Impact of supine exercise on muscle deoxygenation kinetics heterogeneity: mechanistic insights into slow pulmonary oxygen uptake dynamics

The effect of body position on cardiovascular, skeletal muscle and ventilatory responses to submaximal cycling

The completion of exercise in different body positions can impact the function of various components of the oxygen delivery pathway; however, the effect of the haemodynamic conditions induced by a semi-upright body position on the integrative physiological response to exercise is poorly understood. The purpose of this study was to explore the effect of a semi-upright body position on cardiac output (CO), vastus lateralis oxygen saturation (S m O 2 ${{S}_{{\mathrm{m}}{{{\mathrm{O}}}_2}}}$ ), oxygen consumption (V ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_2}}}$ ) and ratings of perceived exertion (Borg RPE) during submaximal cycling. Twenty healthy individuals (22 ± 3 years, 50% female) each completed alternating 5-min bouts of submaximal upright and semi-upright (40° incline) cycling at 50 and 100 W. CO,S m O 2 ${{S}_{{\mathrm{m}}{{{\mathrm{O}}}_2}}}$ ,V ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_2}}}$ and RPE were assessed at rest and at each exercise intensity during steady state. There was a main effect of intensity on the increase in CO,S m O 2 ${{S}_{{\mathrm{m}}{{{\mathrm{O}}}_2}}}$ ,V ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_2}}}$ and RPE (all P < 0.001). In a semi-upright position, the increase in CO (7.9 ± 2.8 vs. 6.4 ± 2.6 L/min, P < 0.001), RPE (median (interquartile range): 11 (9-13) vs. 10 (8-12), P = 0.013) and the decrease inS m O 2 ${{S}_{{\mathrm{m}}{{{\mathrm{O}}}_2}}}$ (-38 ± 23 vs. -21% ± 18%, P < 0.001) were greater than upright, while the increase inV ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_2}}}$ was attenuated (1.030 ± 0.130 vs. 1.154 ± 0.165 L/min, P < 0.001). These results suggest that while a semi-upright body position produces elevations in CO, these elevations do not seem to perfuse the active skeletal muscle. This may explain the elevation in RPE despite a blunting in the increase inV ̇ O 2 ${{\dot{V}}_{{{{\mathrm{O}}}_2}}}$ . Further work is required to understand the effects of a semi-upright exercise position on skeletal muscle activation and lower limb blood flow.

Effect of Supplemental Oxygen on Physiological Responses to Exercise in Fibrotic Interstitial Lung Disease

PURPOSE

We studied the effect of O 2 supplementation on physiological response to exercise in patients with moderate to severe interstitial lung disease (ILD).

METHODS

Thirteen patients (age 66 ± 10 yr, 7 males) with ILD (total lung capacity, 71% ± 22% predicted; carbon monoxide diffusion capacity, 44% ± 16% predicted) and 13 healthy individuals (age 50 ± 17 yr, 7 males) were tested. ILD patients performed symptom-limited cardiopulmonary exercise tests and constant work rate (WR) tests at 80% of the WR at the gas exchange threshold. Tests breathing room air (RA; 21% O 2 ) were compared with tests performed breathing 30% O 2 . Oxygen uptake (V̇O 2 ) kinetics were calculated from the constant WR test results.

RESULTS

In the ILD group, peak WR, peak V̇O 2 , and V̇O 2 at the gas exchange threshold improved significantly when breathing 30% O 2 compared with RA (mean ± SD, 75 ± 26 vs 66 ± 23 W, 17 ± 4 vs 15 ± 2 mL·kg -1 ·min -1 , and 932 ± 245 vs 854 ± 232 mL·min -1 ; P = 0.004, P = 0.001, and P = 0.01, respectively). O 2 saturation (SpO 2 %) at peak exercise was higher with 30% O 2 (97% ± 4% vs 88% ± 9%, P = 0.002). The time constant (tau) of V̇O 2 kinetics was faster in ILD patients while breathing 30% O 2 (41 ± 10 s) compared with RA (52 ± 14 s, P = 0.003). There was a negative linear relation between tau and SpO 2 % with RA ( r = -0.76, P = 0.006) and while breathing 30% O 2 ( r = -0.68, P = 0.02).

