 13%, P = 0.025). In conclusion, oxygen extraction is higher, blood transit time longer, and perfusion more homogeneous in endurance-trained subjects compared with untrained subjects at the same workload. These changes may be associated with improved exercise efficiency in the endurance-trained subjects.

Muscle blood flow has been shown to be heterogeneous at the voxel by voxel level in positron emission tomography (PET) studies using oxygen-15 labelled water. However, the limited spatial resolution of the imaging device does not allow direct measurement of true vascular flow heterogeneity. Fractal dimension (D) obtained by fractal analysis describes the relationship between the relative dispersion and the size of the region studied, and has been used for the assessment of perfusion heterogeneity in microvascular units. This study was undertaken to evaluate fractal characteristics of PET perfusion data and to estimate perfusion heterogeneity in microvascular units. Skeletal muscle blood flow was measured in healthy subjects using [15O]water PET and the fractal characteristics of blood flow in resting and exercising skeletal muscle were analysed. The perfusion heterogeneity in microvascular units was estimated using the measured heterogeneity (relative dispersion, RD = SD/mean) and D values. Heterogeneity due to methodological factors was estimated with phantoms and subtracted from the flow data. The number of aggregated voxels was inversely correlated with RD both in phantoms (Pearson r = -0.96-0.97) and in muscle (Pearson r = -0.94) when both parameters were expressed using a logarithmic scale. Fractal dimension was similar between exercising (1.13) and resting (1.14) muscles and significantly lower than the values in the phantoms with different activity levels (1.27-1.29). Measured flow heterogeneity values were 20% +/- 6% (exercise) and 27% +/- 5% (rest, P < 0.001), whereas estimated flow heterogeneity values in microvascular units (1 mm3) were 35% +/- 14% (exercise) and 49% +/- 14% (rest, P < 0.01). In conclusion, these results show that it is feasible to apply fractal analysis to PET perfusion data. When microvascular flow heterogeneity is estimated using fractals, perfusion appears to be more heterogeneous in microvascular units than when obtained by routine spatial analysis of PET data. Analysis of flow heterogeneity using PET and fractals could provide new insight into physiological conditions and diseases associated with changes in peripheral vascular function.

Blood flow is the main regulator of skeletal muscle's oxygen supply, and several studies have shown heterogeneous blood flow among and within muscles. However, it remains unclear whether exercise changes the heterogeneity of flow in exercising human skeletal muscle. Muscle blood flow and spatial flow heterogeneity were measured simultaneously in exercising and in the contralateral resting quadriceps femoris (QF) muscle in eight healthy men using H2(15)O and positron emission tomography. The relative dispersion (standard deviation/mean) of blood flow was calculated as an index of spatial flow heterogeneity. Average muscle blood flow in QF was 29 (10) ml x (kg muscle)(-1) x min(-1) at rest and 146 (54) ml x (kg muscle)(-1) x min(-1) during exercise (P = 0.008 for the difference). Blood flow was significantly (P < 0.001) higher in the vastus medialis and the vastus intermedius than in the vastus lateralis and the rectus femoris, both in the resting and the exercising legs. Flow was more homogeneous in the exercising vastus medialis and more heterogeneous (P < 0.001) in the exercising vastus lateralis (P = 0.01) than in the resting contralateral muscle. Flow was more homogeneous (P < 0.001) in those exercising muscles in which flow was highest (vastus intermedius and vastus medialis) as compared to muscles with the lowest flow (vastus lateralis and the rectus femoris). These data demonstrate that muscle blood flow varies among different muscles in humans both at rest and during exercise. Muscle perfusion is spatially heterogeneous at rest and during exercise, but responses to exercise are different depending on the muscle.