Contribution of oxygen extraction fraction to maximal oxygen uptake in healthy young men

Oxidative and O2 diffusive function in triceps brachii of recreational to world class swimmers

This study aimed to evaluate in vivo oxidative capacity and relative resistance to O2 diffusion using near-infrared spectroscopy (NIRS) in the m. triceps brachii of recreational to world class swimmers and evaluate their relationships with swimming performance. Twenty-eight swimmers were enrolled and assigned into three subgroups according to their level: 'recreational/trained' (Tier 1/2; n = 8), 'national' (Tier 3; n = 12) and 'international/world class' (Tier 4/5; n = 8). Performance was evaluated by 100 m freestyle trials. Training volume was measured by self-reported distance (km/week). Them V ̇ O 2 ${\mathrm{m}}{{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}}}$ recovery k of m. triceps brachii was non-invasively estimated by NIRS through repeated intermittent occlusions under two conditions: well-oxygenated (kHIGH) and low O2 availability (kLOW). The difference between kHIGH and kLOW (Δk) was calculated as an index of relative resistance to O2 diffusion. FINA points and 100 m performance differed among all groups. Training volume was greater in Tier 4/5 (34.0 ± 5.5 km week-1) and Tier 3 (35.5 ± 11.6 km week-1) than in Tier 1/2 (6.4 ± 1.8 km week-1). kHIGH was greater in Tier 4/5 and Tier 3 (3.18 ± 0.41 and 2.79 ± 0.40 min-1) versus Tier 1/2 (2.10 ± 0.36 min-1; all P < 0.002). kHIGH correlated with FINA points, 100 m performance and training volume. ∆k was not different among tiers and was not associated with training volume or performance. M. triceps brachii oxidative capacity (kHIGH) was positively associated with performance and training volume in swimmers. ∆k, which reflects relative resistance to O2 diffusion, was not different among athletes. These data suggest that m. triceps brachii oxidative capacity is associated with swimming performance and that muscle O2 diffusing capacity exerts a similar relative resistance to O2 diffusive flow across swimmers.

Thirty-second chair stand test in older adults with chronic health conditions is associated with lower limb oxygen extraction capacity

BACKGROUND

The 30-s chair stand test (CS-30) is a well-known measure of muscle strength in older adults. However, factors other than muscle strength may also be involved in older adults with chronic health conditions who require support and care in daily living.

PURPOSE

To test the hypothesis that the CS-30 in older adults with chronic health conditions is associated with lower limb muscle oxygen extraction capacity.

METHODS

Twenty-seven older adults with chronic health conditions (those who needed support and care in daily living because of stroke, musculoskeletal disease, etc.) were recruited. Tissue and percutaneous oxygen saturations of the right vastus lateralis muscle were measured during CS-30 measurements, and muscle oxygen extraction rate (MOER) was calculated. Knee extension strength, skeletal muscle mass index (SMI), and phase angle (PhA) were measured. In a multiple regression analysis with CS-30 as the dependent variable, results were calculated for model 1 with SMI, PhA, and ΔMOER as independent variables and model 2 with knee extension muscle strength added to model 1.

RESULTS

Phase angle (model 1, β = 0.46, p = .014; model 2, β = 0.46, p = .016) and ΔMOER (model 1, β = 0.39, p = .032; model 2, β = 0.40, p = .039) were significantly associated in both models. Adjusted R2 was 0.26 (Model 1) and 0.23 (Model 2).

CONCLUSION

The CS-30 in older adults with chronic health conditions may be related to muscle oxygen extraction capacity. This indicates that CS-30 also considers lower limb endurance assessment in this population.

Genetics, Fitness, and Left Ventricular Remodelling: The Current State of Play

Cardiorespiratory fitness (CRF) exists on a spectrum and is driven by a constellation of factors, including genetic and environmental differences. This results in wide interindividual variation in baseline CRF and the ability to improve CRF with regular endurance exercise training. As opposed to monogenic conditions, CRF is described as a complex genetic trait as it is believed to be influenced by multiple common genetic variants in addition to exogenous factors. Importantly, CRF is an independent predictor of morbidity and mortality, and so understanding the impact of genetic variation on CRF may provide insights into both human athletic performance and personalized risk assessment and prevention. Despite rapidly advancing technology, progress in this field has been restricted by small sample sizes and the limited number of genetic studies using the "gold standard" objective measure of peak oxygen consumption (VO2peak) for CRF assessment. In recent years, there has been increasing interest in the heritability of numerous parameters of cardiac structure and function and how this may relate to both normal cardiac physiology and disease pathology. Regular endurance training can result in exercise-induced cardiac remodelling, which manifests as balanced dilation of cardiac chambers and is associated with superior CRF. This results in a complex relationship between CRF, cardiac size, and exercise, and whether shared genetic pathways may influence this remains unknown. In this review we highlight recent and relevant studies into the genomic predictors of CRF with a unique emphasis on how this may relate to cardiac remodelling and human adaptation to endurance exercise.

Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression

BACKGROUND

Skeletal muscle mitochondria and capillaries are crucial for aerobic fitness, and suppressed levels are associated with chronic and age-related diseases. Currently, evidence-based exercise training recommendations to enhance these characteristics are limited. It is essential to explore how factors, such as fitness level, age, sex, and disease affect mitochondrial and capillary adaptations to different exercise stimuli.

OBJECTIVES

The main aim of this study was to compare the effects of low- or moderate intensity continuous endurance training (ET), high-intensity interval or continuous training (HIT), and sprint interval training (SIT) on changes in skeletal muscle mitochondrial content and capillarization. Secondarily, the effects on maximal oxygen consumption (VO2max), muscle fiber cross-sectional area, and fiber type proportion were investigated.

METHODS

A systematic literature search was conducted in PubMed, Web of Science, and SPORTDiscus databases, with no data restrictions, up to 2 February 2022. Exercise training intervention studies of ET, HIT, and SIT were included if they had baseline and follow-up measures of at least one marker of mitochondrial content or capillarization. In total, data from 5973 participants in 353 and 131 research articles were included for the mitochondrial and capillary quantitative synthesis of this review, respectively. Additionally, measures of VO2max, muscle fiber cross-sectional area, and fiber type proportion were extracted from these studies.

RESULTS

After adjusting for relevant covariates, such as training frequency, number of intervention weeks, and initial fitness level, percentage increases in mitochondrial content in response to exercise training increased to a similar extent with ET (23 ± 5%), HIT (27 ± 5%), and SIT (27 ± 7%) (P > 0.138), and were not influenced by age, sex, menopause, disease, or the amount of muscle mass engaged. Higher training frequencies (6 > 4 > 2 sessions/week) were associated with larger increases in mitochondrial content. Per total hour of exercise, SIT was ~ 2.3 times more efficient in increasing mitochondrial content than HIT and ~ 3.9 times more efficient than ET, while HIT was ~ 1.7 times more efficient than ET. Capillaries per fiber increased similarly with ET (15 ± 3%), HIT (13 ± 4%) and SIT (10 ± 11%) (P = 0.556) after adjustments for number of intervention weeks and initial fitness level. Capillaries per mm2 only increased after ET (13 ± 3%) and HIT (7 ± 4%), with increases being larger after ET compared with HIT and SIT (P < 0.05). This difference coincided with increases in fiber cross-sectional area after ET (6.5 ± 3.5%), HIT (8.9 ± 4.9%), and SIT (11.9 ± 15.1%). Gains in capillarization occurred primarily in the early stages of training (< 4 weeks) and were only observed in untrained to moderately trained participants. The proportion of type I muscle fibers remained unaltered by exercise training (P > 0.116), but ET and SIT exhibited opposing effects (P = 0.041). VO2max increased similarly with ET, HIT, and SIT, although HIT showed a tendency for greater improvement compared with both ET and SIT (P = 0.082), while SIT displayed the largest increase per hour of exercise. Higher training frequencies (6 > 4 > 2 sessions/week) were associated with larger increases in VO2max. Women displayed greater percentage gains in VO2max compared with men (P = 0.008). Generally, lower initial fitness levels were associated with greater percentage improvements in mitochondrial content, capillarization, and VO2max. SIT was particularly effective in improving mitochondrial content and VO2max in the early stages of training, while ET and HIT showed slower but steady improvements over a greater number of training weeks.

CONCLUSIONS

The magnitude of change in mitochondrial content, capillarization, and VO2max to exercise training is largely determined by the initial fitness level, with greater changes observed in individuals with lower initial fitness. The ability to adapt to exercise training is maintained throughout life, irrespective of sex and presence of disease. While training load (volume × intensity) is a suitable predictor of changes in mitochondrial content and VO2max, this relationship is less clear for capillary adaptations.

Left Ventricular Volume as a Predictor of Exercise Capacity and Functional Independence in Individuals with Normal Ejection Fraction

AIMS

Low cardiorespiratory fitness (CRF) is associated with functional disability, heart failure and mortality. Left ventricular (LV) end-diastolic volume (LVEDV) has been linked with CRF, but its utility as a diagnostic marker of low CRF has not been tested.

METHODS

This multi-center international cohort examined the relationship between LV size on echocardiography and CRF (peak oxygen uptake [peak VO2] from cardiopulmonary exercise testing) in individuals with LV ejection fraction ≥50%. Absolute and BSA-indexed LVEDV (LVEDVi) were tested as predictors of low CRF and functional disability (peak VO2 <1100ml/min or <18 ml/kg/min) and compared against candidate measures of cardiac structure and function.

RESULTS

2876 individuals (309 endurance athletes, 251 healthy non-athletes, 1969 individuals with unexplained dyspnea, 347 individuals with heart failure with preserved ejection fraction) were included. For the entire cohort, LVEDV had the strongest univariable association with peak VO2 (R2 =0.45, standardized [std]β 0.67, p<0.001) and remained the strongest independent predictor of peak VO2 after adjusting for age, sex and BMI (stdβ 0.30, p<0.001). LVEDV was better at identifying low CRF than most established echocardiographic measures (LVEDV AUC 0.72; LVEDVi AUC 0.71), but equivalent to the E/e' ratio. The probability of achieving a peak VO2 below the functional independence threshold was highest for smaller ventricular volumes, with LVEDV and LVEDVi of 88ml and 57ml/m2 providing the optimal cut-points, respectively.