CONCLUSIONS

Using a clinically applicable level of O 2 supplementation (30%) improved maximal, aerobic exercise capacity and V̇O 2 kinetics in ILD patients, likely due to increased blood O 2 content subsequently increasing the O 2 delivery to the working muscles.

Larger splenic emptying correlate with slower EPOC kinetics in healthy men and women during supine cycling

PURPOSE

The present study investigated whether larger splenic emptying augments faster excess post-exercise O2 consumption (EPOC) following aerobic exercise cessation.

METHODS

Fifteen healthy participants (age 24 ± 4, 47% women) completed 3 laboratory visits at least 48-h apart. After obtaining medical clearance and familiarizing themselves with the test, they performed a ramp-incremental test in the supine position until task failure. At their final visit, they completed three step-transition tests from 20 W to a moderate-intensity power output (PO), equivalent to [Formula: see text]O2 at 90% gas exchange threshold, where data on metabolic, cardiovascular, and splenic responses were recorded simultaneously. After step-transition test cessation, EPOCfast was recorded, and the first 10 min of the recovery period was used for further analysis. Blood samples were collected before and immediately after the end of exercise.

RESULTS

In response to moderate-intensity supine cycling ([Formula: see text]O2 = ~ 2.1 L·min-1), a decrease in spleen volume of ~ 35% (p = 0.001) was observed, resulting in a transient increase in red cell count of ~ 3-4% (p = 0.001) in mixed venous blood. In parallel, mean blood pressure, heart rate, and stroke volume increased by 30-100%, respectively. During recovery, mean τ[Formula: see text]O2 was 45 ± 18 s, the amplitude was 2.4 ± 0.5 L·min-1, and EPOCfast was 1.69 L·O2. Significant correlations were observed between the percent change in spleen volume and (i) EPOCfast (r = - 0.657, p = 0.008) and (ii) τ[Formula: see text]O2 (r = - 0.619, p = 0.008), but not between the change in spleen volume and (iii) [Formula: see text]O2 peak (r = 0.435, p = 0.105).

CONCLUSION

Apparently, during supine cycling, individuals with larger spleen emptying tend to have slower [Formula: see text] O2 recovery kinetics and a greater EPOCfast.

Relationship between muscle venous blood oxygenation and near-infrared spectroscopy: quantitative analysis of the Hb and Mb contributions

A linear relationship between skeletal muscle venous ([Formula: see text]) and oxygenated (ΔHbMbO2,N) or deoxygenated (ΔHHbMbN) near-infrared spectroscopy (NIRS) signals suggest a main hemoglobin (Hb) contribution to the NIRS signal. However, experimental, and computational evidence supports a significant contribution of myoglobin (Mb) to the NIRS. Venous and NIRS measurements from a canine model of muscle oxidative metabolism (Sun Y, Ferguson BS, Rogatzki MJ, McDonald JR, Gladden LB. Med Sci Sports Exerc 48(10):2013-2020, 2016) were integrated into a computational model of muscle O2 transport and utilization to evaluate whether the relationship between venous and NIRS oxygenation can be affected by a significant Mb contribution to the NIRS signals. The mathematical model predicted well the measure of the changes of [Formula: see text] and NIRS signals for different O2 delivery conditions (blood flow, arterial O2 content) in muscle at rest (T1, T2) and during contraction (T3). Furthermore, computational analysis indicates that for adequate O2 delivery, Mb contribution to NIRS signals was significant (20%-30%) even in the presence of a linear [Formula: see text]-NIRS relationship; for a reduced O2 delivery the nonlinearity of the [Formula: see text]-NIRS relationship was related to the Mb contribution (50%). In this case (T3), the deviation from linearity is observed when O2 delivery is reduced from 1.3 to 0.7 L kg-1·min-1 ([Formula: see text] < 10 mLO2 100 mL-1) and Mb saturation decreased from 85% to 40% corresponding to an increase of the Mb contribution to ΔHHbMbN from 15% to 50% and the contribution to ΔHbMbO2,N from 0% to 30%. In contrast to a common assumption, our model indicates that both NIRS signals (ΔHHbMbN and ΔHbMbO2,N are significantly affected by Hb and Mb oxygenation changes.NEW & NOTEWORTHY Within the near-infrared spectroscopy (NIRS) signal, the contribution from hemoglobin is indistinguishable from that of myoglobin. A computation analysis indicates that a linear relationship between muscle venous oxygen content and NIRS signals does not necessarily indicate a negligible myoglobin contribution to the NIRS signal. A reduced oxygen delivery increases the myoglobin contribution to the NIRS signal. The integrative approach proposed is a powerful way to assist in interpreting the elements from which the NIRS signals are derived.

