The role of vascular function on exercise capacity in health and disease

Association of vitamin D deficiency with post-exercise hypotension and arterial stiffness following prolonged endurance exercise in healthy young men

BACKGROUND

Vitamin D is associated with vascular function; however, the impact of different vitamin D levels on vascular elasticity following prolonged exercise remains uncertain. The primary objective of this study was to investigate the association of vitamin D levels with changes in peripheral pulse wave velocity (pPWV) and the magnitude of acute post-exercise hypotension (PEH) following prolonged endurance exercise in healthy young men.

METHODS

All the participants were divided into two groups: the 25-hydroxyvitamin D (25(OH)D) sufficiency group (25(OH)D ≧50 nmol/L) and the deficiency group (25(OH)D < 50 nmol/L). A cardiopulmonary exercise test for maximal oxygen uptake (V . O2max) was performed on the graded cycling. The prolonged exercise was set at 60% V . O2max for 120 min of continuous riding on a stationary bicycle. The pPWV and blood pressure were measured at baseline and 0, 15, 30, 45, 60 min after prolonged endurance exercise.

RESULTS

Post hoc analysis revealed that the vitamin D sufficient group had a greater magnitude of PEH than the deficiency group at post-45 min. Multiple linear regression analyses showed a significant correlation between 25(OH)D and both pPWV (p = 0.036) and PEH (p = 0.007), after adjusting for V . O2max, weight, height, and physical activity. In addition, the 25(OH)D deficiency group also had higher pPWV at post-15 min (5.41 ± 0.93 vs 4.84 ± 0.75 m/s), post-30 min (5.30 ± 0.77 vs 4.87 ± 0.50 m/s), post-45 min (5.56 ± 0.93 vs 5.05 ± 0.68 m/s) than the sufficiency group.

CONCLUSIONS

There was a positive correlation between 25(OH)D levels and systolic PEH following prolonged endurance exercise. Individuals with sufficient 25(OH)D status may have better vascular elasticity and more efficient blood pressure regulation during exercise.

Wagner diagram for modeling O2pathway-calculation and graphical display by the Helsinki O2Pathway Tool

Objective.Maximal O2uptake (V˙O2max) reflects the individual's maximal rate of O2transport and utilization through the integrated whole-body pathway composed of the lungs, heart, blood, circulation, and metabolically active tissues. As such,V˙O2maxis strongly associated with physical capacity as well as overall health and thus acts as one predictor of physical performance and as a vital sign in determination of status and progress of numerous clinical conditions. Quantifying the contribution of single parts of the multistep O2pathway toV˙O2maxprovides mechanistic insights into exercise (in)tolerance and into therapy-, training-, or disuse-induced adaptations at individual or group levels. We developed a desktop application (Helsinki O2Pathway Tool-HO2PT) to model numerical and graphical display of the O2pathway based on the 'Wagner diagram' originally formulated by Peter D. Wagner and his colleagues.Approach.The HO2PT was developed and programmed in Python to integrate the Fick principle and Fick's law of diffusion into a computational system to import, calculate, graphically display, and export variables of the Wagner diagram.Main results.The HO2PT models O2pathway both numerically and graphically according to the Wagner diagram and pertains to conditions under which the mitochondrial oxidative capacity of metabolically active tissues exceeds the capacity of the O2transport system to deliver O2to the mitochondria. The tool is based on the Python open source code and libraries and freely and publicly available online for Windows, macOS, and Linux operating systems.Significance.The HO2PT offers a novel functional and demonstrative platform for those interested in examiningV˙O2maxand its determinants by using the Wagner diagram. It will improve access to and usability of Wagner's and his colleagues' integrated physiological model and thereby benefit users across the wide spectrum of contexts such as scientific research, education, exercise testing, sports coaching, and clinical medicine.

NIRS-Derived Muscle-Deoxygenation and Microvascular Reactivity During Occlusion-Reperfusion at Rest Are Associated With Whole-Body Aerobic Fitness

Purpose: Near-infrared spectroscopy (NIRS) indices during arterial occlusion-reperfusion maneuver have been used to examine the muscle's oxidative metabolism and microvascular function-important determinants of whole-body aerobic-fitness. The association of NIRS-derived parameters with whole-body VO2max was previously examined using a method requiring exercise (or electrical stimulation) followed by multiple arterial occlusions. We examined whether NIRS-derived indices of muscle deoxygenation and microvascular reactivity assessed during a single occlusion-reperfusion at rest are (a) associated with maximal/submaximal indices of whole-body aerobic-fitness and (b) could discriminate individuals with different VO2max. We, also, investigated which NIRS-parameter during occlusion-reperfusion correlates best with whole-body aerobic-fitness. Methods: Twenty-five young individuals performed an arterial occlusion-reperfusion at rest. Changes in oxygenated- and deoxygenated-hemoglobin (O2Hb and HHb, respectively) in vastus-lateralis were monitored; adipose tissue thickness (ATT) at NIRS-application was assessed. Participants also underwent a maximal incremental exercise test for VO2max, maximal aerobic velocity (MAV), and ventilatory-thresholds (VTs) assessments. Results: The HHbslope and HHbmagnitude of increase (occlusion-phase) and O2Hbmagnitude of increase (reperfusion-phase) were strongly correlated with VO2max (r = .695-.763, p < .001) and moderately with MAV (r = .468-.530; p < .05). O2Hbmagnitude was moderately correlated with VTs (r = .399-.414; p < .05). After controlling for ATT, the correlations remained significant for VO2max (r = .672-.704; p < .001) and MAV (r = .407; p < .05). Individuals in the high percentiles after median and tritile splits for HHbslope and O2Hbmagnitude had significantly greater VO2max vs. those in low percentiles (p < .01-.05). The HHbslope during occlusion was the best predictor of VO2max. Conclusion: NIRS-derived muscle deoxygenation/reoxygenation indices during a single arterial occlusion-reperfusion maneuver are strongly associated with whole-body maximal indices of aerobic-fitness (VO2max, MAV) and may discriminate individuals with different VO2max.