CONCLUSIONS

Small resting ventricular size is associated with a higher probability of low CRF and functional disability. LV size is the strongest independent echocardiographic predictor of CRF across the health-disease continuum.

Characterization of muscle oxygenation response in well-trained handcyclists

PURPOSE

 Peripheral responses might be important in handcycling, given the involvement of small muscles compared to other exercise modalities. Therefore, the goal of this study was to compare changes in muscle oxygen saturation (∆SmO2) and deoxyhemoglobin level (∆[HHb]) between different efforts and muscles.

METHODS

 Handcyclists participated in a Wingate, a maximal incremental test and a 20-min time-trial (TT). Oxygen uptake (VO2) as well as ∆SmO2, ∆[HHb], deoxygenation and reoxygenation rates in the triceps brachii (TB), biceps brachii (BB), anterior deltoid (AD) and extensor carpi radialis brevis (ER) were measured.

RESULTS

 ER ∆[HHb]max was 37% greater in the incremental test than in the Wingate (ES = 0.392, P = 0.031). TT mean power (W/kg) was associated with BB ∆SmO2min measured in the incremental test (r = -0.998 [-1.190, -0.806], P = 0.002) and in the Wingate (r = -0.994 [-1.327, -0.661], P = 0.006). MAP (W/kg) was associated with Wingate BB ∆SmO2min (r = -0.983 [-0.999, -0.839], P = 0.003), and Wingate peak (r = 0.649 [0.379, 0.895], P = 0.008) and mean power (W/kg) (r = 0.925 [0.752, 0.972], P = 0.003) was associated with right handgrip force. The strongest physiological predictor for TT performance was BB ∆SmO2min in the incremental test (P = 0.002, r2 = 0.993, SEE 0.016 W/kg), Wingate BB ∆SmO2min for MAP (P = 0.003, r2 = 0.956, SEE 0.058 W/kg) and right handgrip force for Wingate peak power (P = 0.005, r2 = 0.856, SEE 0.551 W/kg).

CONCLUSION

 Peripheral aerobic responses (muscle oxygenation) were predictive of handcycling performance.

The effects of endurance training on muscle oxygen desaturation during incremental exercise tests: a systematic review and meta-analysis

Objective

Minimum muscle oxygen saturation (SmO2min) measured via near-infrared spectroscopy (NIRS) is a common measure during incremental exercise testing (IET). Our objective was to determine the effects of pre-to-post endurance training on SmO2min (ΔSmO2min) during an IET, using a meta-analysis.

Data sources

MEDLINE, EMBASE, and SPORTDiscus.

Study selection

Studies including healthy individuals had to meet the following criteria: (1) endurance training intervention; (2) peripheral muscle NIRS; (3) incremental exercise test pre/post training; (4) SmO2 or analogous saturation parameter measured.

Analysis

A PEDro scale was used for risk of bias analysis. A random effect meta-analysis model was used to synthesize the effect of training on ΔSmO2min in individual studies. Statistical heterogeneity was quantified using I2 statistic. A meta-regression was used to estimate the effect of training on the relationship between peak cycling power output (Wpeak), peak pulmonary oxygen uptake (V˙O2peak), and ΔSmO2min. A mixed-effect model was used to estimate categorical variables.

Results

Five studies met the inclusion criteria. No difference in SmO2min was detected following training pre- and post-intervention IETs. A trend for an effect of training on the relationship between Wpeak and ΔSmO2min was observed (p = 0.06).

Conclusion

This meta-analysis showed no effects of endurance training on SmO2min during an IET. Our results showed a trend for an effect of training on the relationship between Wpeak and ΔSmO2min, with no effect for V˙O2peak and ΔSmO2min. It is possible that SmO2min is not affected by endurance training, and may be used as a physiological marker for improvements in submaximal performance rather than at peak.

The metabolic and physiologic impairments underlying long COVID associated exercise intolerance

Data from invasive CPET (iCPET) revealed long COVID patients have impaired systemic oxygen extraction (EO2), suggesting impaired mitochondrial ATP production. However, it remains uncertain whether the initial severity of SARS-CoV-2 infection has implications on EO2 and exercise capacity (VO2) nor has there been assessment of anerobic ATP generation in long COVID patients. iCPET was performed on 47 long COVID patients (i.e., full cohort; n = 8 with severe SARS-CoV-2 infection). In a subset of patients (i.e., metabolomic cohort; n = 26) metabolomics on venous and arterial blood samples during iCPET was performed. In the full cohort, long COVID patients exhibited reduced peak EO2 with reduced peak VO2 (90 ± 17% predicted) relative to cardiac output (118 ± 23% predicted). Peak VO2 [88% predicted (IQR 81% - 108%) vs. 70% predicted (IQR 64% - 89%); p = 0.02] and EO2 [0.59(IQR 0.53-0.62) vs. 0.53(IQR 0.50-0.48); p = 0.01) were lower in severe versus mild infection. In the metabolomic cohort, 12 metabolites were significantly consumed, and 41 metabolites were significantly released (p-values < 0.05). Quantitative metabolomics demonstrated significant increases in inosine and succinate arteriovenous gradients during exercise. Peak VO2 was significantly correlated with peak venous succinate (r = 0.68; p = 0.0008) and peak venous lactate (r = 0.49; p = 0.0004). Peak EO2 and consequently peak VO2 impact long COVID patients in a severity dependent manner. Exercise intolerance associated with long COVID is defined by impaired aerobic and anaerobic energy production. Peak venous succinate may serve as a potential biomarker in long COVID.

Training Intensity Distribution of a 7-Day HIIT Shock Microcycle: Is Time in the "Red Zone" Crucial for Maximizing Endurance Performance? A Randomized Controlled Trial

BACKGROUND

Various studies have shown that the type of intensity measure affects training intensity distribution (TID) computation. These conclusions arise from studies presenting data from meso- and macrocycles, while microcycles, e.g., high-intensity interval training shock microcycles (HIIT-SM) have been neglected so far. Previous literature has suggested that the time spent in the high-intensity zone, i.e., zone 3 (Z3) or the "red zone", during HIIT may be important to achieve improvements in endurance performance parameters. Therefore, this randomized controlled trial aimed to compare the TID based on running velocity (TIDV), running power (TIDP) and heart rate (TIDHR) during a 7-day HIIT-SM. Twenty-nine endurance-trained participant were allocated to a HIIT-SM consisting of 10 HIIT sessions without (HSM, n = 9) or with (HSM + LIT, n = 9) additional low-intensity training or a control group (n = 11). Moreover, we explored relationships between time spent in Z3 determined by running velocity (Z3V), running power (Z3P), heart rate (Z3HR), oxygen uptake ( Z 3 V ˙ O 2 ) and changes in endurance performance.

RESULTS

Both intervention groups revealed a polarized pattern for TIDV (HSM: Z1: 38 ± 17, Z2: 16 ± 17, Z3: 46 ± 2%; HSM + LIT: Z1: 59 ± 18, Z2: 14 ± 18, Z3: 27 ± 2%) and TIDP (Z1: 50 ± 8, Z2: 14 ± 11, Z3: 36 ± 7%; Z1: 62 ± 15, Z2: 12 ± 16, Z3: 26 ± 2%), while TIDHR (Z1: 48 ± 13, Z2: 26 ± 11, Z3: 26 ± 7%; Z1: 65 ± 17, Z2: 22 ± 18, Z3: 13 ± 4%) showed a pyramidal pattern. Time in Z3HR was significantly less compared to Z3V and Z3P in both intervention groups (all p < 0.01). There was a time x intensity measure interaction for time in Z3 across the 10 HIIT sessions for HSM + LIT (p < 0.001, pη2 = 0.30). Time in Z3V and Z3P within each single HIIT session remained stable over the training period for both intervention groups. Time in Z3HR declined in HSM from the first (47%) to the last (28%) session, which was more pronounced in HSM + LIT (45% to 16%). A moderate dose-response relationship was found for time in Z3V and changes in peak power output (rs = 0.52, p = 0.028) as well as time trial performance (rs = - 0.47, p = 0.049) with no such associations regarding time in Z3P, Z3HR, and Z 3 V ˙ O 2 .

CONCLUSION

The present study reveals that the type of intensity measure strongly affects TID computation during a HIIT-SM. As heart rate tends to underestimate the intensity during HIIT-SM, heart rate-based training decisions should be made cautiously. In addition, time in Z3V was most closely associated with changes in endurance performance. Thus, for evaluating a HIIT-SM, we suggest integrating a comprehensive set of intensity measures. Trial Registration Trial register: Clinicaltrials.gov, registration number: NCT05067426.

Lifelong physiology of a former marathon world-record holder: the pros and cons of extreme cardiac remodeling

In a 77-year-old former world-record-holding male marathoner (2:08:33.6), this study sought to investigate the impact of lifelong intensive endurance exercise on cardiac structure, function, and the trajectory of functional capacity (determined by maximal oxygen consumption, V̇o2max) throughout the adult lifespan. As a competitive runner, our athlete (DC) reported performing up to 150-300 miles/wk of moderate-to-vigorous exercise and sustained 10-15 h/wk of endurance exercise after retirement from competition. DC underwent maximal cardiopulmonary exercise testing in 1970 (aged 27 yr), 1991 (aged 49 yr), and 2020 (aged 77 yr) to determine V̇o2max. At his evaluation in 2020, DC also underwent comprehensive cardiac assessments including resting echocardiography, and resting and exercise cardiac magnetic resonance to quantify cardiac structure and function at rest and during peak supine exercise. DC's V̇o2max showed minimal change from 27 yr (69.7 mL/kg/min) to 49 yr (68.1 mL/kg/min), although it eventually declined by 36% by the age of 77 yr (43.6 mL/kg/min). DC's V̇o2max at 77 yr, was equivalent to the 50th percentile for healthy 20- to 29-yr-old males and 2.4 times the requirement for maintaining functional independence. This was partly due to marked ventricular dilatation (left-ventricular end-diastolic volume: 273 mL), which facilitates a large peak supine exercise stroke volume (200 mL) and cardiac output (22.2 L/min). However, at the age of 78 yr, DC developed palpitations and fatigue and was found to be in atrial fibrillation requiring ablation procedures to revert his heart to sinus rhythm. Overall, this life study of a world champion marathon runner exemplifies the substantial benefits and potential side effects of many decades of intense endurance exercise.NEW & NOTEWORTHY This life study of a 77-yr-old former world champion marathon runner exemplifies the impact of lifelong high-volume endurance exercise on functional capacity (V̇o2max equivalent to a 20- to 29-yr-old), partly due to extreme ventricular remodeling that facilitates a large cardiac output during exercise despite reduced maximal heart rate. Although it is possible that this extreme remodeling may contribute to developing atrial fibrillation, the net benefits of extreme exercise throughout this athlete's lifespan favor increased health span and expected longevity.