How Priming Exercise Affects Oxygen Uptake Kinetics: From Underpinning Mechanisms to Endurance Performance

The observation that prior heavy or severe-intensity exercise speeds overall oxygen uptake ([Formula: see text]O₂) kinetics, termed the "priming effect", has garnered significant research attention and its underpinning mechanisms have been hotly debated. In the first part of this review, the evidence for and against (1) lactic acidosis, (2) increased muscle temperature, (3) O₂ delivery, (4) altered motor unit recruitment patterns and (5) enhanced intracellular O₂ utilisation in underpinning the priming effect is discussed. Lactic acidosis and increased muscle temperature are most likely not key determinants of the priming effect. Whilst priming increases muscle O₂ delivery, many studies have demonstrated that an increased muscle O₂ delivery is not a prerequisite for the priming effect. Motor unit recruitment patterns are altered by prior exercise, and these alterations are consistent with some of the observed changes in [Formula: see text]O₂ kinetics in humans. Enhancements in intracellular O₂ utilisation likely play a central role in mediating the priming effect, probably related to elevated mitochondrial calcium levels and parallel activation of mitochondrial enzymes at the onset of the second bout. In the latter portion of the review, the implications of priming on the parameters of the power-duration relationship are discussed. The effect of priming on subsequent endurance performance depends critically upon which phases of the [Formula: see text]O₂ response are altered. A reduced [Formula: see text]O₂ slow component or increased fundamental phase amplitude tend to increase the work performable above critical power (i.e. W´), whereas a reduction in the fundamental phase time constant following priming results in an increased critical power.

Determination of exercise intensity domains during upright versus supine cycling: a methodological study

Background

There is a growing interest among the research community and clinical practitioners to investigate cardiopulmonary exercise test (CPET) procedures and protocols utilized in supine cycling.

Materials and Methods

The current study investigated the effects of posture on indicators of exercise intensity including gas exchange threshold (GET), respiratory compensation point (RCP), and the rate of peak oxygen uptake (V̇O₂ peak), as well as the role of V̇O₂ mean response time (MRT) in determining exercise intensity domains in nineteen healthy men (age: 22 ± 3 years). Two moderate-intensity step-transitions from 20 to 100 Watt (W) were completed, followed by a maximal CPET. After completing the ramp test, participants performed a constant-load at 90% of their attained peak power output (PPO).

Results

No differences were observed in the V̇O₂ MRT between the two positions, although the phase II-time constant (τV̇O_2p) was 7 s slower in supine position compared to upright (p = 0.001). The rate of O₂ uptake in the supine position at GET and RCP were lower compared to the upright position (208 ± 200 mL·min^-1 (p = 0.007) and 265 ± 235 mL·min^-1 (p = 0.012) respectively). Besides, V̇O₂ peak was significantly decreased (by 6%, p = 0.002) during supine position. These findings were confirmed by the wide limits of agreement between the measures of V̇O₂ in different postures (V̇O₂ peak: -341 to 859; constant-load test: -528 to 783; GET: -375 to 789; RCP: -520 to 1021 all in mL·min^-1).