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.

Overdot and overline annotation must be understood to accurately interpret V.O2MAX physiology with the Fick formula

Few formulas have been used in exercise physiology as extensively as the Fick formula, which calculates the rate of oxygen consumption (i.e., V.O2) as the product of cardiac output (Q.) and the difference in oxygen content in arterial and mixed venous blood (Δav ¯ O2). Unfortunately, the physiology of maximum V.O2 (V.O2MAX) is often misinterpreted due to a lack of appreciation for the limitations represented by the oft-ignored superscript annotations in the Fick formula. The purpose of this perspective is to explain the meaning of the superscript annotations and highlight how such annotations influence proper interpretation of V.O2MAX physiology with the Fick formula. First, we explain the significance of the overdots above V.O2 and Q., which indicate a measure per unit of time. As we will show, the presence of an overdot above Q. and lack of one above Δav ¯ O2 denotes they are different types of ratios and should be interpreted in the context of one another-not in contrast to each other as is commonplace. Second, we discuss the significance of the overline above the "v ¯ " in Δav ¯ O2, which indicates the venous sample is an average of blood that comes from mixed sources. The mixed nature of the venous sample has major implications for interpreting the influence of oxygen diffusion and blood flow heterogeneity on V.O2MAX. Ultimately, we give recommendations and insights for using the Fick formula to calculate V.O2 and interpret V.O2MAX physiology.

Vascular dysfunction and the age-related decline in critical power

Ageing results in lower exercise tolerance, manifested as decreased critical power (CP). We examined whether the age-related decrease in CP occurs independently of changes in muscle mass and whether it is related to impaired vascular function. Ten older (63.1 ± 2.5 years) and 10 younger (24.4 ± 4.0 years) physically active volunteers participated. Physical activity was measured with accelerometry. Leg muscle mass was quantified with dual X-ray absorptiometry. The CP and maximum power during a graded exercise test (PGXT ) of single-leg knee-extension exercise were determined over the course of four visits. During a fifth visit, vascular function of the leg was assessed with passive leg movement (PLM) hyperaemia and leg blood flow and vascular conductance during knee-extension exercise at 10 W, 20 W, slightly below CP (90% CP) and PGXT . Despite not differing in leg lean mass (P = 0.901) and physical activity (e.g., steps per day, P = 0.735), older subjects had ∼30% lower mass-specific CP (old = 3.20 ± 0.94 W kg-1 vs. young = 4.60 ± 0.87 W kg-1 ; P < 0.001). The PLM-induced hyperaemia and leg blood flow and/or conductance were blunted in the old at 20 W, 90% CP and PGXT (P < 0.05). When normalized for leg muscle mass, CP was strongly correlated with PLM-induced hyperaemia (R2  = 0.52; P < 0.001) and vascular conductance during knee-extension exercise at 20 W (R2  = 0.34; P = 0.014) and 90% CP (R2  = 0.39; P = 0.004). In conclusion, the age-related decline in CP is not only an issue of muscle quantity, but also of impaired muscle quality that corresponds to impaired vascular function.

Pulmonary hypertension alters blood flow distribution and impairs the hyperemic response in the rat diaphragm

Pulmonary hypertension (PH) is characterized by pulmonary vascular remodeling, respiratory muscle and cardiac impairments, and exercise intolerance. Specifically, impaired gas exchange increases work of the diaphragm; however, compromised contractile function precludes the diaphragm from meeting the increased metabolic demand of chronic hyperventilation in PH. Given that muscle contractile function is in part, dependent upon adequate blood flow (Q ˙ ), diaphragmatic dysfunction may be predicated by an inability to match oxygen delivery with oxygen demand. We hypothesized that PH rats would demonstrate a decreased hyperemic response to contractions compared to healthy controls. Methods: Sprague-Dawley rats were randomized into healthy (HC, n = 7) or PH (n = 7) groups. PH rats were administered monocrotaline (MCT) while HC rats received vehicle. Disease progression was monitored via echocardiography. Regional and total diaphragm blood flow and vascular conductance at baseline and during 3 min of electrically-stimulated contractions were determined using fluorescent microspheres. Results: PH rats displayed morphometric and echocardiographic criteria for disease (i.e., acceleration time/ejection time, right ventricular hypertrophy). In all rats, total costal diaphragm Q ˙ increased during contractions and did not differ between groups. In HC rats, there was a greater increase in medial costal Q ˙ compared to PH rats (55% ± 3% vs. 44% ± 4%, p < 0.05), who demonstrated a redistribution of Q ˙ to the ventral costal region. Conclusion: These findings support a redistribution of regional diaphragm perfusion and an impaired medial costal hyperemic response in PH, suggesting that PH alters diaphragm vascular function and oxygen delivery, providing a potential mechanism for PH-induced diaphragm contractile dysfunction.