Sex-Specific Differences in Cardiovascular Adaptations and Risks in Elite Athletes: Bridging the Gap in Sports Cardiology

BACKGROUND

The growing participation of women in competitive sports necessitates a comprehensive understanding of sex-specific cardiovascular adaptations and risks. Historically, research has predominantly focused on male athletes, leaving a gap in knowledge about the unique cardiovascular dynamics of female peers.

HYPOTHESIS

we hypothesized that female athletes exhibit distinct cardiovascular adaptations and face different risks, influenced by physiological, hormonal, and structural differences.

METHODS

A systematic review of the literature was conducted, analyzing studies on cardiovascular responses and adaptations in athletes. Data were extracted on hemodynamic changes, autonomic and neural reflex regulation, cardiac remodeling, and arrhythmias. Comparative analyses were performed to identify sex-specific patterns and discrepancies in cardiovascular health outcomes.

RESULTS

We revealed considerable sex differences in cardiovascular adaptations to athletic training. Female athletes generally have longer QT intervals, greater sinoatrial node automaticity, and enhanced atrioventricular node function compared to males. They also exhibit lower sympathetic activity, lower maximal stroke volumes, and a tendency toward eccentric cardiac remodeling. Conversely, male athletes are more prone to concentric hypertrophy and higher incidences of bradyarrhythmia and accessory pathway arrhythmias. Female athletes are more likely to experience symptomatic atrial fibrillation and face higher procedural complications during catheter ablation.

CONCLUSIONS

Our findings underscore the necessity for sex-specific approaches in sports cardiology. Recognizing and addressing these differences could enhance performance and reduce adverse cardiac events in athletes. Future research should focus on developing tailored screening, prevention, and treatment strategies to bridge the knowledge gap and promote cardiovascular health in both male and female athletes.

Assessing Leg Blood Flow and Cardiac Output During Running Using Thermodilution

Cardiac output (Q̇C) and leg blood flow (Q̇LEG) can be measured simultaneously with high accuracy using transpulmonary and femoral vein thermodilution with a single-bolus injection. The invasive measure has offered important insight into leg hemodynamics and blood flow distribution during exercise. Despite being the natural modality of exercise in humans, there has been no direct measure of Q̇LEG while running in humans. We sought to determine the feasibility of the thermodilution technique for measuring Q̇LEG and conductance during high-intensity running, in an exploratory case study. A trained runner (30 years male) completed two maximal incremental tests on a cycle ergometer and motorized treadmill. Q̇LEG and Q̇C were determined using the single-bolus thermodilution technique. Arterial and venous blood were sampled throughout exercise, with continuous monitoring of metabolism, intra-arterial and venous pressure, and temperature. The participant reached a greater peak oxygen uptake (V̇O2peak) during running relative to cycling (74 vs. 68 mL/kg/min) with comparable Q̇LEG (19.0 vs. 19.5 L/min) and Q̇C (27.4 vs. 26.2 L/min). Leg vascular conductance was greater during high-intensity running relative to cycling (82 vs. 70 mL/min/mmHg @ ~80% V̇O2peak). The "beat phenomenon" was apparent in femoral flow while running, producing large gradients in conductance (62-90 mL/min/mmHg @ 70% V̇O2peak). In summary, we present the first direct measure of Q̇LEG and conductance in a running human. Our findings corroborate several assumptions about Q̇LEG during running compared with cycling. Importantly, we demonstrate that using thermodilution in running exercise can be completed effectively and safely.

Oxygen Extraction Ratios to Guide Red Blood Cell Transfusion

Hemoglobin-based red blood cell transfusion (RBC) triggers do not clearly identify which patients with moderate anemia (hemoglobin 7-10 g/dL) will benefit from RBC transfusion. The National Heart, Lung, and Blood Institute has recognized the need for bedside oxygenation measures to enhance transfusion decision-making. This narrative review uses four studies to explore the potential of the oxygen extraction ratio (O2ER)-the ratio of consumed oxygen to delivered oxygen in a critical tissue bed as a more physiologically relevant indicator for guiding RBC transfusions in patients with moderate anemia. The aim of this review is to present existing data on the relationship between O2ER and responsiveness to RBC transfusion, as well as the feasibility of O2ER as bedside measure of tissue oxygenation. This review presents a narrative appraisal of three critical papers that investigate the relationship between O2ER and transfusion outcomes, and one paper that demonstrates proof-of-concept for a noninvasive device to measure O2ER at the bedside. Despite limitations in the existing studies, including small sample sizes and observational designs, the evidence collectively suggests that O2ER has the potential to enhance transfusion decision accuracy. The development of noninvasive measurement devices could facilitate widespread implementation in many kinds of care settings.

Neurovascular coupling is optimized to compensate for the increase in proton production from nonoxidative glycolysis and glycogenolysis during brain activation and maintain homeostasis of pH, pCO2, and pO2

During transient brain activation cerebral blood flow (CBF) increases substantially more than cerebral metabolic rate of oxygen consumption (CMRO2) resulting in blood hyperoxygenation, the basis of BOLD-fMRI contrast. Explanations for the high CBF versus CMRO2 slope, termed neurovascular coupling (NVC) constant, focused on maintenance of tissue oxygenation to support mitochondrial ATP production. However, paradoxically the brain has a 3-fold lower oxygen extraction fraction (OEF) than other organs with high energy requirements, like heart and muscle during exercise. Here, we hypothesize that the NVC constant and the capillary oxygen mass transfer coefficient (which in combination determine OEF) are co-regulated during activation to maintain simultaneous homeostasis of pH and partial pressure of CO2 and O2 (pCO2 and pO2). To test our hypothesis, we developed an arteriovenous flux balance model for calculating blood and brain pH, pCO2, and pO2 as a function of baseline OEF (OEF0), CBF, CMRO2, and proton production by nonoxidative metabolism coupled to ATP hydrolysis. Our model was validated against published brain arteriovenous difference studies and then used to calculate pH, pCO2, and pO2 in activated human cortex from published calibrated fMRI and PET measurements. In agreement with our hypothesis, calculated pH, pCO2, and pO2 remained close to constant independently of CMRO2 in correspondence to experimental measurements of NVC and OEF0. We also found that the optimum values of the NVC constant and OEF0 that ensure simultaneous homeostasis of pH, pCO2, and pO2 were remarkably similar to their experimental values. Thus, the high NVC constant is overall determined by proton removal by CBF due to increases in nonoxidative glycolysis and glycogenolysis. These findings resolve the paradox of the brain's high CBF yet low OEF during activation, and may contribute to explaining the vulnerability of brain function to reductions in blood flow and capillary density with aging and neurovascular disease.

Near-InfraRed Spectroscopy Provides a Reproducible Estimate of Muscle Aerobic Capacity, but Not Whole-Body Aerobic Power

Near-infrared spectroscopy (NIRS) during repeated limb occlusions is a noninvasive tool for assessing muscle oxidative capacity. However, the method's reliability and validity remain under investigation. This study aimed to determine the reliability of the NIRS-derived mitochondrial power of the musculus vastus lateralis and its correlation with whole-body (cycling) aerobic power (V̇O2 peak). Eleven healthy active men (28 ± 10 y) twice (2 days apart) underwent repeated arterial occlusions to induce changes in muscle oxygen delivery after 15 s of electrical muscle stimulation. The muscle oxygen consumption (mV̇O2) recovery time and rate (k) constants were calculated from the NIRS O2Hb signal. We assessed the reliability (coefficient of variation and intraclass coefficient of correlation [ICC]) and equivalency (t-test) between visits. The results showed high reproducibility for the mV̇O2 recovery time constant (ICC = 0.859) and moderate reproducibility for the k value (ICC = 0.674), with no significant differences between visits (p > 0.05). NIRS-derived k did not correlate with the V̇O2 peak relative to body mass (r = 0.441, p = 0.17) or the absolute V̇O2 peak (r = 0.366, p = 0.26). In conclusion, NIRS provides a reproducible estimate of muscle mitochondrial power, which, however, was not correlated with whole-body aerobic capacity in the current study, suggesting that even if somewhat overlapping, not the same set of factors underpin these distinct indices of aerobic capacity at the different (peripheral and whole-body systemic) levels.