Conclusion

Since an accurate identification of an appropriate power output (PO) from a single-visit CPET remains a matter of debate, especially for supine cycling, we propose that moderate-intensity step-transitions preceding a ramp CPET could be a viable addition to ensure appropriate exercise-intensity domain determination, in particular upon GET-based prescription.

No differences in splenic emptying during on-transient supine cycling between aerobically trained and untrained participants

Purpose

The role of splenic emptying in O₂ transport during aerobic exercise still remains a matter of debate. Our study compared the differences in spleen volume changes between aerobically trained and untrained individuals during step-transition supine cycling exercise at moderate-intensity. We also examined the relationship between spleen volume changes, erythrocyte release, and O₂ uptake parameters.

Methods

Fourteen healthy men completed all study procedures, including a detailed medical examination, supine maximal O₂ uptake (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}\text{O}}_{2}$$\end{document}V˙O2 max.) test, and three step-transitions from 20 W to a moderate-intensity power output, equivalent to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}\text{O}}_{2}$$\end{document}V˙O2 uptake at 90% gas exchange threshold. During these step-transitions pulmonary \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}\text{O}}_{{2{\text{p}}}}$$\end{document}V˙O2p, near-infrared spectroscopy of the vastus lateralis, and cardiovascular responses were continuously measured. In parallel, minute-by-minute ultrasonic measurements of the spleen were performed. Blood samples were taken before and immediately after step-transition cycling.

Results

On average, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}\text{O}}_{2}$$\end{document}V˙O2 max. was 10 mL kg min⁻¹ (p = 0.001) higher in trained compared to their aerobically untrained peers. In response to supine step-transition cycling, the splenic volume was significantly reduced, and the largest reduction (~ 106 to 115 mL, ~ 38%, p = 0.001) was similar in both aerobically trained and untrained individuals. Erythrocyte concentration and platelet count transiently increased after exercise cessation, with no differences observed between groups. However, the vastus lateralis deoxygenation amplitude was 30% (p = 0.001) greater in trained compared to untrained individuals. No associations existed between: (i) spleen volumes at rest (ii) spleen volume changes (%), (iii) resting hematocrit and oxygen uptake parameters.

Conclusion

Greater splenic emptying and subsequent erythrocyte release do not lead to a slower \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\tau {\dot{\text{V}}\text{O}}_{{2{\text{p}}}}$$\end{document}τV˙O2p, regardless of individual \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{\text{V}}\text{O}}_{2}$$\end{document}V˙O2 max. readings.

Dissociation between exercise intensity thresholds: mechanistic insights from supine exercise

This study tested the hypothesis that the respiratory compensation point (RCP) and breakpoint in deoxygenated [heme] [deoxy[heme]_BP, assessed via near-infrared spectroscopy (NIRS)] during ramp incremental exercise would occur at the same metabolic rate in the upright (U) and supine (S) body positions. Eleven healthy men completed ramp incremental exercise tests in U and S. Gas exchange was measured breath-by-breath and time-resolved-NIRS was used to measure deoxy[heme] in the vastus lateralis (VL) and rectus femoris (RF). RCP (S: 2.56 ± 0.39, U: 2.86 ± 0.40 L·min^-1, P = 0.02) differed from deoxy[heme]_BP in the VL in U (3.10 ± 0.44 L·min^-1, P = 0.002), but was not different in S in the VL (2.70 ± 0.50 L·min^-1, P = 0.15). RCP was not different from the deoxy[heme]_BP in the RF for either position (S: 2.34 ± 0.48 L·min^-1, U: 2.76 ± 0.53 L·min^-1, P > 0.05). However, the deoxy[heme]_BP differed between muscles in both positions (P < 0.05), and changes in deoxy[heme]_BP did not relate to ΔRCP between positions (VL: r = 0.55, P = 0.080, RF: r = 0.26, P = 0.44). The deoxy[heme]_BP was consistently preceded by a breakpoint in total[heme], and was, in turn, itself preceded by a breakpoint in muscle surface electromyography (EMG). RCP and the deoxy[heme]_BP can be dissociated across muscles and different body positions and, therefore, do not represent the same underlying physiological phenomenon. The deoxy[heme]_BP may, however, be mechanistically related to breakpoints in total[heme] and muscle activity.

