Neuromuscular recovery from severe- and extreme-intensity exercise in men and women.

Maximal voluntary contraction force (MVC), potentiated twitch force (Qpot), and voluntary activation (%VA) recover to baseline within 90 s following extreme-intensity exercise. However, methodological limitations mask important recovery kinetics. We hypothesized reductions in MVC, Qpot, and %VA at task failure following extreme-intensity exercise would be less than following severe-intensity exercise, and Qpot and MVC following extreme-intensity exercise would show significant recovery within 120 s but remain depressed following severe-intensity exercise. Twelve subjects (6 men) completed 2 severe-intensity (40, 50% MVC) and 2 extreme-intensity (70, 80% MVC) isometric knee-extension exercise bouts to task failure (Tlim). Neuromuscular function was measured at baseline, Tlim, and through 150 s of recovery. Each intensity significantly reduced MVC and Qpot compared with baseline. MVC was greater at Tlim (p < 0.01) and at 150 s of recovery (p = 0.004) following exercise at 80% MVC compared with severe-intensity exercise. Partial recovery of MVC and Qpot were detected within 150 s following Tlim for each exercise intensity; Qpot recovered to baseline values within 150 s of recovery following exercise at 80% MVC. No differences in %VA were detected pre- to post-exercise or across recovery for any intensity. Although further analysis showed sex-specific differences in MVC and Qpot, future studies should closely examine sex-dependent responses to extreme-intensity exercise. It is clear, however, that these data reinforce that mechanisms limiting exercise tolerance during extreme-intensity exercise recover quickly. Novelty: Severe- and extreme-intensity exercise cause independent responses in fatigue accumulation and the subsequent recovery time courses. Recovery of MVC and Qpot occurs much faster following extreme-intensity exercise in both men and women.

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

This study compared the brachial artery blood flow (Q̇BA) and microvascular oxygen delivery responses during handgrip exercise above vs. below critical force (CF; the isometric analog of critical power). Q̇BA 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 Q̇BA and microvascular oxygen delivery responses above vs. below CF remain unknown. 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. Seven men and six women performed 1) a 5-min rhythmic isometric-handgrip maximal-effort test (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, Q̇BA 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 Q̇BA during both tests above CF was not different from Q̇BA estimated at CF (392 ± 37 mL/min). At end-exercise, deoxy-[heme] was not different between tests above CF (S1: 150 ± 50 µM; S2: 155 ± 57 µM), but was greater during tests above CF compared to tests below CF (H1: 101 ± 24 µM; H2: 111 ± 21 µM; all p < 0.05). At end-exercise, total-[heme] was not different between tests above CF (S1: 404 ± 58 µM; S2: 397 ± 73 µM), but was greater during tests above CF compared to H1 (352 ± 58 µM; p < 0.01) but not H2 (371 ± 57 µM). 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.

Effect of assuming constant tissue scattering on measured tissue oxygenation values during tissue ischemia and vascular reperfusion.

The purpose of this study was to determine the effects of assuming constant tissue scattering properties on tissue oxygenation measurements during a vascular occlusion test (VOT). Twenty-one subjects (21.8 ± 1.9 yr) completed a VOT [1 min baseline (BL), 5 min of tissue ischemia (TI), and 3 min of vascular reperfusion (VR)]. Absolute concentrations of oxygenated heme (oxy-[heme]), deoxygenated heme (deoxy-[heme]), total heme (total [heme), tissue oxygen saturation (StO2), and heme difference [heme]diff) were measured using frequency domain near-infrared spectroscopy (FD-NIRS) while 1) continuously measuring and incorporating tissue scattering ([Formula: see text]) and 2) assuming scattering remained constant. FD-NIRS measured [Formula: see text] increased during TI at 692 nm (P < 0.001) and decreased at 834 nm (P < 0.001). During VR, [Formula: see text] decreased at 692 nm (P < 0.001) and increased at 834 nm (P < 0.001). When assuming constant scattering, oxy-[heme] was significantly less at TIpeak (P < 0.05) while deoxy-[heme] and StO2 were significantly altered at BL, TIpeak, and VRpeak (all P < 0.001). Total [heme] did not change during the VOT. Absolute changes in deoxy-[heme], oxy-[heme], and StO2 in response to TI and VR were significantly exaggerated (all P < 0.001) and the rates of change during TI (slope 1) and VR (slope 2) in deoxy-[heme], oxy-[heme], StO2, and [heme]diff were significantly increased (all P < 0.05) when constant tissue scattering was assumed. These findings demonstrate the need for caution when interpreting NIRS data without continuously measuring tissue optical properties. Further, assuming tissue optical properties remain constant may have important consequences to experimental data and clinical conclusions made using NIRS.NEW & NOTEWORTHY NIRS measurements provide significant experimental and clinical insight. We demonstrate that absolute changes in tissue oxygenation measurements made with NIRS are overestimated and the kinetic responses of NIRS measurements are exaggerated by varying degrees among individuals if tissue scattering characteristics are assumed to remain constant during vascular occlusion tests.