Muscle Oxygen Extraction during Vascular Occlusion Test in Physically Very Active versus Inactive Healthy Men: A Comparative Study

An increase in the delivery and use of oxygen to the musculature in physically active subjects are determinants of improving health-related aerobic capacity. Additional health benefits, such as an increase in the muscle mass and a decrease in fat mass, principally in the legs, could be achieved with weekly global physical activity levels of more than 300 min. The objective was to compare the muscle vascular and metabolic profiles of physically very active and inactive subjects. Twenty healthy men participated in the study; ten were assigned to the physically very active group (25.5 ± 4.2 years; 72.7 ± 8.1 kg; 173.7 ± 7.6 cm) and ten to the physically inactive group (30.0 ± 7.4 years; 74.9 ± 11.8 kg; 173.0 ± 6.4 cm). The level of physical activity was determined by the Global Physical Activity Questionnaire (GPAQ). A resting vascular occlusion test (5 min of an ischemic phase and 3 min of a reperfusion phase) was used, whereas a near-field infrared spectroscopy (NIRS) device was used to evaluate the muscle oxygenation in the right vastus lateralis of the quadriceps muscle. The area under the curve of the deoxyhemoglobin (HHb) during the ischemic phase and above the curve of the tissue saturation index (TSI) during the reperfusion phase were obtained to determine muscle metabolic and vascular responses, respectively. Physically very active group showed a higher absolute HHb (3331.9 ± 995.7 vs. 6182.7 ± 1632.5 mmol/s) and lower TSI (7615.0 ± 1111.9 vs. 5420.0 ± 781.4 %/s) and relative to body weight (46.3 ± 14.6 vs. 84.4 ± 27.1 mmol/s/kg and 106.0 ± 20.6 vs. 73.6 ± 13.8 %/s/kg, respectively), muscle mass (369.9 ± 122.2 vs. 707.5 ± 225.8 mmol/kg and 829.7 ± 163.4 vs. 611.9 ± 154.2 %/s/kg) and fat mass (1760.8 ± 522.9 vs. 2981.0 ± 1239.9 mmol/s/kg and 4160.0 ± 1257.3 vs. ±2638.4 ± 994.3 %/s/kg, respectively) than physically inactive subjects. A negative correlation was observed between HHb levels and TSI (r = -0.6; p < 0.05). Physically very active men (>300 min/week) present better muscle oxidative metabolism and perfusion and perform significantly more physical activity than physically inactive subjects. Extra benefits for vascular health and muscle oxidative metabolism are achieved when a subject becomes physically very active, as recommended by the World Health Organization. In addition, a higher level of physical activity determined by GPAQ is related to better vascular function and oxidative metabolism of the main locomotor musculature, i.e., the quadriceps.

The Improvement in Exercise Performance during Reduced Muscle Mass Exercise is Associated with an Increase in Femoral Blood Flow in Older and Younger Endurance-Trained Athletes

This study investigated whether the improved performance observed with maximal self-paced single-leg (SL), compared with double-leg (DL) cycling, is associated with enhanced femoral blood flow and/or altered tissue oxygenation. The hyperaemic response to exercise was assessed in younger and older athletes. Power output was measured in 12 older (65 ± 4 y) and 12 younger (35 ± 5 y) endurance-trained individuals performing 2 x 3 min maximal self-paced exercise using SL and DL cycling. Blood flow (BF) in the femoral artery was assessed using Doppler ultrasound and muscle oxygenation was measured using near-infrared spectroscopy on the vastus lateralis. SL cycling elicited a greater power output (295 ± 83 vs 265 ± 70 W, P < 0.001) and peak femoral BF (1749.1 ± 533.3 vs 1329.7 ± 391.7 ml/min, P < 0.001) compared with DL cycling. Older individuals had a lower peak BF in response to exercise (1355.4 ± 385.8 vs 1765.2 ± 559.6 ml/min, P = 0.019) compared with younger individuals. Peak BF in response to exercise was correlated with power output during SL (r = 0.655, P = 0.002) and DL (r = 0.666, P = 0.001) cycling. The greater exercise performance during SL compared with DL cycling may be partly explained by a greater hyperaemic response when reducing active muscle mass. Despite regular endurance training, older athletes had a lower femoral BF in response to maximal self-paced exercise compared with younger athletes.

Erythrocyte metabolism

Our aim is to present an updated overview of the erythrocyte metabolism highlighting its richness and complexity. We have manually collected and connected the available biochemical pathways and integrated them into a functional metabolic map. The focus of this map is on the main biochemical pathways consisting of glycolysis, the pentose phosphate pathway, redox metabolism, oxygen metabolism, purine/nucleoside metabolism, and membrane transport. Other recently emerging pathways are also curated, like the methionine salvage pathway, the glyoxalase system, carnitine metabolism, and the lands cycle, as well as remnants of the carboxylic acid metabolism. An additional goal of this review is to present the dynamics of erythrocyte metabolism, providing key numbers used to perform basic quantitative analyses. By synthesizing experimental and computational data, we conclude that glycolysis, pentose phosphate pathway, and redox metabolism are the foundations of erythrocyte metabolism. Additionally, the erythrocyte can sense oxygen levels and oxidative stress adjusting its mechanics, metabolism, and function. In conclusion, fine-tuning of erythrocyte metabolism controls one of the most important biological processes, that is, oxygen loading, transport, and delivery.

Low- and high-volume blood-flow restriction treadmill walking both improve maximal aerobic capacity independently of blood volume

AIM

Assess the effect of low- and high-volume blood flow restriction training (BFR) on maximal aerobic capacity (VO2 max) and determine if alteration in VO2 max is mediated through changes in hemoglobin mass (Hbmass) and blood volume.

METHODS

Participants' Hbmass (CO-rebreathe), single, and double-leg VO2 max and blood volume regulating hormonal responses (renin and copeptin) were measured before and after BFR training. Training consisted of treadmill walking either (1) twice-daily for 4week (CON and BFRHV ) or (2) twice-weekly for 6week (BFRLV ). Each session consisted of five intervals (3 min, 5% incline, 5 km/h, 100% of lowest occlusion pressure), with 1 min of standing rest between sets.

RESULTS

VO2 max increased using both training exposures, in as quickly as 2-weeks (BFRLV baseline to 4week: +315 ± 241 mL (8.7%), p = 0.02; BFRHV baseline to 2week: +360 ± 261 mL (7.9%), p < 0.01), for the BFRLV and BFRHV groups, with no change in CON. Single- and double-leg VO2 max improved proportionately (single/double-leg VO2 max ratio: BFRLV 78 ± 4.9-78 ± 5.8%, BFRHV 79 ± 6.5-77 ± 6.5%), suggesting that the mechanism for increased VO2 max is not solely limited to central or peripheral adaptations. Hbmass remained unchanged across groups (CON: +10.2 ± 34 g, BFRLV : +6.6 ± 42 g, BFRHV : +3.2 ± 44 g; p = 0.9), despite a significant release of blood volume regulating hormones after initial BFR exposure (renin +20.8 ± 21.9 ng/L, p < 0.01; copeptin +22.0 ± 23.8 pmol/L, p < 0.01), which was blunted following BFRHV training (renin: +13.4 ± 12.4 ng/L, p = 0.09; copeptin: +1.9 ± 1.7 pmol/L, p = 0.98).

CONCLUSION

BFR treadmill walking increases VO2 max irrespective of changes in Hbmass or blood volume despite a large release of blood volume regulating hormones in response to BFR treadmill walking.

Peripheral limitations for performance: Muscle capillarization

Sufficient delivery of oxygen and metabolic substrates, together with removal of waste products, are key elements of muscle performance. Capillaries are the primary site for this exchange in skeletal muscle and the degree of muscle capillarization affects diffusion conditions by influencing mean transit time, capillary surface area and diffusion distance. Muscle capillarization may thus represent a limiting factor for performance. Exercise training increases the number of capillaries per muscle fiber by about 10%-20% within a few weeks in untrained subjects, whereas capillary growth progresses more slowly in well-trained endurance athletes. Studies show that capillaries are tortuous, situated along and across the length of the fibers with an arrangement related to muscle fascicles. Although direct data is lacking, it is possible that years of training not only enhances capillary density but also optimizes the positioning of capillaries, to further improve the diffusion conditions. Muscle capillarization has been shown to increase oxygen extraction during exercise in humans, but direct evidence for a causal link between increased muscle capillarization and performance is scarce. This review covers current knowledge on the implications of muscle capillarization for oxygen and glucose uptake as well as performance. A brief overview of the process of capillary growth and of physical factors, inherent to exercise, which promote angiogenesis, provides the foundation for a discussion on how different training modalities may influence muscle capillary growth. Finally, we identify three areas for future research on the role of capillarization for exercise performance.

The effect of cardiac rehabilitation on cardiopulmonary function after coronary artery bypass grafting: A systematic review and meta-analysis

We carried out a meta-analysis on the effect of cardiac rehabilitation (CR) on cardiopulmonary function after coronary artery bypass grafting (CABG). Four databases were searched for studies comparing CR with control. A random-effects model was used to pool mean difference (MD). The meta-analysis showed an increase in peak oxygen consumption (peak VO2) (MD = 1.93 mL/kg/min, p = 0.0006), and 6-min walk distance (6MWD) (MD = 59.21 m, p < 0.00001), and a decrease in resting heart rate (resting HR) (MD = 5.68 bpm, p < 0.0001) in the CR group. The subgroup analysis revealed aerobic exercise could further improve resting HR and peak HR, and physical/combination with aerobic exercise could further increase 6MWD. The improvement of peak VO2, workload, resting HR, peak HR, and 6MWD regarding CR performed within one week after CABG is greater than that one week after CABG. CR after CABG can improve the cardiopulmonary function, which is reflected by the improvement of peak VO2, 6MWD, and resting HR.

Understanding Exercise Capacity: From Elite Athlete to HFpEF

Exercise capacity is a spectrum that reflects an individual's functional capacity and the dynamic nature of cardiac remodelling along with respiratory and skeletal muscle systems. The relationship of increasing physical activity, increased cardiac mass and volumes, and improved cardiorespiratory fitness (CRF) is well established in the endurance athlete. However, less emphasis has been placed on the other end of the spectrum, which includes individuals with a more sedentary lifestyle and small hearts who are at increased risk of functional disability and poor clinical outcomes. Reduced CRF is an independent predictor of all-cause mortality and cardiovascular events determined by multiple inter-related exogenous and endogenous factors. In this review, we explore the relationship of physical activity, cardiac remodelling, and CRF across the exercise spectrum, emphasising the critical role of cardiac size in determining exercise capacity. In contrast to the large compliant left ventricle of the endurance athlete, an individual with a lifetime of physical inactivity is likely to have a small, stiff heart with reduced cardiac reserve. We propose that this might contribute to the development of heart failure with preserved ejection fraction in certain individuals, and is key to understanding the link between low CRF and increased risk of heart failure.

