Influence of exercise intensity on skeletal muscle blood flow, O₂ extraction and O₂ uptake on-kinetics

Following the start of low-intensity exercise in healthy humans, it has been established that the kinetics of skeletal muscle O₂ delivery is faster than, and does not limit, the kinetics of muscle O₂ uptake. Direct data are lacking, however, on the question of whether O₂ delivery might limit kinetics during high-intensity exercise. Using multiple exercise transitions to enhance confidence in parameter estimation, we therefore investigated the kinetics of, and inter-relationships between, muscle blood flow, a- difference and following the onset of low-intensity (LI) and high-intensity (HI) exercise. Seven healthy males completed four 6 min bouts of LI and four 6 min bouts of HI single-legged knee-extension exercise. Blood was frequently drawn from the femoral artery and vein during exercise and, a- difference and were calculated and subsequently modelled using non-linear regression techniques. For LI, the fundamental component mean response time (MRT_p) for kinetics was significantly shorter than kinetics (mean ± SEM, 18 ± 4 vs. 30 ± 4 s; P < 0.05), whereas for HI, the MRT_p for and was not significantly different (27 ± 5 vs. 29 ± 4 s, respectively). There was no difference in the MRT_p for either or between the two exercise intensities; however, the MRT_p for a- difference was significantly shorter for HI compared with LI (17 ± 3 vs. 28 ± 4 s; P < 0.05). Excess O₂, i.e. oxygen not taken up (×), was significantly elevated within the first 5 s of exercise and remained unaltered thereafter, with no differences between LI and HI. These results indicate that bulk O₂ delivery does not limit kinetics following the onset of LI or HI knee-extension exercise.