Anthracycline chemotherapy, vascular dysfunction and cognitive impairment: burgeoning topics and future directions

Anthracyclines, chemotherapeutic agents used to treat common forms of cancer, increase cardiovascular (CV) complications, thereby necessitating research regarding interventions to improve the health of cancer survivors. Vascular dysfunction, which is induced by anthracycline chemotherapy, is an established antecedent to overt CV diseases. Potential treatment options for ameliorating vascular dysfunction have largely been understudied. Furthermore, patients treated with anthracyclines have impaired cognitive function and vascular dysfunction is an independent risk factor for the development of mild cognitive impairment. Here, we will focus on: anthracycline chemotherapy associated CV diseases risk; how targeting mechanisms underlying vascular dysfunction may be a means to improve both CV and cognitive health; and research gaps and potential future directions for the field of cardio-oncology.

Aerobic capacity and [Formula: see text] kinetics adaptive responses to short-term high-intensity interval training and detraining in untrained females

PURPOSE

This study investigated the physical fitness and oxygen uptake kinetics (τ[Formula: see text]) along with the O2 delivery and utilization (heart rate kinetics, τHR; deoxyhemoglobin/[Formula: see text] ratio, ∆[HHb]/[Formula: see text]) adaptations of untrained female participants responding to 4 weeks of high-intensity interval training (HIIT) and 2 weeks of detraining.

METHODS

Participants were randomly assigned to HIIT (n = 11, 4 × 4 protocol) or nonexercising control (n = 9) groups. Exercising group engaged 4 weeks of treadmill HIIT followed by 2 weeks of detraining while maintaining daily activity level. Ramp-incremental (RI) tests and step-transitions to moderate-intensity exercise were performed. Aerobic capacity and performance (maximal oxygen uptake, [Formula: see text]; gas-exchange threshold, GET; power output, PO), body composition (skeletal muscle mass, SMM; body fat percentage, BF%), muscle oxygenation status (∆[HHb]), [Formula: see text], and HR kinetics were assessed.

RESULTS

HIIT elicited improvements in aerobic capacity ([Formula: see text], + 0.17 ± 0.04 L/min; GET, + 0.18 ± 0.05 L/min, P < 0.01; PO-[Formula: see text], ± 23.36 ± 8.37 W; PO-GET, + 17.18 ± 3.07 W, P < 0.05), body composition (SMM, + 0.92 ± 0.17 kg; BF%, - 3.08% ± 0.58%, P < 0.001), and speed up the τ[Formula: see text] (- 8.04 ± 1.57 s, P < 0.001) significantly, extending to better ∆[HHb]/[Formula: see text] ratio (1.18 ± 0.08 to 1.05 ± 0.14). After a period of detraining, the adaptation in body composition and aerobic capacity, as well as the accelerated τ[Formula: see text] were maintained in the HIIT group, but the PO-[Formula: see text] and PO-GET declined below the post-training level (P < 0.05), whereas no changes were reported in controls (P > 0.05). Four weeks of HIIT induced widespread physiological adaptations in females, and the majority of improvements were preserved after 2 weeks of detraining except for power output corresponding to [Formula: see text] and GET.

Effects of cooling glove on the human body's recovery after exercise and improvement of exercise ability

BACKGROUND

After high-intensity exercises, the body's core temperature increases, affecting the body's metabolism, increasing thermal stress and muscle fatigue. The most popular technique to maximize post-workout recovery is cryotherapy. However, the cooling effect may vary depending on the body part being cooled since body tissues do not process the same perfusion.

OBJECTIVE

This study investigates the effects of hand cooling on human body functional recovery and exercise ability improvement by comparing normal rest and rest with hand cooling gloves after high-intensity exercise.

METHODS

Thirty healthy subjects participated in this study wherein they exercised and used normal rest for one session and hand cooling rest for the next. Blood lactate concentration, heart rate recovery rate, VO2 max measurement, and the degree of recovery of muscle strength, muscular endurance, and muscle fatigue were investigated in both groups to determine the efficacy of hand cooling gloves for postexercise recovery.

RESULTS

When hands were cooled after exercise, blood lactate concentration and body temperature significantly decreased, and cardiopulmonary function, muscle strength, and muscular endurance significantly recovered.

CONCLUSION

Using hand cooling gloves after exercise could attenuate core temperature elevation and improve postexercise recovery. It could also effectively improve athletic performance without using large-scale facilities.

Walking Endurance and Oxygen Uptake On-Kinetics in Individuals With Parkinson Disease Following Overground Locomotor Training

BACKGROUND AND PURPOSE

Poor walking endurance in Parkinson disease (PD) may be attributable to both bioenergetic and biomechanical factors, but locomotor training methods addressing both these factors simultaneously are understudied. Our objective was to examine the effects of overground locomotor training (OLT) on walking endurance in individuals with mild-to-moderate PD, and to further explore potential cardiorespiratory contributions.