Anaerobic Speed Reserve and Performance Relationships Between International and World-Class Short-Track Speed Skating

PURPOSE

Short-track speed skating race distances of 500, 1000, and 1500 m that last ∼40 seconds to ∼2.5 minutes and require a maximal intensity at speeds beyond maximal oxygen uptake (VO2max). Recently, the anaerobic speed reserve (ASR) has been applied by scientists and coaches in middle-distance sports to deepen understanding of 1- to 5-minute event performance where different physiological profiles (speed, hybrid, and endurance) can have success.

METHODS

World-class (women, n = 2; men, n = 3) and international-level (women, n = 4; men, n = 5) short-track speed skaters completed maximal aerobic speed and maximal skating speed tests. ASR characteristics were compared between profiles and associated with on-ice performance.

RESULTS

World-class athletes raced at a lower %ASR in the 1000- (3.1%; large; almost certainly) and 1500-m (1.8%; large; possibly) events than international athletes. Men's and women's speed profiles operated at a higher %ASR in the 500-m than hybrid and endurance profiles, whereas in the 1500-m, endurance profiles worked at a substantially lower %ASR than hybrid and speed profiles. Women's 500-m performance is very largely associated with maximal skating speed, while women's maximal aerobic speed appears to be a key determining factor in the 1000- and 1500-m performance.

CONCLUSION

World-class short-track speed skaters can be developed in speed, hybrid, and endurance profiles but achieve their performance differently by leveraging their strongest characteristics. These results show nuanced differences between men's and women's 500-, 1000- and 1500-m event performance across ASR profile that unlock new insights for individualizing athlete performance in these disciplines.

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.

The Emerging Role of Hypoxic Training for the Equine Athlete

This paper provides a comprehensive discussion on the physiological impacts of hypoxic training, its benefits to endurance performance, and a rationale for utilizing it to improve performance in the equine athlete. All exercise-induced training adaptations are governed by genetics. Exercise prescriptions can be tailored to elicit the desired physiological adaptations. Although the application of hypoxic stimuli on its own is not ideal to promote favorable molecular responses, exercise training under hypoxic conditions provides an optimal environment for maximizing physiological adaptations to enhance endurance performance. The combination of exercise training and hypoxia increases the activity of the hypoxia-inducible factor (HIF) pathway compared to training under normoxic conditions. Hypoxia-inducible factor-1 alpha (HIF-1α) is known as a master regulator of the expression of genes since over 100 genes are responsive to HIF-1α. For instance, HIF-1-inducible genes include those critical to erythropoiesis, angiogenesis, glucose metabolism, mitochondrial biogenesis, and glucose transport, all of which are intergral in physiological adaptations for endurance performance. Further, hypoxic training could conceivably have a role in equine rehabilitation when high-impact training is contraindicated but a quality training stimulus is desired. This is achievable through purpose-built equine motorized treadmills inside commercial hypoxic chambers.

Endothelial dysfunction in autoimmune, pulmonary, and kidney systems, and exercise tolerance following SARS-CoV-2 infection

Long COVID is characterized by persistent symptoms beyond 3-months of severe acute respiratory syndrome Coronavirus-2 (SARS-CoV-2) infection that last for at least 2 months and cannot be explained by an alternative diagnosis. Autonomic, immunologic, endothelial, and hypercoagulation are implicated as possible mechanisms of long COVID symptoms. Despite recognition of the public health challenges posed by long COVID, the current understanding of the pathophysiological underpinnings is still evolving. In this narrative review, we explore the long-term effects of SARS-CoV-2 infection on T cell activation such as autoimmune disorders and endothelial cell dysfunction involving vascular impairments within pulmonary and renal architecture. We have described how endothelial dysfunction and vascular abnormalities may underscore findings of exercise intolerance by way of impaired peripheral oxygen extraction in individuals with long COVID.

The molecular athlete: exercise physiology from mechanisms to medals

Human skeletal muscle demonstrates remarkable plasticity, adapting to numerous external stimuli including the habitual level of contractile loading. Accordingly, muscle function and exercise capacity encompass a broad spectrum, from inactive individuals with low levels of endurance and strength to elite athletes who produce prodigious performances underpinned by pleiotropic training-induced muscular adaptations. Our current understanding of the signal integration, interpretation, and output coordination of the cellular and molecular mechanisms that govern muscle plasticity across this continuum is incomplete. As such, training methods and their application to elite athletes largely rely on a "trial-and-error" approach, with the experience and practices of successful coaches and athletes often providing the bases for "post hoc" scientific enquiry and research. This review provides a synopsis of the morphological and functional changes along with the molecular mechanisms underlying exercise adaptation to endurance- and resistance-based training. These traits are placed in the context of innate genetic and interindividual differences in exercise capacity and performance, with special consideration given to aging athletes. Collectively, we provide a comprehensive overview of skeletal muscle plasticity in response to different modes of exercise and how such adaptations translate from "molecules to medals."

Can affinity of hemoglobin to oxygen to be a prognostic marker in critically ill COVID-19 patients?

Background

This study's objective is to determine the slope of the hemoglobin oxygen dissociation curve in critically ill patients who have COVID-19 along with blood gas measurements and how mortality might be impacted by this circumstance.

Aim

It has been reported that the hemoglobin oxygen dissociation curve is not different from healthy patients in COVID-19. However, there are insufficient data on the behavior of the curve in patients who require intensive care.

Patients and Methods

This retrospective study was conducted between 01.03.2021 and 01.07.2021 with patients who were followed up due to COVID-19 in adult intensive care unit. P50 and lactate value obtained from in vitro calculated blood gas analysis. The survival status of the patients was recorded.

Results

The mean P50 value at the admission of nonsurvivors was significantly higher than survivors. In correlation analysis, a significant positive correlation was seen between P50, mortality, and lactate level at admission. SpO₂, PaO₂/FiO₂ ratio, and length of stay in intensive care unit were significantly negatively correlated with P50 levels.

Conclusion

A right shift in the hemoglobin oxygen dissociation curve is associated with mortality. Lactate levels were also associated with a right shift. Prospective experimental studies are needed to provide a better understanding of this process.

Effect of Glycerol-Induced Hyperhydration on a 5-kilometer Running Time-Trial Performance in the Heat in Recreationally Active Individuals

Maximal oxygen consumption (V˙O_2max) is a major determinant of 5-km running time-trial (TT) performance. Glycerol-induced hyperhydration (GIH) could improve V˙O_2max in recreationally active persons through an optimal increase in plasma volume. Moreover, ingestion of a large bolus of cold fluid before exercise could decrease thermal stress during exercise, potentially contributing to improved performance. We determined the effect of GIH on 5-km running TT performance in 10 recreationally active individuals (age: 24 ± 4 years; V˙O_2max: 48 ± 3 mL/kg/min). Using a randomized and counterbalanced protocol, participants underwent two, 120-min hydration protocols where they ingested a 1) 30 mL/kg fat-free mass (FFM) of cold water (~4 °C) with an artificial sweetener + 1.4 g glycerol/kg FFM over the first 60 min (GIH) or 2) 7.5 mL/kg FFM of cold water with an artificial sweetener over the first 20 min (EUH). Following GIH and EUH, participants underwent a 5-km running TT at 30 °C and 50% relative humidity. After 120 min, GIH was associated with significantly greater fluid retention (846 ± 415 mL) and plasma volume changes (10.1 ± 8.4%) than EUH, but gastrointestinal (GI) temperature did not differ. During exercise, 5-km running TT performance (GIH: 22.95 ± 2.62; EUH: 22.52 ± 2.74 min), as well as heart rate, GI temperature and perceived exertion did not significantly differ between conditions. This study demonstrates that the additional body water and plasma volume gains provided by GIH do not improve 5-km running TT performance in the heat in recreationally active individuals.

Computational Modeling and Imaging of the Intracellular Oxygen Gradient

Computational modeling can provide a mechanistic and quantitative framework for describing intracellular spatial heterogeneity of solutes such as oxygen partial pressure (pO₂). This study develops and evaluates a finite-element model of oxygen-consuming mitochondrial bioenergetics using the COMSOL Multiphysics program. The model derives steady-state oxygen (O₂) distributions from Fickian diffusion and Michaelis–Menten consumption kinetics in the mitochondria and cytoplasm. Intrinsic model parameters such as diffusivity and maximum consumption rate were estimated from previously published values for isolated and intact mitochondria. The model was compared with experimental data collected for the intracellular and mitochondrial pO₂ levels in human cervical cancer cells (HeLa) in different respiratory states and under different levels of imposed pO₂. Experimental pO₂ gradients were measured using lifetime imaging of a Förster resonance energy transfer (FRET)-based O₂ sensor, Myoglobin-mCherry, which offers in situ real-time and noninvasive measurements of subcellular pO₂ in living cells. On the basis of these results, the model qualitatively predicted (1) the integrated experimental data from mitochondria under diverse experimental conditions, and (2) the impact of changes in one or more mitochondrial processes on overall bioenergetics.

Cardiorespiratory Coordination in Collegiate Rowing: A Network Approach to Cardiorespiratory Exercise Testing

Collegiate rowing performance is often assessed by a cardiopulmonary exercise test (CPET). Rowers' on-water performance involves non-linear dynamic interactions and synergetic reconfigurations of the cardiorespiratory system. Cardiorespiratory coordination (CRC) method measures the co-variation among cardiorespiratory variables. Novice (n = 9) vs. Intermediate (n = 9) rowers' CRC (H0: Novice CRC = Intermediate CRC; HA: Novice CRC < Intermediate CRC) was evaluated through principal components analysis (PCA). A female NCAA Division II team (N = 18) grouped based on their off-water performance on 6000 m time trial. Rowers completed a customized CPET to exhaustion and a variety of cardiorespiratory values were recorded. The number of principal components (PCs) and respective PC eigenvalues per group were computed on SPSS vs28. Intermediate (77%) and Novice (33%) groups showed one PC1. Novice group formed an added PC2 due to the shift of expired fraction of oxygen or, alternatively, heart rate/ventilation, from the PC1 cluster of examined variables. Intermediate rowers presented a higher degree of CRC, possible due to their increased ability to utilize the bicarbonate buffering system during the CPET. CRC may be an alternative measure to assess aerobic fitness providing insights to the complex cardiorespiratory interactions involved in rowing during a CPET.

