To test the hypothesis that, during exercise, substantial heterogeneity of muscle hemoglobin and myoglobin deoxygenation [deoxy(Hb + Mb)] dynamics exists and to determine whether such heterogeneity is associated with the speed of pulmonary O(2) uptake (pVo(2)) kinetics, we adapted multi-optical fibers near-infrared spectroscopy (NIRS) to characterize the spatial distribution of muscle deoxygenation kinetics at exercise onset. Seven subjects performed cycle exercise transitions from unloaded to moderate [GET) work rates and the relative changes in deoxy(Hb + Mb), at 10 sites in the quadriceps, were sampled by NIRS. At exercise onset, the time delays in muscle deoxy(Hb + Mb) were spatially inhomogeneous [intersite coefficient of variation (CV), 3~56% for GET]. The primary component kinetics (time constant) of muscle deoxy(Hb + Mb) reflecting increased O(2) extraction were also spatially inhomogeneous (intersite CV, 6~48% for GET) and faster (P < 0.05) than those of phase 2 pVo(2). However, the degree of dynamic intersite heterogeneity in muscle deoxygenation did not correlate significantly with phase 2 pVo(2) kinetics. In conclusion, the dynamics of quadriceps microvascular oxygenation demonstrates substantial spatial heterogeneity that must arise from disparities in the relative kinetics of Vo(2) and O(2) delivery increase across the regions sampled.

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-The conventional continuous wave near-infrared spectroscopy (CW-NIRS) has enabled identification of regional differences in muscle deoxygenation following onset of exercise. However, assumptions of constant optical factors (e.g., path length) used to convert the relative changes in CW-NIRS signal intensity to values of relative concentration, bring the validity of such measurements into question. Furthermore, to justify comparisons among sites and subjects, it is essential to correct the amplitude of deoxygenated hemoglobin plus myoglobin [deoxy(HbϩMb)] for the adipose tissue thickness (ATT). We used two time-resolved NIRS systems to measure the distribution of the optical factors directly, thereby enabling the determination of the absolute concentrations of deoxy(HbϩMb) simultaneously at the distal and proximal sites within the vastus lateralis (VL) and the rectus femoris muscles. Eight subjects performed cycle exercise transitions from unloaded to heavy work rates (Ͼgas exchange threshold). Following exercise onset, the ATT-corrected amplitudes (Ap), time delay (TDp), and time constant (p) of the primary component kinetics in muscle deoxy(Hb ϩ Mb) were spatially heterogeneous (intersite coefficient of variation range for the subjects: 10 -50 for Ap, 16 -58 for TDp, 14 -108% for p). The absolute and relative amplitudes of the deoxy(HbϩMb) responses were highly dependent on ATT, both within subjects and between measurement sites. The present results suggest that regional heterogeneity in the magnitude and temporal profile of muscle deoxygenation is a consequence of differential matching of O 2 delivery and O2 utilization, not an artifact caused by changes in optical properties of the tissue during exercise or variability in the overlying adipose tissue. near-infrared spectroscopy; oxygen uptake kinetics; muscle oxygen delivery; muscle oxygen utilization MUSCLE OXYGENATION/DEOXYGENATION reflects the balance between O 2 delivery (Q o 2 ) and O 2 utilization (V O 2 ), i.e., Q o 2 /V O 2 ratio (or V O 2 /Q o 2 as deoxygenation). Thus, the profile of muscle deoxygenation, for example, following the onset of exercise can provide important information regarding the adequacy of the vascular response and the O 2 pressures essential for driving blood-muscle O 2 flux (1, 4 -8, 11, 13, 15-17, 25, 32, 39, 43, 47).Recently, we found that the dynamics of muscle microvascular deoxygenation [deoxy(Hb ϩ Mb)] measured by continuous wave near-infrared spectroscopy (CW-NIRS) were spatially heterogeneous within the quadriceps muscles in transient states where metabolic rate was changing rapidly (25, 43). However, the spatial heterogeneity of the muscle deoxygenation dynamics may have been related to intra-and intersubject variability in unmeasured optical factors such as path length, absorption, and scattering coefficients inherent in CW-NIRS technology (15). Thus, it remains unknown to what extent the absolute amplitude of muscle deoxygenation of the different regions reflects the temporal profile of the mean muscle oxygen pressur...