METHODS

A single-arm, longitudinal design was used to examine the effects of 24 biweekly sessions of OLT in people with mild-to-moderate PD (n = 12). Walking endurance was measured as total distance walked during a 10-minute walk test (10minWT). Oxygen uptake (V˙ o2 ) on-kinetic profiles were determined using a monoexponential function. Perceived fatigability was assessed following the 10minWT using a self-report scale. Magnitude of change in primary outcomes was assessed using Cohen's d and adjusted for sample size (Cohen's d(unbiased) ).

RESULTS

Participants executed 3036 (297) steps and maintained 65.5% (8%) age-predicted heart rate maximum in a typical session lasting 56.9 (2.5) minutes. Medium effects in total distance walked-885.9 (157.2) versus 969.5 (140.9); Cohen's d(unbiased) = 0.54-and phase II time constant of the V˙ o2 on-kinetic profile-33.7 (12.3) versus 25.9 (15.3); Cohen's d(unbiased) = 0.54-were observed alongside trivial effects for perceived fatigability-4.7 (1.4) versus 4.8 (1.5); Cohen's d(unbiased) = 0.11-following OLT.

DISCUSSION AND CONCLUSIONS

These preliminary findings may demonstrate the potential for moderate-intensity OLT to improve walking endurance and enhance cardiorespiratory adjustments to walking activity in adults with mild-to-moderate PD.Video Abstract available for more insights from the authors (see the Video, Supplemental Digital Content 1, http://links.lww.com/JNPT/A407 ).

Lactate as a myokine and exerkine: drivers and signals of physiology and metabolism

No longer viewed as a metabolic waste product and cause of muscle fatigue, a contemporary view incorporates the roles of lactate in metabolism, sensing and signaling in normal as well as pathophysiological conditions. Lactate exists in millimolar concentrations in muscle, blood, and other tissues and can rise more than an order of magnitude as the result of increased production and clearance limitations. Lactate exerts its powerful driver-like influence by mass action, redox change, allosteric binding, and other mechanisms described in this article. Depending on the condition, such as during rest and exercise, following carbohydrate nutrition, injury, or pathology, lactate can serve as a myokine or exerkine with autocrine-, paracrine-, and endocrine-like functions that have important basic and translational implications. For instance, lactate signaling is: involved in reproductive biology, fueling the heart, muscle adaptation, and brain executive function, growth and development, and a treatment for inflammatory conditions. Lactate also works with many other mechanisms and factors in controlling cardiac output and pulmonary ventilation during exercise. Ironically, lactate can be disruptive of normal processes such as insulin secretion when insertion of lactate transporters into pancreatic β-cell membranes is not suppressed, and in carcinogenesis when factors that suppress carcinogenesis are inhibited, whereas factors that promote carcinogenesis are upregulated. Lactate signaling is important in areas of intermediary metabolism, redox biology, mitochondrial biogenesis, neurobiology, gut physiology, appetite regulation, nutrition, and overall health and vigor. The various roles of lactate as a myokine and exerkine are reviewed.NEW & NOTEWORTHY Lactate sensing and signaling is a relatively new and rapidly changing field. As a physiological signal lactate works both independently and in concert with other signals. Lactate operates via covalent binding and canonical signaling, redox change, and lactylation of DNA. Lactate can also serve as an element of feedback loops in cardiopulmonary regulation. From conception through aging lactate is not the only a myokine or exerkine, but it certainly deserves consideration as a physiological signal.

Dynamics of capillary blood flow responses to acute local changes in oxygen and carbon dioxide concentrations

Objectives: We aimed to quantify the magnitude and time transients of capillary blood flow responses to acute changes in local oxygen concentration ([O₂]), and carbon dioxide concentration ([CO₂]) in skeletal muscle. Additionally, we sought to quantify the combined response to both low [O₂] and high [CO₂] to mimic muscle microenvironment changes at the onset of exercise. Methods: 13 Sprague Dawley rats were anaesthetized, mechanically ventilated, and instrumented with indwelling catheters for systemic monitoring. The extensor digitorum longus muscle was blunt dissected, and reflected over a microfluidic gas exchange chamber in the stage of an inverted microscope. Four O₂ challenges, four CO₂ challenges, and a combined low O₂ (7-2%) and high CO₂ (5-10%) challenges were delivered to the surface with simultaneous visualization of capillary blood flow responses. Recordings were made for each challenge over a 1-min baseline period followed by a 2-min step change. The combined challenge employed a 1-min [O₂] challenge followed by a 2-min change in [CO₂]. Mean data for each sequence were fit using least-squared non-linear exponential models to determine the dynamics of each response. Results: 7-2% [O₂] challenges decreased capillary RBC saturation within 2 s following the step change (46.53 ± 19.56% vs. 48.51 ± 19.02%, p < 0.0001, τ = 1.44 s), increased RBC velocity within 3 s (228.53 ± 190.39 μm/s vs. 235.74 ± 193.52 μm/s, p < 0.0003, τ = 35.54 s) with a 52% peak increase by the end of the challenge, hematocrit and supply rate show similar dynamics. 5-10% [CO₂] challenges increased RBC velocity within 2 s following the step change (273.40 ± 218.06 μm/s vs. 276.75 ± 215.94 μm/s, p = 0.007, τ = 79.34s), with a 58% peak increase by the end of the challenge, supply rate and hematocrit show similar dynamics. Combined [O₂] and [CO₂] challenges resulted in additive responses to all microvascular hemodynamic measures with a 103% peak velocity increase by the end of the collection period. Data for mean responses and exponential fitting parameters are reported for all challenges. Conclusion: Microvascular level changes in muscle [O₂] and [CO₂] provoked capillary hemodynamic responses with differing time transients. Simulating exercise via combined [O₂] and [CO₂] challenges demonstrated the independent and additive nature of local blood flow responses to these agents.

