Limb blood flow and muscle oxygenation responses during handgrip exercise above vs. below critical force

This study compared the brachial artery blood flow ((Q) over dot(BA)) and microvascular oxygen delivery responses during handgrip exercise above vs. below critical force (CF; the isometric analog of critical power). (Q) over dot(BA) and microvascular oxygen delivery are important determinants of oxygen utilization and metabolite accumulation during exercise, both of which increase progressively during exercise above CF. However the (Q) over dot(BA) and microvascular oxygen delivery responses above vs. below CF remain unknown. We hypothesized that (Q) over dot(BA), 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. Seven men and six women performed 1) a 5-min rhythmic isometric-handgrip maximal-effort WA (MET) to determine CF and 2) two constant target-force tests above (severe-intensity; S1 and S2) and two constant target-force tests below (heavy-intensity; H1 and H2) CF. CF was 189.3 +/- 16.7 N (29.7 +/- 1.6%MVC). At end-exercise, (Q) over dot(BA) was greater for tests above CF (S1: 418 +/- 147 mL/min; S2: 403 +/- 137 mL/min) compared to tests below CF (H1: 287 +/- 97 mL/min; H2: 340 +/- 116 mL/min; all p < 0.05) but was not different between S1 and S2. Further, end-test (Q) over dot(BA) during both tests above CF was not different from (Q) over dot(BA) estimated at CF (392 +/- 37 mL/min). At end-exercise, deoxy- [heme] was not different between tests above CF (S1: 150 +/- 50 mu M; S2: 155 +/- 57 mu M), but was greater during tests above CF compared to tests below CF (H1: 101 +/- 24 mu M; H2: 111 +/- 21 mu M; all p < 0.05). M end-exercise, total-[heme] was not different between tests above CF (S1: 404 +/- 58 mu M; S2: 397 +/- 73 mu M), but was greater during tests above CF compared to H1 (352 +/- 58 mu M; p < 0.01) but not H2 (371 +/- 57 mu M). These data suggest limb blood flow limitations exist and maximal levels of muscle microvascular oxygen delivery and extraction occur during exercise above, but not below, CF.