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

Abstract

Following the start of low-intensity exercise in healthy humans, it has been established that the kinetics of skeletal muscle O(2) delivery is faster than, and does not limit, the kinetics of muscle O(2) uptake (V(O(2)(m))). Direct data are lacking, however, on the question of whether O(2) delivery might limit (V(O(2)(m))) 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 (Q(m)), a-(V(O(2))) difference and (V(O(2)(m))) 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 Q(m), a-(V(O(2))) difference and (V(O(2)(m))) were calculated and subsequently modelled using non-linear regression techniques. For LI, the fundamental component mean response time (MRT(p)) for Q(m) kinetics was significantly shorter than (V(O(2)(m))) kinetics (mean ± SEM, 18 ± 4 vs. 30 ± 4 s; P < 0.05), whereas for HI, the MRT(p) for Q(m) and (V(O(2)(m))) was not significantly different (27 ± 5 vs. 29 ± 4 s, respectively). There was no difference in the MRT(p) for either Q(m) or (V(O(2)(m))) between the two exercise intensities; however, the MRT(p)for a-(V(O(2)) difference was significantly shorter for HI compared with LI (17 ± 3 vs. 28 ± 4 s; P < 0.05). Excess O(2), i.e. oxygen not taken up (Q(m) x (V(O(2))), 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 O(2) delivery does not limit (V(O(2)(m))) kinetics following the onset of LI or HI knee-extension exercise.

Figure 1. Schematic representation of the experimental protocol

Four consecutive 6 min low-intensity single-legged knee-extension exercise bouts (LI, EX1–4) were performed interspersed with 30 min rest periods, followed by four 6 min high-intensity single-legged knee-extension exercise bouts (HI, EX5–8) interspersed with 45 min rest periods. During EX1–2 and EX5–6, femoral arterial blood samples (BSa) and venous blood samples (BSv) were collected and thigh blood flow was measured. During EX3 and EX7, the vein-to-artery transit time (TTv–a) was determined and venous blood samples were collected. During EX4 and EX8, the artery-to-vein transit time (TTa–v) was determined.

Figure 2. Thigh blood flow (A), arterial and venous O₂ content (B) and muscle O₂ extraction (C) before and during 6 min of low-intensity (LI, filled symbols) and high-intensity (HI, open symbols) single-legged knee-extension exercise

Values are mean ± SEM. ^#LI significantly different from HI.

Figure 3. Muscle oxygen uptake before and during 6 min of low-intensity (LI, filled symbols) and high-intensity (HI, open symbols) single-legged knee-extension exercise

Values are mean ± SEM. ^#LI significantly different from HI.

Figure 4. Excess muscle oxygen before and during 6 min of low-intensity (LI, filled symbols) and high-intensity (HI, open symbols) single-legged knee-extension exercise

Values are mean ± SEM.

Figure 5. Schematic illustration, based on the group mean model fits, of the relative changes in muscle blood flow, O₂ uptake and arterio-venous O₂ difference for the initial phase of low-intensity (A) and high-intensity (B) exercise

Notice that muscle blood flow kinetics are faster than muscle O₂ uptake kinetics for low-intensity exercise but that the kinetics of muscle blood flow and muscle O₂ uptake are similar for high-intensity exercise. Notice also that the arterio-venous O₂ difference falls more rapidly following the onset of HI compared with LI exercise.