ATPase Inhibitory Factor 1-A Novel Marker of Cellular Fitness and Exercise Capacity?

ATPase inhibitory factor 1 is a myokine inhibiting the hydrolytic activity of mitochondrial adenosine triphosphate synthase and ecto-F1-ATPase on the surface of many cells. IF1 affects ATP metabolism in mitochondria and the extracellular space and upregulates glucose uptake in myocytes; these processes are essential in physical activity. It is unknown whether the IF1 serum concentration is associated with exercise capacity. This study explored the association between resting IF1 serum concentration and exercise capacity indices in healthy people. IF1 serum concentration was measured in samples collected at rest in 97 healthy amateur cyclists. Exercise capacity was assessed on a bike ergometer at the successive stages of the progressive cardiopulmonary exercise test (CPET). IF1 serum concentration was negatively and significantly correlated with oxygen consumption, oxygen pulse, and load at various CPET stages. A better exercise capacity was associated with lower circulating IF1. IF1 may reflect better cellular/mitochondrial energetic fitness, but there is uncertainty regarding how IF1 is released into the intravascular space. We speculate that lower IF1 concentration may reflect a better cellular/mitochondrial integrity, as this protein is bound more strongly with ATPases in mitochondria and cellular surfaces in people with higher exercise capacity.

Optical method to determine in vivo capillary hematocrit, hemoglobin concentration, and 3-D network geometry in skeletal muscle

OBJECTIVE

The aim of this study was to develop a tool to visualize and quantify hemodynamic information, such as hemoglobin concentration and hematocrit, within microvascular networks recorded in vivo using intravital video microscopy. Additionally, we aimed to facilitate the 3-D reconstruction of the microvascular networks.

METHODS

Digital images taken from an intravital video microscopy preparation of the extensor digitorum longus muscle in rats for 25 capillary segments were used. The developed algorithm was used to delineate capillaries of interest, calculate the optical density for each pixel in the image, and reconstruct the 3-D capillary geometry using the calculated light path-lengths. Subsequently, the mean corpuscular hemoglobin concentration (MCHC), hemoglobin concentration, and hematocrit for these capillaries were calculated. We evaluated the hematocrit values determined by our methodology by comparing them to those obtained using a previously published method.

RESULTS

The hematocrit values from the proposed optical method were strongly correlated with those calculated using published methods r² (25) = .92, p < .001, and demonstrated excellent agreement with a mean difference of 1.3% and a coefficient of variation (CV) of 11%. The average MCHC, hemoglobin concentration, and light path-lengths were 23.83 g/dl, 8.06 g/dl, and 3.92 µm, respectively.

CONCLUSION

The proposed methodology can quantify hemodynamic measurements and produce functional images for visualization of the microcirculation in vivo.

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.

The Impact of Obesity on the Regulation of Muscle Blood Flow during Exercise in Patients with Heart Failure with a Preserved Ejection Fraction

Obesity is now considered a primary comorbidity in heart failure with preserved ejection fraction (HFpEF) pathophysiology, mediated largely by systemic inflammation. While there is accumulating evidence for a disease-related dysregulation of blood flow during exercise in this patient group, the role of obesity in the hemodynamic response to exercise remain largely unknown. Small muscle mass handgrip (HG) exercise was utilized to evaluate exercising muscle blood flow in non-obese (BMI < 30 kg/m²,n=14) and obese (BMI > 30 kg/m²,n=40) patients with HFpEF. Heart rate (HR), stroke index (SI), cardiac index (CI), mean arterial pressure (MAP), forearm blood flow (FBF) and vascular conductance (FVC) were assessed during progressive intermittent HG exercise (15-30-45% maximal voluntary contraction, MVC). Blood biomarkers of inflammation (C-reactive protein (CRP) and Interleukin-6 (IL-6)) were also determined. Exercising FBF was reduced in obese patients with HFpEF at all work rates (15%: 304±42 vs. 229±15ml/min; 30%: 402±46 vs. 300±18ml/min; 45%: 484±55 vs. 380±23ml/min, non-obese vs. obese, p=0.025), and was negatively correlated with BMI (R=-.47, p<0.01). In contrast, no differences in central hemodynamics (HR, SI, CI, MAP) were found between groups. Proinflammatory biomarkers were markedly elevated in obese patients (CRP: 2133±418 vs. 4630±590ng/ml, p=0.02; IL-6: 2.9±0.3 vs. 5.2±0.7pg/ml, p = 0.04, non-obese vs. obese), and both biomarkers were positively correlated with BMI (CRP: R=0.40, p=0.03; IL-6: R=0.57, p<0.01). Together, these findings demonstrate the presence of obesity and an accompanying milieu of systemic inflammation as important factors in the dysregulation of exercising muscle blood flow in patients with HFpEF.

