Limb blood flow and muscle oxygenation responses to rhythmic exercise below and above critical force

Abstract

This study aimed to determine brachial artery blood flow (Q̇BA) and microvascular oxygen delivery responses to rhythmic exercise above and below critical force (CF; the isometric analog of critical power). We hypothesized that Q̇BA, deoxygenated‐heme (deoxy‐[heme]; an estimate of microvascular fractional oxygen extraction), and total‐heme concentrations (total‐[heme]; an estimate of changes in microvascular hematocrit) would demonstrate physiological maximums above CF despite increases in exercise intensity. 13 healthy subjects performed 1) a 5‐min rhythmic isometric‐handgrip maximal‐effort test (MET) to determine CF and 2) constant target‐force tests above (severe‐intensity; S1 and S2) and below (heavy‐intensity; H1 and H2) CF. CF was 189.3 ± 16.7 N (29.7 ± 1.6 %MVC). At end‐exercise, Q̇BA was greater during S1 (418 ± 147 mL/min) and S2 (403 ± 137 mL/min) than during H1 (287 ± 97 mL/min) and H2 (340 ± 116 mL/min; p < 0.05) but was not different between S1 and S2. Further, Q̇BA for S1 and S2 were not significantly different from the Q̇BA estimated for CF (392 ± 37 mL/min). At end‐exercise, deoxy‐[heme] was not different between S1 (150 ± 50 μM) and S2 (155 ± 57 μM), but was greater during S1 and S2 than during H1 (101 ± 24 μM) and H2 (111 ± 21 μM; p < 0.05). Total‐[heme] was not different between S1 (404 ± 58 μM) and S2 (397 ± 73 μM), but was greater during S1 and S2 than H1 (352 ± 58 μM; p < 0.01) but not H2 (371 ± 57 μM). These data suggest limb blood flow limitations and maximal levels of muscle microvascular oxygen delivery and extraction during exercise above, but not below, CF.End‐exercise brachial artery blood flow (Q̇BA) values. Q̇BA at end‐exercise below (● H1 and ○ H2) and above (▼ S1 and Δ S2) critical force (CF), and Q̇BA predicted at CF by linear regression (♦). * Significantly greater than Q̇BA at end‐exercise during H1 (p < 0.05). ** Significantly greater than Q̇BA at end‐exercise during H2 (p < 0.05). Note no differences among end‐exercise Q̇BA values at (estimated) and above CF, suggesting a maximal response in the severe‐intensity domain.Figure 1Frequency‐domain near‐infrared spectroscopy (FD‐NIRS) measurements of muscle oxygenation during exercise below (● H1 and ○ H2) and above (▼ S1 and Δ S2) critical force (CF). Dashed lines indicate average endexercise values of respective FD‐NIRS signals during the maximal‐effort test (MET). * Significantly different from end‐exercise during H1 and H2 (p < 0.05). ** Significantly greater than total‐[heme] at end‐exercise during H2 (p < 0.05).Figure 2