Muscle deoxygenation (i.e., deoxy[Hb + Mb]) during exercise assesses the matching of oxygen delivery (Q̇O2) to oxygen utilization (V̇O2). Until now limitations in near-infrared spectroscopy (NIRS) technology did not permit discrimination of deoxy[Hb + Mb] between superficial and deep muscles. In humans, the deep quadriceps is more highly vascularized and oxidative than the superficial quadriceps. Using high-power time-resolved NIRS, we tested the hypothesis that deoxygenation of the deep quadriceps would be less than in superficial muscle during incremental cycling exercise in eight males. Pulmonary V̇O2 was measured and muscle deoxy[Hb + Mb] was determined in the superficial vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF-s) and the deep rectus femoris (RF-d). deoxy[Hb + Mb] in RF-d was significantly less than VL at 70% (67.2 ± 7.0 vs. 75.5 ± 10.7 μM) and 80% (71.4 ± 11.0 vs. 79.0 ± 15.4 μM) of peak work rate (WR(peak)), but greater than VL and VM at WR(peak) (87.7 ± 32.5 vs. 76.6 ± 17.5 and 75.1 ± 19.9 μM). RF-s was intermediate at WR(peak) (82.6 ± 18.7 μM). Total hemoglobin and myoglobin concentration and tissue oxygen saturation were significantly greater in RF-d than RF-s throughout exercise. The slope of deoxy[Hb + Mb] increase (proportional to Q̇O2/V̇O2) in VL and VM slowed markedly above 70% WR(peak), whereas it became greater in RF-d. This divergent deoxygenation pattern may be due to a greater population of slow-twitch muscle fibers in the RF-d muscle and the differential recruitment profiles and vascular and metabolic control properties of specific fiber populations within superficial and deeper muscle regions.

Near-infrared assessment of skeletal muscle is restricted to superficial tissues due to power limitations of spectroscopic systems. We reasoned that understanding of muscle deoxygenation may be improved by simultaneously interrogating deeper tissues. To achieve this, we modified a high-power (∼8 mW), time-resolved, near-infrared spectroscopy system to increase depth penetration. Precision was first validated using a homogenous optical phantom over a range of inter-optode spacings (OS). Coefficients of variation from 10 measurements were minimal (0.5-1.9%) for absorption (μa), reduced scattering, simulated total hemoglobin, and simulated O2 saturation. Second, a dual-layer phantom was constructed to assess depth sensitivity, and the thickness of the superficial layer was varied. With a superficial layer thickness of 1, 2, 3, and 4 cm (μa = 0.149 cm(-1)), the proportional contribution of the deep layer (μa = 0.250 cm(-1)) to total μa was 80.1, 26.9, 3.7, and 0.0%, respectively (at 6-cm OS), validating penetration to ∼3 cm. Implementation of an additional superficial phantom to simulate adipose tissue further reduced depth sensitivity. Finally, superficial and deep muscle spectroscopy was performed in six participants during heavy-intensity cycle exercise. Compared with the superficial rectus femoris, peak deoxygenation of the deep rectus femoris (including the superficial intermedius in some) was not significantly different (deoxyhemoglobin and deoxymyoglobin concentration: 81.3 ± 20.8 vs. 78.3 ± 13.6 μM, P > 0.05), but deoxygenation kinetics were significantly slower (mean response time: 37 ± 10 vs. 65 ± 9 s, P ≤ 0.05). These data validate a high-power, time-resolved, near-infrared spectroscopy system with large OS for measuring the deoxygenation of deep tissues and reveal temporal and spatial disparities in muscle deoxygenation responses to exercise.