From heart to muscle: pathophysiological mechanisms underlying long-term physical sequelae from SARS-CoV-2 infection

The long-term sequelae of the coronavirus disease 2019 (COVID-19) are multifaceted and, besides the lungs, impact other organs and tissues, even in cases of mild infection. Along with commonly reported symptoms such as fatigue and dyspnea, a significant proportion of those with prior COVID-19 infection also exhibit signs of cardiac damage, muscle weakness, and ultimately, poor exercise tolerance. This review provides an overview of evidence indicating cardiac impairments and persistent endothelial dysfunction in the peripheral vasculature of those previously infected with COVID-19, irrespective of the severity of the acute phase of illness. In addition, V̇o2peak appears to be lower in convalescent patients, which may stem, in part, from alterations in O2 transport such as impaired diffusional O2 conductance. Together, the persistent multi-organ dysfunction induced by COVID-19 may set previously healthy individuals on a trajectory towards frailty and disease. Given the large proportion of individuals recovering from COVID-19, it is critically important to better understand the physical sequelae of COVID-19, the underlying biological mechanisms contributing to these outcomes, and the long-term effects on future disease risk. This review highlights relevant literature on the pathophysiology post-COVID-19 infection, gaps in the literature, and emphasizes the need for the development of evidence-based rehabilitation guidelines.

Cross-sectional associations between cardiorespiratory fitness and NMR-derived metabolic biomarkers in children - the PANIC study

OBJECTIVE

Cardiorespiratory fitness has been inversely associated with cardiovascular risk across the lifespan. Some studies in adults suggest that higher cardiorespiratory fitness is associated with cardioprotective metabolite profile, but the evidence in children is lacking. Therefore, we investigated the cross-sectional association of cardiorespiratory fitness with serum nuclear magnetic resonance derived metabolic biomarkers in children.

METHODS

A population sample of 450 children aged 6-8 years was examined. Cardiorespiratory fitness was assessed by a maximal exercise test on a cycle ergometer and quantified as maximal power output normalised for lean body mass assessed by dual-energy X-ray absorbtiometry. Serum metabolites were assessed using a high throughput nuclear magnetic resonance platform. The data were analysed using linear regression analyses adjusted for age and sex and subsequently for body fat percentage (BF%) assessed by DXA.

RESULTS

Cardiorespiratory fitness was directly associated with high density lipoprotein (HDL) cholesterol (β=0.138, 95% CI=0.042 to 0.135, p=0.005), average HDL particle diameter (β=0.102, 95% CI=0.004 to 0.199, p=0.041), and the concentrations of extra-large HDL particles (β=0.103, 95% CI=0.006 to 0.201, p=0.038), large HDL particles (β=0.122, 95% CI=0.025 to 0.220, p=0.014), and medium HDL particles (β=0.143, 95% CI=0.047 to 0.239, p=0.004) after adjustment for age and sex. Higher cardiorespiratory fitness was also associated with higher concentrations of ApoA1 (β=0.145, 95% CI=0.047 to 0.242, p=0.003), glutamine (β=0.161, 95% CI=0.064 to 0.257, p=0.001), and phenylalanine (β=0.187, 95% CI=0.091 to 0.283, p<0.001). However, only the direct associations of cardiorespiratory fitness with the concentrations of HDL cholesterol (β=0.114, 95% CI=0.018 to 0.210, p=0.021), medium HDL particles (β=0.126, 95% CI=0.030 to 0.223, p=0.010), ApoA1 (β=0.126, 95% CI=0.030 to 0.223, p=0.011), glutamine (β=0.147, 95% CI=0.050 to 0.224, p=0.003), and phenylalanine (β=0.217, 95% CI=0.122 to 0.311, p<0.001) remained statistically significant after further adjustment for BF%.

CONCLUSIONS

Higher cardiorespiratory fitness was associated with a cardioprotective biomarker profile in children. Most associations were independent of BF% suggesting that the differences in serum metabolites between children are driven by cardiorespiratory fitness and not adiposity.

Contrast Ultrasound Assessment of Skeletal Muscle Recruitable Perfusion after Permanent Left Ventricular Assist Device Implantation: Implications for Functional Recovery

BACKGROUND

In heart failure with reduced ejection fraction (HFrEF), abnormal regulation of skeletal muscle perfusion contributes to reduced exercise tolerance. The aim of this study was to test the hypothesis that improvement in functional status after permanent left ventricular assist device (LVAD) implantation in patients with HFrEF is related to improvement in muscle perfusion during work, which was measured using contrast-enhanced ultrasound (CEUS).