Impairment in maximal lactate steady state after carbon monoxide inhalation is related to training status

NEW FINDINGS

What is the central question of this study? What is the effect of an elevated COHb concentration following carbon monoxide inhalation on the maximal lactate steady state (MLSS) in humans and is this effect dependent on aerobic fitness? What is the main finding and its importance? An elevated COHb concentration intensified physiological responses to exercise at the MLSS- including heart rate, ventilation, and peripheral fatigue-in all participants and reduced the MLSS (i.e., destabilized the blood lactate concentration) in trained but not untrained males and females.

ABSTRACT

This study investigated whether a lower effective [Hb], induced by carbon monoxide (CO) inhalation, reduces the peak oxygen uptake (V̇O₂ peak) and the maximal lactate steady state (MLSS) and whether training status explains individual variation in these impairments. Healthy young participants completed two ramp incremental tests (n = 20 [10 female]) and two trials at MLSS (n = 16 [8 female]) following CO rebreathe tests and sham procedures (SHAM) in random orders. All fitness variables were normalized to fat-free mass (FFM) to account for sex-related differences in body composition, and males and females were matched for aerobic fitness. The V̇O₂ peak (mean [SD]: -4.2 [3.7]%), peak power output (-3.3 [2.2]%), and respiratory compensation point (-6.3 [4.5]%) were reduced in CO compared with SHAM (P < 0.001 for all), but the gas exchange threshold (-3.3 [7.1]%) was not (P = 0.077). Decreases in V̇O₂ peak (r = -0.45; P = 0.047) and peak power output (r = -0.49; P = 0.029) in CO were correlated with baseline aerobic fitness. Compared to SHAM, physiological and perceptual indicators of exercise-related stress were exacerbated by CO while cycling at MLSS. Notably, the mean blood lactate concentration ([La]) increased (i.e., Δ[La] > 1.0 mM) between 10 min (5.5 [1.4] mM) and 30 min (6.8 [1.3] mM; P = 0.026) in CO, with 9/16 participants classified as unstable. These unstable participants had a higher V̇O₂ peak (66.2 [8.5] vs. 56.4 [8.8] mL·kg FFM^-1 ·min^-1 , P = 0.042) and V̇O₂ at MLSS (55.8 vs. 44.3 mL·kg FFM^-1 ·min^-1 , P = 0.006) compared to the stable group. In conclusion, a reduced O₂ -carrying capacity decreased maximal and submaximal exercise performance, with higher aerobic fitness associated with greater impairments in both. This article is protected by copyright. All rights reserved.

Limits to Submaximal and Maximal Exercise in Patients with Hypertrophic Cardiomyopathy

BACKGROUND

Patients with hypertrophic cardiomyopathy (HCM) often have reduced exercise capacity, and it is unclear whether cardiovascular regulation during exercise is intact in these patients. We aimed to determine the relationship between cardiac output (Q̇_c) and oxygen uptake (V̇O₂), and stroke volume (SV) reserve in HCMcompared to healthy participants and participants with left ventricular hypertrophy (LVH) but not HCM.

METHODS

Sixteen patients with HCM (48±7 years, 44% female), 16 participants with LVH (49±5 years, 44% female), and 61 healthy controls (CON: 52±5 years, 52% female) completed submaximal steady-state treadmill exercise followed by a maximal exercise test. V̇O₂, Q̇_c,SV and arterio-venous oxygen difference were measured during rest and exercise, and Q̇_c/V̇O₂ slopes were constructed.

RESULTS

The Q̇_c/V̇O₂ slopewas blunted in HCM compared to CON and LVH (HCM 4.9±0.7 vs. CON 5.5± 1.0 [P = 0.027], vs LVH 6.0±1.0AU [P = 0.002]) and participants with HCM had a lower SV reserve (HCM 53±33%, controls 83±33%, LVH 82±22%; HCM vs. controls P = 0.002; HCM vs. LVH P = 0.015). Despite a blunted Q̇_c/V̇O₂ slope, 75% of patients with HCM achieved ≥80% predicted V̇O₂max by augmenting a-vO₂ difference at maximal exercise (16.0±0.8 mL/100mL vs 13.8±2.7 mL/100mL, P = 0.021).

CONCLUSIONS

Patients with HCM do not appropriately match Q̇_c to metabolic demand, primarily due to inadequate stroke volume augmentation. Despite this central limitation, many patients achieve normal exercise capacities by significantly increasing peripheral oxygen extraction.

Resting Rates of Blood Flow and Glucose Use per Neuron Are Proportional to Number of Endothelial Cells Available per Neuron Across Sites in the Rat Brain

We report in a companion paper that in the mouse brain, in contrast to the 1,000-fold variation in local neuronal densities across sites, capillary density (measured both as capillary volume fraction and as density of endothelial cells) show very little variation, of the order of only fourfold. Here we confirm that finding in the rat brain and, using published rates of local blood flow and glucose use at rest, proceed to show that what small variation exists in capillary density across sites in the rat brain is strongly and linearly correlated to variations in local rates of brain metabolism at rest. Crucially, we show that such variations in local capillary density and brain metabolism are not correlated with local variations in neuronal density, which contradicts expectations that use-dependent self-organization would cause brain sites with more neurons to have higher capillary densities due to higher energetic demands. In fact, we show that the ratio of endothelial cells per neuron serves as a linear indicator of average blood flow and glucose use per neuron at rest, and both increase as neuronal density decreases across sites. In other words, because of the relatively tiny variation in capillary densities compared to the large variation in neuronal densities, the anatomical infrastructure of the brain is such that those sites with fewer neurons have more energy supplied per neuron, which matches a higher average rate of energy use per neuron, compared to sites with more neurons. Taken together, our data support the interpretation that resting brain metabolism is not demand-based, but rather limited by its capillary supply, and raise multiple implications for the differential vulnerability of diverse brain areas to disease and aging.

Matching of O2 Utilization and O2 Delivery in Contracting Skeletal Muscle in Health, Aging, and Heart Failure

Skeletal muscle is one of the most dynamic metabolic organs as evidenced by increases in metabolic rate of >150-fold from rest to maximal contractile activity. Because of limited intracellular stores of ATP, activation of metabolic pathways is required to maintain the necessary rates of ATP re-synthesis during sustained contractions. During the very early phase, phosphocreatine hydrolysis and anaerobic glycolysis prevails but as activity extends beyond ∼1 min, oxidative phosphorylation becomes the major ATP-generating pathway. Oxidative metabolism of macronutrients is highly dependent on the cardiovascular system to deliver O₂ to the contracting muscle fibres, which is ensured through a tight coupling between skeletal muscle O₂ utilization and O₂ delivery. However, to what extent O₂ delivery is ideal in terms of enabling optimal metabolic and contractile function is context-dependent and determined by a complex interaction of several regulatory systems. The first part of the review focuses on local and systemic mechanisms involved in the regulation of O₂ delivery and how integration of these influences the matching of skeletal muscle O₂ demand and O₂ delivery. In the second part, alterations in cardiovascular function and structure associated with aging and heart failure, and how these impact metabolic and contractile function, will be addressed. Where applicable, the potential of exercise training to offset/reverse age- and disease-related cardiovascular declines will be highlighted in the context of skeletal muscle metabolic function. The review focuses on human data but also covers animal observations.

From a Demand-Based to a Supply-Limited Framework of Brain Metabolism

What defines the rate of energy use by the brain, as well as per neurons of different sizes in different structures and animals, is one fundamental aspect of neuroscience for which much has been theorized, but very little data are available. The prevalent theories and models consider that energy supply from the vascular system to different brain regions is adjusted both dynamically and in the course of development and evolution to meet the demands of neuronal activity. In this perspective, we offer an alternative view: that regional rates of energy use might be mostly constrained by supply, given the properties of the brain capillary network, the highly stable rate of oxygen delivery to the whole brain under physiological conditions, and homeostatic constraints. We present evidence that these constraints, based on capillary density and tissue oxygen homeostasis, are similar between brain regions and mammalian species, suggesting they derive from fundamental biophysical limitations. The same constraints also determine the relationship between regional rates of brain oxygen supply and usage over the full physiological range of brain activity, from deep sleep to intense sensory stimulation, during which the apparent uncoupling of blood flow and oxygen use is still a predicted consequence of supply limitation. By carefully separating "energy cost" into energy supply and energy use, and doing away with the problematic concept of energetic "demands," our new framework should help shine a new light on the neurovascular bases of metabolic support of brain function and brain functional imaging. We speculate that the trade-offs between functional systems and even the limitation to a single attentional spot at a time might be consequences of a strongly supply-limited brain economy. We propose that a deeper understanding of brain energy supply constraints will provide a new evolutionary understanding of constraints on brain function due to energetics; offer new diagnostic insight to disturbances of brain metabolism; lead to clear, testable predictions on the scaling of brain metabolic cost and the evolution of brains of different sizes; and open new lines of investigation into the microvascular bases of progressive cognitive loss in normal aging as well as metabolic diseases.

NIRS-derived muscle V̇O2 kinetics after moderate running exercise in healthy males: reliability and associations with parameters of aerobic fitness

NEW FINDINGS

What is the central question of this study? In vivo muscle oxidative capacity has been evaluated through the mV̇O₂ kinetics following single joint exercise using NIRS system. Here, we demonstrated its utility following running exercise. What is the main finding and its importance? We demonstrated that time constant of mV̇O₂ kinetics in gastrocnemius following moderate running exercise presents good to excellent reliability. In addition, it was well correlated with parameters of aerobic fitness, such as maximal speed of the incremental test, ventilatory threshold and pulmonary V̇O₂ on-kinetics. Therefore, NIRS-derived muscle oxidative capacity together with other physiological measurements may allow a concomitant local and systemic analysis of the components of the oxidative system.

