Resolving the bases for different physiological functioning or exercise performance within a population is dependent on our understanding of control mechanisms. For example, when most young healthy individuals run or cycle at moderate intensities, oxygen uptake (VO2) kinetics are rapid and the amplitude of the VO2 response is not constrained by O2 delivery. For this to occur, muscle O2 delivery (i.e., blood flow × arterial O2 concentration) must be coordinated superbly with muscle O2 requirements (VO2), the efficacy of which may differ among muscles and distinct fiber types. When the O2 transport system succumbs to the predations of aging or disease (emphysema, heart failure, and type 2 diabetes), muscle O2 delivery and O2 delivery-VO2 matching and, therefore, muscle contractile function become impaired. This forces greater influence of the upstream O2 transport pathway on muscle aerobic energy production, and the O2 delivery-VO2 relationship(s) assumes increased importance. This review is the first of its kind to bring a broad range of available techniques, mostly state of the art, including computer modeling, radiolabeled microspheres, positron emission tomography, magnetic resonance imaging, near-infrared spectroscopy, and phosphorescence quenching to resolve the O2 delivery-VO2 relationships and inherent heterogeneities at the whole body, interorgan, muscular, intramuscular, and microvascular/myocyte levels. Emphasis is placed on the following: 1) intact humans and animals as these provide the platform essential for framing and interpreting subsequent investigations, 2) contemporary findings using novel technological approaches to elucidate O2 delivery-VO2 heterogeneities in humans, and 3) future directions for investigating how normal physiological responses can be explained by O2 delivery-VO2 heterogeneities and the impact of aging/disease on these processes.

To date our knowledge of skeletal muscle deoxygenation as measured by near-infrared spectroscopy (NIRS) is predicated almost exclusively on sampling of superficial muscle(s), most commonly the vastus lateralis (VL-s). Recently developed high power NIRS facilitates simultaneous sampling of deep (i.e., rectus femoris, RF-d) and superficial muscles of RF (RF-s) and VL-s. Because deeper muscle is more oxidative with greater capillarity and sustains higher blood flows than superficial muscle, we used time-resolved NIRS to test the hypotheses that, following exercise onset, the RF-d has slower deoxy[Hb+Mb] kinetics with reduced amplitude than superficial muscles. Thirteen participants performed cycle exercise transitions from unloaded to heavy work rates. Within the same muscle (RF-s vs. RF-d) deoxy[Hb+Mb] kinetics (mean response time, MRT) and amplitudes were not different. However, compared with the kinetics of VL-s, deoxy[Hb+Mb] of RF-s and RF-d were slower (MRT: RF-s, 51 ± 23; RF-d, 55 ± 29; VL-s, 18 ± 6 s; P < 0.05). Moreover, the amplitude of total[Hb+Mb] was greater for VL-s than both RF-s and RF-d (P < 0.05). Whereas pulmonary V˙O2 kinetics (i.e., on vs. off) were symmetrical in heavy exercise, there was a marked on-off asymmetry of deoxy[Hb+Mb] for all three sites i.e., MRT-off > MRT-on (P < 0.05). Collectively these data reveal profoundly different O2 transport strategies, with the RF-s and RF-d relying proportionately more on elevated perfusive and the VL-s on diffusive O2 transport. These disparate O2 transport strategies and their temporal profiles across muscles have previously been concealed within the "global" pulmonary V˙O2 response.

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.

It is axiomatic that greater aerobic fitness (V˙O2peak) derives from enhanced perfusive and diffusive O2 conductances across active muscles. However, it remains unknown how these conductances might be reflected by regional differences in fractional O2 extraction (i.e., deoxy [Hb+Mb] and tissue O2 saturation [StO2]) and diffusive O2 potential (i.e., total[Hb+Mb]) among muscles spatially heterogeneous in blood flow, fiber type, and recruitment (vastus lateralis, VL; rectus femoris, RF). Using quantitative time-resolved near-infrared spectroscopy during ramp cycling in 24 young participants (V˙O2peak range: ~37.4-66.4 mL kg-1 min-1), we tested the hypotheses that (1) deoxy[Hb+Mb] and total[Hb+Mb] at V˙O2peak would be positively correlated with V˙O2peak in both VL and RF muscles; (2) the pattern of deoxygenation (the deoxy[Hb+Mb] slopes) during submaximal exercise would not differ among subjects differing in V˙O2peak Peak deoxy [Hb+Mb] and StO2 correlated with V˙O2peak for both VL (r = 0.44 and -0.51) and RF (r = 0.49 and -0.49), whereas for total[Hb+Mb] this was true only for RF (r = 0.45). Baseline deoxy[Hb+Mb] and StO2 correlated with V˙O2peak only for RF (r = -0.50 and 0.54). In addition, the deoxy[Hb+Mb] slopes were not affected by aerobic fitness. In conclusion, while the pattern of deoxygenation (the deoxy[Hb+Mb] slopes) did not differ between fitness groups the capacity to deoxygenate [Hb+Mb] (index of maximal fractional O2 extraction) correlated significantly with V˙O2peak in both RF and VL muscles. However, only in the RF did total[Hb+Mb] (index of diffusive O2 potential) relate to fitness.