METHODS

CEUS perfusion imaging of calf muscle at rest and during low-intensity plantar flexion exercise (20 W, 0.2 Hz) was performed in patients with HFrEF (n = 22) at baseline and 3 months after placement of permanent LVADs. Parametric analysis of CEUS data was used to quantify muscle microvascular blood flow (MBF), blood volume index, and red blood cell flux rate. For subjects alive at 3 months, comparisons were made between those with New York Heart Association functional class I or II (n = 13) versus III or IV (n = 7) status after LVAD. Subjects were followed for a median of 5.7 years for mortality.

RESULTS

Echocardiographic data before and after LVAD placement and LVAD parameters were similar in subjects classified with New York Heart Association functional class I-II versus functional class III-IV after LVAD. Skeletal muscle MBF at rest and during exercise before LVAD implantation was also similar between groups. After LVAD placement, resting MBF remained similar between groups, but during exercise those with New York Heart Association functional class I or II had greater exercise MBF (111 ± 60 vs 52 ± 38 intensity units/sec, P = .03), MBF reserve (median, 4.45 [3.95 to 6.80] vs 2.22 [0.98 to 3.80]; P = .02), and percentage change in exercise MBF (median, 73% [-28% to 83%] vs -45% [-80% to 26%]; P = .03). During exercise, increases in MBF were attributable to faster microvascular flux rate, with little change in blood volume index, indicating impaired exercise-mediated microvascular recruitment. The only clinical or echocardiographic feature that correlated with post-LVAD exercise MBF was a history of diabetes mellitus. There was a trend toward better survival in patients who demonstrated improvement in muscle exercise MBF after LVAD placement (P = .05).

CONCLUSIONS

CEUS perfusion imaging can quantify peripheral vascular responses to advanced therapies for HFrEF. After LVAD implantation, improvement in functional class is seen in patients with improvements in skeletal muscle exercise perfusion and flux rate, implicating a change in vasoactive substances that control resistance arteriolar tone.

Heterogeneity of human adaptations to bed rest and hypoxia: a retrospective analysis within the skeletal muscle oxidative function

This retrospective study was designed to analyze the interindividual variability in the responses of different variables characterizing the skeletal muscle oxidative function to normoxic (N-BR) and hypoxic (H-BR) bed rests and to a hypoxic ambulatory confinement (H-AMB) of 10 and 21 days. We also assessed whether and how the addition of hypoxia to bed rest might influence the heterogeneity of the responses. In vivo measurements of O₂ uptake and muscle fractional O₂ extraction were carried out during an incremental one-leg knee-extension exercise. Mitochondrial respiration was assessed in permeabilized muscle fibers. A total of 17 subjects were included in this analysis. This analysis revealed a similar variability among subjects in the alterations induced by N-BR and H-BR both in peak O₂ uptake (SD: 4.1% and 3.3% after 10 days; 4.5% and 8.1% after 21 days, respectively) and peak muscle fractional O₂ extraction (SD: 5.9% and 7.3% after 10 days; 6.5% and 7.3% after 21 days), independently from the duration of the exposure. The individual changes measured in these variables were significantly related (r = 0.66, P = 0.004 after N-BR; r = 0.61, P = 0.009 after H-BR). Mitochondrial respiration showed a large variability of response after both N-BR (SD: 25.0% and 15.7% after 10 and 21 days) and H-BR (SD: 13.0% and 19.8% after 10 and 21 days); no correlation was found between N-BR and H-BR changes. When added to bed rest, hypoxia altered the individual adaptations within the mitochondria but not those intrinsic to the muscle oxidative function in vivo, both after the short- and medium-term exposures.

Locomotor Muscle Microvascular Dysfunction in Heart Failure With Preserved Ejection Fraction

[Figure: see text].

Altered central and peripheral haemodynamics during rhythmic handgrip exercise in young adults with SARS-CoV-2

New Findings

What is the central question of this study?

Are central and peripheral haemodynamics during handgrip exercise different in young adults 3–4 weeks following infection with of SARS‐CoV‐2 compared with young healthy adults.

What is the main finding and its importance?

Exercising heart rate was higher while brachial artery blood flow and vascular conductance were lower in the SARS‐CoV‐2 compared with the control group. These findings provide evidence for peripheral impairments to exercise among adults with SARS‐CoV‐2, which may contribute to exercise limitations.