Spleen Emptying Does Not Correlate With Faster Oxygen Kinetics During a Step-Transition Supine Cycling

This manuscript quantified spleen volume changes and examined the relationship between those changes and V̇O2 kinetics during supine cycling. Ten volunteers (age=22±3), completed three step-transitions from 20 W to their power output at 90% gas exchange threshold. Ultrasonic measurements of the spleen were performed each minute. The largest spleen volume reduction was 105 mL (p=.001). No associations existed between: i) spleen volumes at rest ii) spleen volume changes (%) and τV̇O2p. Larger resting spleen volume and greater emptying do not correlate with a faster τV̇O2p. Novelty: • Greater splenic contractions do not augment τV̇O2p, irrespective of spleen emptying and subsequent erythrocyte release.

Impact of supine versus upright exercise on muscle deoxygenation heterogeneity during ramp incremental cycling is site specific

PURPOSE

We tested the hypothesis that incremental ramp cycling exercise performed in the supine position (S) would be associated with an increased reliance on muscle deoxygenation (deoxy[heme]) in the deep and superficial vastus lateralis (VLd and VLs, respectively) and the superficial rectus femoris (RFs) when compared to the upright position (U).

METHODS

11 healthy men completed ramp incremental exercise tests in S and U. Pulmonary [Formula: see text]O₂ was measured breath-by-breath; deoxy[heme] was determined via time-resolved near-infrared spectroscopy in the VLd, VLs and RFs.

RESULTS

Supine exercise increased the overall change in deoxy[heme] from baseline to maximal exercise in the VLs (S: 38 ± 23 vs. U: 26 ± 15 μM, P < 0.001) and RFs (S: 36 ± 21 vs. U: 25 ± 15 μM, P < 0.001), but not in the VLd (S: 32 ± 23 vs. U: 29 ± 26 μM, P > 0.05).

CONCLUSIONS

The present study supports that the impaired balance between O₂ delivery and O₂ utilization observed during supine exercise is a regional phenomenon within superficial muscles. Thus, deep muscle defended its O₂ delivery/utilization balance against the supine-induced reductions in perfusion pressure. The differential responses of these muscle regions may be explained by a regional heterogeneity of vascular and metabolic control properties, perhaps related to fiber type composition.

High-intensity exercise impairs extradiaphragmatic respiratory muscle perfusion in patients with COPD