ABSTRACT

NIRS-derived muscle oxygen uptake (mV̇O₂ ) kinetics following single-joint exercise has been used to assess muscle oxidative capacity. However, little evidence is available on the use of this technique following whole-body exercises. Therefore, this study aimed to assess the reliability of the NIRS-derived mV̇O₂ kinetics following running exercise and to investigate the relationship between the time constant of mV̇O₂ off-kinetics (τmV̇O₂ ) with parameters of aerobic fitness. After an incremental test to determine V̇O₂ max, first (VT₁ ) and second (VT₂ ) ventilatory thresholds, and maximal speed (Smax), thirteen males (age = 21 ± 4 years; V̇O₂ max = 55.9 ± 3.4 mlꞏkg-1ꞏmin-1) performed three sets (two in the first day and one on a subsequent day) of two repetitions of 6-min running exercise at 90%VT₁ . The pulmonary V̇O₂ on-kinetics (pV̇O₂ ) and mV̇O₂ off-kinetics in gastrocnemius were assessed. τmV̇O₂ presented no systematic change and satisfactory reliability (SEM and ICC of 4.21 s and 0.49 for between transitions; and 2.65 s and 0.74 averaging τmV̇O₂ within each time-set), with no difference (p > 0.3) between the within- (SEM = 2.92 s) and between-day variability (SEM = 2.78 s and 2.19 s between first vs. third set, and second vs. third set, respectively). τmV̇O₂ (28.5 ± 4.17 s) correlated significantly (p < 0.05) with Smax (r = -0.66), VT₁ (r = -0.64) and time constant of the pV̇O2 on-kinetics (r = 0.69). These findings indicate that NIRS-derived mV̇O₂ kinetics in the gastrocnemius following moderate running exercise is a useful and reliable method to assess muscle oxidative capacity. This article is protected by copyright. All rights reserved.

Effects of Short-Term Phosphate Loading on Aerobic Capacity under Acute Hypoxia in Cyclists: A Randomized, Placebo-Controlled, Crossover Study

The aim of this study was to evaluate the effects of sodium phosphate (SP) supplementation on aerobic capacity in hypoxia. Twenty-four trained male cyclists received SP (50 mg·kg-1 of FFM/day) or placebo for six days in a randomized, crossover study, with a three-week washout period between supplementation phases. Before and after each supplementation phase, the subjects performed an incremental exercise test to exhaustion in hypoxia (FiO2 = 16%). Additionally, the levels of 2,3-diphosphoglycerate (2,3-DPG), hypoxia-inducible factor 1 alpha (HIF-1α), inorganic phosphate (Pi), calcium (Ca), parathyroid hormone (PTH) and acid-base balance were determined. The results showed that phosphate loading significantly increased the Pi level by 9.0%, whereas 2,3-DPG levels, hemoglobin oxygen affinity, buffering capacity and myocardial efficiency remained unchanged. The aerobic capacity in hypoxia was not improved following SP. Additionally, our data revealed high inter-individual variability in response to SP. Therefore, the participants were grouped as Responders and Non-Responders. In the Responders, a significant increase in aerobic performance in the range of 3-5% was observed. In conclusion, SP supplementation is not an ergogenic aid for aerobic capacity in hypoxia. However, in certain individuals, some benefits can be expected, but mainly in athletes with less training-induced central and/or peripheral adaptation.

The Determination of Step Frequency in 3-min Incremental Step-in-Place Tests for Predicting Maximal Oxygen Uptake from Heart Rate Response in Taiwanese Adults

The maximal oxygen uptake (VO_2max) prediction models established by step tests are often used for evaluating cardiorespiratory fitness (CRF). However, it is unclear which type of stepping frequency sequence is more suitable for the public to assess the CRF. Therefore, the main purpose of this study was to test the effectiveness of two 3-min incremental step-in-place (3MISP) tests (i.e., 3MISP_30s and 3MISP_60s) with the same total number of steps but different step-frequency sequences in predicting VO_2max. In this cross-sectional study, a total of 200 healthy adults in Taiwan completed 3MISP_30s and 3MISP_60s tests, as well as cardiopulmonary exercise testing. The 3MISP_30s and 3MISP_60s models were established through multiple stepwise regression analysis by gender, age, percent body fat, and 3MISP-heart rate. The statistical analysis included Pearson's correlations, the standard errors of estimate, the predicted residual error sum of squares, and the Bland-Altman plot to compare the measured VO_2max values and those estimated. The results of the study showed that the exercise intensity of the 3MISP_30s test was higher than that of the 3MISP_60s test (% heart rate reserve (HRR) during 3MISP_30s vs. %HRR during 3MISP_60s = 81.00% vs. 76.81%, p < 0.001). Both the 3MISP_30s model and the 3MISP_60s model explained 64.4% of VO_2max, and the standard errors of the estimates were 4.2043 and 4.2090 mL·kg^-1·min^-1, respectively. The cross-validation results also indicated that the measured VO_2max values and those predicted by the 3MISP_30s and 3MISP_60s models were highly correlated (3MISP_30s model: r = 0.804, 3MISP_60s model: r = 0.807, both p < 0.001). There was no significant difference between the measured VO_2max values and those predicted by the 3MISP_30s and 3MISP_60s models in the testing group (p > 0.05). The results of the study showed that when the 3MISP_60s test was used, the exercise intensity was significantly reduced, but the predictive effectiveness of VO_2max did not change. We concluded that the 3MISP_60s test was physiologically less stressful than the 3MISP_30s test, and it could be a better choice for CRF evaluation.

The Road to the Beijing Winter Olympics and Beyond: Opinions and Perspectives on Physiology and Innovation in Winter Sport

Beijing will host the 2022 Winter Olympics, and China strengthens research on various aspects to allow their athletes to compete successfully in winter sport. Simultaneously, Government-directed initiatives aim to increase public participation in recreational winter sport. These parallel developments allow research to advance knowledge and understanding of the physiological determinants of performance and health related to winter sport. Winter sport athletes often conduct a substantial amount of training with high volumes of low-to-moderate exercise intensity and lower volumes of high-intensity work. Moreover, much of the training occur at low ambient temperatures and winter sport athletes have high risk of developing asthma or asthma-related conditions, such as exercise-induced bronchoconstriction. The high training volumes require optimal nutrition with increased energy and dietary protein requirement to stimulate muscle protein synthesis response in the post-exercise period. Whether higher protein intake is required in the cold should be investigated. Cross-country skiing is performed mostly in Northern hemisphere with a strong cultural heritage and sporting tradition. It is expected that innovative initiatives on recruitment and training during the next few years will target to enhance performance of Chinese athletes in classical endurance-based winter sport. The innovation potential coupled with resourcing and population may be substantial with the potential for China to become a significant winter sport nation. This paper discusses the physiological aspects of endurance training and performance in winter sport highlighting areas where innovation may advance in athletic performance in cold environments. In addition, to ensure sustainable development of snow sport, a quality ski patrol and rescue system is recommended for the safety of increasing mass participation.

Reduced exercise capacity in patients with systemic sclerosis is associated with lower peak tissue oxygen extraction: a cardiovascular magnetic resonance-augmented cardiopulmonary exercise study

BACKGROUND

Exercise intolerance in systemic sclerosis (SSc) is typically attributed to cardiopulmonary limitations. However, problems with skeletal muscle oxygen extraction have not been fully investigated. This study used cardiovascular magnetic resonance (CMR)-augmented cardiopulmonary exercise testing (CMR-CPET) to simultaneously measure oxygen consumption and cardiac output. This allowed calculation of arteriovenous oxygen content gradient, a recognized marker of oxygen extraction. We performed CMR-CPET in 4 groups: systemic sclerosis (SSc); systemic sclerosis-associated pulmonary arterial hypertension (SSc-PAH); non-connective tissue disease pulmonary hypertension (NC-PAH); and healthy controls.

METHODS

We performed CMR-CPET in 60 subjects (15 in each group) using a supine ergometer following a ramped exercise protocol until exhaustion. Values for oxygen consumption, cardiac output and oxygen content gradient, as well as ventricular volumes, were obtained at rest and peak-exercise for all subjects. In addition, T1 and T2 maps were acquired at rest, and the most recent clinical measures (hemoglobin, lung function, 6-min walk, cardiac and catheterization) were collected.

RESULTS

All patient groups had reduced peak oxygen consumption compared to healthy controls (p < 0.022). The SSc and SSc-PAH groups had reduced peak oxygen content gradient compared to healthy controls (p < 0.03). Conversely, the SSc-PAH and NC-PH patients had reduced peak cardiac output compared to healthy controls and SSc patients (p < 0.006). Higher hemoglobin was associated with higher peak oxygen content gradient (p = 0.025) and higher myocardial T1 was associated with lower peak stroke volume (p = 0.011).

CONCLUSIONS

Reduced peak oxygen consumption in SSc patients is predominantly driven by reduced oxygen content gradient and in SSc-PAH patients this was amplified by reduced peak cardiac output. Trial registration The study is registered with ClinicalTrials.gov Protocol Registration and Results System (ClinicalTrials.gov ID: 100358).

Responders and non-responders to aerobic exercise training: beyond the evaluation of V˙O2max

The evaluation of the maximal oxygen uptake ( V ˙ O 2 max ) following exercise training is the classical assessment of training effectiveness. Research has lacked in investigating whether individuals that do not respond to the training intervention ( V ˙ O 2 max ), also do not improve in other health-related parameters. We aimed to investigate the cardiovascular and metabolic adaptations (i.e., performance, body composition, blood pressure, vascular function, fasting blood markers, and resting cardiac function and morphology) to exercise training among participants who showed different levels of V ˙ O 2 max responsiveness. Healthy sedentary participants engaged in a 6-week exercise training program, three times a week. Our results showed that responders had a greater increase in peak power output, second lactate threshold, and microvascular responsiveness, whereas non-responders had a greater increase in cycling efficiency. No statistical differences were observed in body composition, blood pressure, fasting blood parameters, and resting cardiac adaptations. In conclusion, our study showed, for the first time, that in addition to the differences in the V ˙ O 2 max , a greater increase in microvascular responsiveness in responders compared to non-responders was observed. Additionally, responders and non-responders did not show differences in the adaptations on metabolic parameters. There is an increasing need for personalized training prescription, depending on the target clinical outcome.