It remains unclear whether an overshoot in skeletal muscle deoxygenation (HHb; reflecting a microvascular kinetic mismatch of O2 delivery to consumption) contributes to the slowed adjustment of oxidative energy provision at the onset of exercise. We progressively reduced the fractional inspired O2 concentration (F(I,O2)) to investigate the relationship between slowed pulmonary O2 uptake (V(O2)) kinetics and the dynamics and spatial distribution of absolute[HHb]. Seven healthy men performed 8 min cycling transitions during normoxia (F(I,O2) = 0.21),moderate hypoxia (F(I,O2) = 0.16) and severe hypoxia (F(I,O2)= 0.12). V(O2) uptake was measured using a flowmeter and gas analyser system. Absolute [HHb] was quantified by multichannel,time-resolved near-infrared spectroscopy from the rectus femoris and vastus lateralis (proximal and distal regions), and corrected for adipose tissue thickness. The phase II V(O2) time constant was slowed (P <0.05) as F(I,O2) decreased (normoxia, 17 ± 3 s;moderate hypoxia, 22 ± 4 s; and severe hypoxia, 29 ± 9 s). The [HHb] overshoot was unaffected by hypoxia, but the transient peak [HHb] increased with the reduction in F(I,O2) (P <0.05). Slowed V(O2) kinetics in hypoxia were positively correlated with increased peak [HHb] in the transient (r(2) = 0.45; P <0.05), but poorly related to the [HHb] overshoot. A reduction of spatial heterogeneity in peak [HHb]was inversely correlated with slowed V(O2) kinetics (r(2) = 0.49; P <0.05). These data suggest that aerobic energy provision at the onset of exercise may be limited by the following factors: (i) the absolute ratio (i.e. peak [HHb]) rather than the kinetic ratio (i.e. [HHb] overshoot) of microvascular O2 delivery to consumption; and (ii) a reduced spatial distribution in the ratio of microvascular O2 delivery to consumption across the muscle.

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.

Skeletal muscle deoxygenated hemoglobin and myoglobin concentration ([HHb]), assessed by near-infrared spectroscopy (NIRS), is commonly used as a surrogate of regional O2 extraction (reflecting the O2 delivery-to-consumption ratio, Q̇/V̇o2). However, [HHb] change (Δ[HHb]) is also influenced by capillary-venous heme concentration, and/or small blood vessel volume (reflected in total heme; [THb]). We tested the hypotheses that Δ[HHb] is associated with O2 extraction, and insensitive to [THb], over a wide range of Q̇/V̇o2 elicited by passive head-up tilt (HUT; 10-min, 15° increments, between -10° and 75°). Steady-state common femoral artery blood flow (FBF) was measured by echo-Doppler, and time-resolved NIRS measured [HHb] and [THb] of vastus lateralis (VL) and gastrocnemius (GS) in 13 men. EMG confirmed muscles were inactive. During HUT in VL [HHb] increased linearly (57 ± 10 to 101 ± 16 μM; P < 0.05 above 15°) and was associated (r(2) ∼ 0.80) with the reduction in FBF (618 ± 75 ml/min at 0° to 268 ± 52 ml/min at 75°; P < 0.05 above 30°) and the increase in [THb] (228 ± 30 vs. 252 ± 32 μM; P < 0.05 above 15°). GS response was qualitatively similar to VL. However, there was wide variation within and among individuals, such that the overall limits of agreement between Δ[HHb] and ΔFBF ranged from -35 to +19% across both muscles. Neither knowledge of tissue O2 saturation nor vascular compliance could appropriately account for the Δ[HHb]-ΔFBF dissociation. Thus, under passive tilt, [HHb] is influenced by Q̇/V̇o2, as well as microvascular hematocrit and/or tissue blood vessel volume, complicating its use as a noninvasive surrogate for muscle microvascular O2 extraction.