The study investigated whether high-intensity exercise impairs inspiratory and expiratory muscle perfusion in patients with chronic obstructive pulmonary disease (COPD). We compared respiratory local muscle perfusion between constant-load cycling[sustained at 80% peak work rate (WRpeak)] and voluntary normocapnic hyperpnea reproducing similar work of breathing (WoB) in 18 patients [forced expiratory volume in the first second (FEV₁): 58 ± 24% predicted]. Local muscle blood flow index (BFI), using indocyanine green dye, and fractional oxygen saturation (%StiO₂) were simultaneously assessed by near-infrared spectroscopy (NIRS) over the intercostal, scalene, rectus abdominis, and vastus lateralis muscles. Cardiac output (impedance cardiography), WoB (esophageal/gastric balloon catheter), and diaphragmatic and extradiaphragmatic respiratory muscle electromyographic activity (EMG) were also assessed throughout cycling and hyperpnea. Minute ventilation, breathing pattern, WoB, and respiratory muscle EMG were comparable between cycling and hyperpnea. During cycling, cardiac output and vastus lateralis BFI were significantly greater compared with hyperpnea [by +4.2 (2.6-5.9) L/min and +4.9 (2.2-7.8) nmol/s, respectively] (P < 0.01). Muscle BFI and %StiO₂ were, respectively, lower during cycling compared with hyperpnea in scalene [by -3.8 (-6.4 to -1.2) nmol/s and -6.6 (-8.2 to -5.1)%], intercostal [by -1.4 (-2.4 to -0.4) nmol/s and -6.0 (-8.6 to -3.3)%], and abdominal muscles [by -1.9 (-2.9 to -0.8) nmol/s and -6.3 (-9.1 to -3.4)%] (P < 0.001). The difference in respiratory (scalene and intercostal) muscle BFI between cycling and hyperpnea was associated with greater dyspnea (Borg CR10) scores (r = -0.54 and r = -0.49, respectively, P < 0.05). These results suggest that in patients with COPD, 1) locomotor muscle work during high-intensity exercise impairs extradiaphragmatic respiratory muscle perfusion and 2) insufficient adjustment in extradiaphragmatic respiratory muscle perfusion during high-intensity exercise may partly explain the increased sensations of dyspnea.NEW & NOTEWORTHY We simultaneously assessed the blood flow index (BFI) in three respiratory muscles during hyperpnea and high-intensity constant-load cycling sustained at comparable levels of work of breathing and respiratory neural drive in patients with COPD. We demonstrated that high-intensity exercise impairs respiratory muscle perfusion, as intercostal, scalene, and abdominal BFI increased during hyperpnea but not during cycling. Insufficient adjustment in respiratory muscle perfusion during exercise was associated with greater dyspnea sensations in patients with COPD.

Effect of priming exercise and body position on pulmonary oxygen uptake and muscle deoxygenation kinetics during cycle exercise

We hypothesized that the performance of prior heavy exercise would speed pulmonary oxygen uptake (V̇o₂) kinetics (i.e., as described by the time constant, [Formula: see text]) and reduce the amplitude of muscle deoxygenation (deoxy[heme]) kinetics in the supine (S) but not upright (U) body position. Seventeen healthy men completed heavy-intensity constant-work rate exercise tests in S and U consisting of two bouts of 6-min cycling separated by 6-min cycling at 20 W. Pulmonary V̇o₂ was measured breath by breath; total and deoxy[heme] were determined via time-resolved near-infrared spectroscopy (NIRS) at three muscle sites. Priming exercise reduced [Formula: see text] in S (bout 1: 36 ± 10 vs. bout 2: 28 ± 10 s, P < 0.05) but not U (bout 1: 27 ± 8 s vs. bout 2: 25 ± 7 s, P > 0.05). Deoxy[heme] amplitude was increased after priming in S (bout 1: 25-28 μM vs. bout 2: 30-35 μM, P < 0.05) and U (bout 1: 13-18 μM vs. bout 2: 17-25 μM, P > 0.05), whereas baseline total[heme] was enhanced in S (bout 1: 110-179 μM vs. bout 2: 121-193 μM, P < 0.05) and U (bout 1: 123-186 μM vs. bout 2: 137-197 μM, P < 0.05). Priming exercise increased total[heme] in both S and U, likely indicating enhanced diffusive O₂ delivery. However, the observation that after priming the amplitude of the deoxy[heme] response was increased in S suggests that the reduction in [Formula: see text] subsequent to priming was related to a combination of both enhanced intracellular O₂ utilization and increased O₂ delivery.NEW & NOTEWORTHY Here we show that oxygen uptake (V̇o₂) kinetics are slower in the supine compared with upright body position, an effect that is associated with an increased amplitude of skeletal muscle deoxygenation in the supine position. After priming in the supine position, the amplitude of muscle deoxygenation remained markedly elevated above that observed during upright exercise. Hence, the priming effect cannot be solely attributed to enhanced O₂ delivery, and enhancements to intracellular O₂ utilization must also be contributory.