Twelve weeks of sprint interval training increases peak cardiac output in previously untrained individuals

INTRODUCTION

Sprint interval training (SIT), characterized by brief bouts of 'supramaximal' exercise interspersed with recovery periods, increases peak oxygen uptake ([Formula: see text]) despite a low total exercise volume. Per the Fick principle, increased [Formula: see text] is attributable to increased peak cardiac output ([Formula: see text]) and/or peak arterio-venous oxygen difference (a-vO_2diff). There are limited and equivocal data regarding the physiological basis for SIT-induced increases in [Formula: see text], with most studies lasting ≤ 6 weeks.

PURPOSE

To determine the effect of 12 weeks of SIT on [Formula: see text], measured using inert gas rebreathing, and the relationship between changes in [Formula: see text] and [Formula: see text].

METHODS

15 healthy untrained adults [6 males, 9 females; 21 ± 2 y (mean ± SD)] performed 28 ± 3 training sessions. Each session involved a 2-min warm-up at 50 W, 3 × 20-s 'all-out' cycling bouts (581 ± 221 W) interspersed with 2-min of recovery, and a 3-min cool-down at 50 W.

RESULTS

Measurements performed before and after training showed that 12 weeks of SIT increased [Formula: see text] (17.0 ± 3.7 vs 18.1 ± 4.6 L/min, p = 0.01, partial η² = 0.28) and [Formula: see text] (2.63 ± 0.78 vs 3.18 ± 1.1 L/min, p < 0.01, partial η² = 0.58). The changes in these two variables were correlated (r² = 0.46, p < 0.01). Calculated peak a-vO_2diff also increased after training (154 ± 22 vs 174 ± 23 ml O₂/L; p < 0.01) and was correlated with the change in [Formula: see text] (r² = 0.33, p = 0.03). Exploratory analyses revealed an interaction (p < 0.01) such that [Formula: see text] increased in male (+ 10%, p < 0.01) but not female participants (+ 0.6%, p = 0.96), suggesting potential sex-specific differences.

CONCLUSION

Twelve weeks of SIT increased [Formula: see text] by 6% in previously untrained participants and the change was correlated with the larger 21% increase in [Formula: see text].

Individual cardiovascular responsiveness to work-matched exercise within the moderate- and severe-intensity domains

Purpose

We investigated the cardiovascular individual response to 6 weeks (3×/week) of work-matched within the severe-intensity domain (high-intensity interval training, HIIT) or moderate-intensity domain (moderate-intensity continuous training, MICT). In addition, we analyzed the cardiovascular factors at baseline underlying the response variability.

Methods

42 healthy sedentary participants were randomly assigned to HIIT or MICT. We applied the region of practical equivalence-method for identifying the levels of responders to the maximal oxygen uptake (V̇O_2max) response. For investigating the influence of cardiovascular markers, we trained a Bayesian machine learning model on cardiovascular markers.

Results

Despite that HIIT and MICT induced significant increases in V̇O_2max, HIIT had greater improvements than MICT (p < 0.001). Greater variability was observed in MICT, with approximately 50% classified as “non-responder” and “undecided”. 20 “responders”, one “undecided” and no “non-responders” were observed in HIIT. The variability in the ∆V̇O_2max was associated with initial cardiorespiratory fitness, arterial stiffness, and left-ventricular (LV) mass and LV end-diastolic diameter in HIIT; whereas, microvascular responsiveness and right-ventricular (RV) excursion velocity showed a significant association in MICT.

Conclusion

Our findings highlight the critical influence of exercise-intensity domains and biological variability on the individual V̇O_2max response. The incidence of “non-responders” in MICT was one third of the group; whereas, no “non-responders” were observed in HIIT. The incidence of “responders” was 11 out of 21 participants in MICT, and 20 out of 21 participants in HIIT. The response in HIIT showed associations with baseline fitness, arterial stiffness, and LV-morphology; whereas, it was associated with RV systolic function in MICT.

Blood flow restriction training and the high-performance athlete: science to application

The manipulation of blood flow in conjunction with skeletal muscle contraction has greatly informed the physiological understanding of muscle fatigue, blood pressure reflexes, and metabolism in humans. Recent interest in using intentional blood flow restriction (BFR) has focused on elucidating how exercise during periods of reduced blood flow affects typical training adaptations. A large initial appeal for BFR training was driven by studies demonstrating rapid increases in muscle size, strength, and endurance capacity, even when notably low intensities and resistances, which would typically be incapable of stimulating change in healthy populations, were used. The incorporation of BFR exercise into the training of strength- and endurance-trained athletes has recently been shown to provide additive training effects that augment skeletal muscle and cardiovascular adaptations. Recent observations suggest BFR exercise alters acute physiological stressors such as local muscle oxygen availability and vascular shear stress, which may lead to adaptations that are not easily attained with conventional training. This review explores these concepts and summarizes both the evidence base and knowledge gaps regarding the application of BFR training for athletes.

Fluctuations in cardiac stroke volume during rowing

Preload to the heart may be limited during rowing because both blood pressure and central venous pressure increase when force is applied to the oar. Considering that only the recovery phase of the rowing stroke allows for unhindered venous return, rowing may induce large fluctuations in stroke volume (SV). Thus, the purpose of this study was to evaluate SV continuously during the rowing stroke. Eight nationally competitive oarsmen (mean ± standard deviation: age 21 ± 2 years, height 190 ± 9 cm, and weight 90 ± 10 kg) rowed on an ergometer at a targeted heart rate of 130 and 160 beats per minute. SV was derived from arterial pressure waveform by pulse contour analysis, while ventilation and force on the handle were measured. Mean arterial pressure was elevated during the stroke at both work rates (to 133 ± 10 [P < .001] and 145 ± 11 mm Hg [P = .024], respectively). Also, SV fluctuated markedly during the stroke with deviations being largest at the higher work rate. Thus, SV decreased by 27 ± 10% (31 ± 11 mL) at the beginning of the stroke and increased by 25 ± 9% (28 ± 10 mL) in the recovery (P = .013), while breathing was entrained with one breath during the drive of the stroke and one prior to the next stroke. These observations indicate that during rowing cardiac output depends critically on SV surges during the recovery phase of the stroke.

Contribution of oxygen extraction fraction to maximal oxygen uptake in healthy young men

We analysed the importance of systemic and peripheral arteriovenous O₂ difference ( a-v¯O2 difference and a‐v_fO₂ difference, respectively) and O₂ extraction fraction for maximal oxygen uptake ( V˙O2max). Fick law of diffusion and the Piiper and Scheid model were applied to investigate whether diffusion versus perfusion limitations vary with V˙O2max. Articles (n = 17) publishing individual data (n = 154) on V˙O2max, maximal cardiac output ( Q˙max; indicator‐dilution or the Fick method), a-v¯O2 difference (catheters or the Fick equation) and systemic O₂ extraction fraction were identified. For the peripheral responses, group‐mean data (articles: n = 27; subjects: n = 234) on leg blood flow (LBF; thermodilution), a‐v_fO₂ difference and O₂ extraction fraction (arterial and femoral venous catheters) were obtained. Q˙max and two‐LBF increased linearly by 4.9‐6.0 L · min^–1 per 1 L · min^–1 increase in V˙O2max (R² = .73 and R² = .67, respectively; both P < .001). The a-v¯O2 difference increased from 118‐168 mL · L^–1 from a V˙O2max of 2‐4.5 L · min^–1 followed by a reduction (second‐order polynomial: R² = .27). After accounting for a hypoxemia‐induced decrease in arterial O₂ content with increasing V˙O2max (R² = .17; P < .001), systemic O₂ extraction fraction increased up to ~90% ( V˙O2max: 4.5 L · min^–1) with no further change (exponential decay model: R² = .42). Likewise, leg O₂ extraction fraction increased with V˙O2max to approach a maximal value of ~90‐95% (R² = .83). Muscle O₂ diffusing capacity and the equilibration index Y increased linearly with V˙O2max (R² = .77 and R² = .31, respectively; both P < .01), reflecting decreasing O₂ diffusional limitations and accentuating O₂ delivery limitations. In conclusion, although O₂ delivery is the main limiting factor to V˙O2max, enhanced O₂ extraction fraction (≥90%) contributes to the remarkably high V˙O2max in endurance‐trained individuals.

Progress Update and Challenges on V . O2max Testing and Interpretation

The maximal oxygen uptake ( V . O2max) is the primary determinant of endurance performance in heterogeneous populations and has predictive value for clinical outcomes and all-cause mortality. Accurate and precise measurement of V . O2max requires the adherence to quality control procedures, including combustion testing and the use of standardized incremental exercise protocols with a verification phase preceded by an adequate familiarization. The data averaging strategy employed to calculate the V . O2max from the breath-by-breath data can change the V . O2max value by 4-10%. The lower the number of breaths or smaller the number of seconds included in the averaging block, the higher the calculated V . O2max value with this effect being more prominent in untrained subjects. Smaller averaging strategies in number of breaths or seconds (less than 30 breaths or seconds) facilitate the identification of the plateau phenomenon without reducing the reliability of the measurements. When employing metabolic carts, averaging intervals including 15-20 breaths or seconds are preferable as a compromise between capturing the true V . O2max and identifying the plateau. In training studies, clinical interventions and meta-analysis, reporting of V . O2max in absolute values and inclusion of protocols and the averaging strategies arise as imperative to permit adequate comparisons. Newly developed correction equations can be used to normalize V . O2max to similar averaging strategies. A lack of improvement of V . O2max with training does not mean that the training program has elicited no adaptations, since peak cardiac output and mitochondrial oxidative capacity may be increased without changes in V . O2max.