Abstract

The novel severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) can have a profound impact on vascular function. While exercise intolerance may accompany a variety of symptoms associated with SARS‐CoV‐2 infection, the impact of SARS‐CoV‐2 on exercising blood flow (BF) remains unclear. Central (photoplethysmography) and peripheral (Doppler ultrasound) haemodynamics were determined at rest and during rhythmic handgrip (HG) exercise at 30% and 45% of maximal voluntary contraction (MVC) in young adults with mild symptoms 25 days after testing positive for SARS‐CoV‐2 (SARS‐CoV‐2: n = 8M/5F; age: 21 ± 2 years; height: 176 ± 11 cm; mass: 71 ± 11 kg) and were cross‐sectionally compared with control subjects (Control: n = 8M/5F; age: 27 ± 6 years; height: 178 ± 8 cm; mass: 80 ± 25 kg). Systolic blood pressure, end systolic arterial pressure and rate pressure product were higher in the SARS‐CoV‐2 group during exercise at 45% MVC compared with controls. Brachial artery BF was lower in the SARS‐CoV‐2 group at both 30% MVC (Control: 384.8 ± 93.3 ml min^–1; SARS‐CoV‐2: 307.8 ± 105.0 ml min^–1; P = 0.041) and 45% MVC (Control: 507.4 ± 109.9 ml min^–1; SARS‐CoV‐2: 386.3 ± 132.5 ml min^–1; P = 0.002). Brachial artery vascular conductance was lower at both 30% MVC (Control: 3.93 ± 1.07 ml min^–1 mmHg^–1; SARS‐CoV‐2: 3.11 ± 0.98 ml min^–1 mmHg^–1; P = 0.022) and 45% MVC (Control: 4.74 ± 1.02 ml min^–1 mmHg^–1; SARS‐CoV‐2: 3.46 ± 1.10 ml min^–1 mmHg^–1; P < 0.001) in the SARS‐CoV‐2 group compared to control group. The shear‐induced dilatation of the brachial artery increased similarly across exercise intensities in the two groups, suggesting the decrease in exercising BF may be due to microvascular impairments. Brachial artery BF is attenuated during HG exercise in young adults recently diagnosed with mild SARS‐CoV‐2, which may contribute to diminished exercise capacity among those recovering from SARS‐CoV‐2 like that seen in severe cases.

Sexual dimorphism in vascular ATP-sensitive K+ channel function supporting interstitial PO2 via convective and/or diffusive O2 transport

KEY POINTS

Inhibition of pancreatic ATP-sensitive K+ (KATP ) channels is the intended effect of oral sulphonylureas to increase insulin release in diabetes. However, pertinent to off-target effects of sulphonylurea medication, sex differences in cardiac KATP channel function exist, whereas potential sex differences in vascular KATP channel function remain unknown. In the present study, we assessed vascular KATP channel function (topical glibenclamide superfused onto fast-twitch oxidative skeletal muscle) supporting blood flow and interstitial O2 delivery-utilization matching ( P O 2 is) during twitch contractions in male, female during pro-oestrus and ovariectomized female (F+OVX) rats. Glibenclamide decreased blood flow (convective O2 transport) and interstitial P O 2 in male and female, but not F+OVX, rats. Compared to males, females also demonstrated impaired diffusive O2 transport and a faster fall in interstitial P O 2 . Our demonstration, in rats, that sex differences in vascular KATP channel function exist support the tentative hypothesis that oral sulphonylureas may exacerbate exercise intolerance and morbidity, especially in premenopausal females.

ABSTRACT

Vascular ATP-sensitive K+ (KATP ) channels support skeletal muscle blood flow ( Q ̇ m ), interstitial O2 delivery ( Q ̇ O 2 )-utilization ( V ̇ O 2 ) matching (i.e. interstitial-myocyte O2 flux driving pressure; P O 2 is) and exercise tolerance. Potential sex differences in skeletal muscle vascular KATP channel function remain largely unexplored. We hypothesized that local skeletal muscle KATP channel inhibition via glibenclamide superfusion (5 mg kg-1 GLI; sulphonylurea diabetes medication) in anaesthetized female Sprague-Dawley rats, compared to males, would demonstrate greater reductions in contracting (1 Hz, 7 V, 180 s) fast-twitch oxidative mixed gastrocnemius (97% type IIA+IID/X+IIB) Q ̇ m (15 μm microspheres) and P O 2 is (phosphorescence quenching), resulting from more compromised convective ( Q ̇ O 2 ) and diffusive ( D O 2   ) O2 conductances. Furthermore, these GLI-induced reductions in ovary-intact females measured during pro-oestrus would be diminished following ovariectomy (F+OVX). GLI similarly impaired mixed gastrocnemius V ̇ O 2 in both males (↓28%) and females (↓33%, both P < 0.032) via reduced Q ̇ m (male: ↓31%, female: ↓35%, both P < 0.020), Q ̇ O 2 (male: 5.6 ± 0.5 vs. 4.0 ± 0.5, female: 6.4 ± 1.1 vs. 4.2 ± 0.6 mL O2  min-1 100 g tissue-1 , P < 0.022) and the resulting P O 2 is, with females also demonstrating a reduced D O 2   (0.40 ± 0.07 vs. 0.30 ± 0.04 mL O2  min-1 100 g tissue-1 , P < 0.042) and a greater GLI-induced speeding of P O 2 is fall (mean response time: Sex × Drug interaction, P = 0.026). Conversely, GLI did not impair the mixed gastrocnemius of F+OVX rats. Therefore, in patients taking sulphonylureas, these results support the potential for impaired vascular KATP channel function to compromise muscle Q ̇ m and therefore exercise tolerance. Such an effect, if present, would likely contribute to adverse cardiovascular events in premenopausal females more than males.

Exercise intolerance in pulmonary arterial hypertension: insight into central and peripheral pathophysiological mechanisms

Exercise intolerance is a cardinal symptom of pulmonary arterial hypertension (PAH) and strongly impacts patients' quality of life (QoL). Although central cardiopulmonary impairments limit peak oxygen consumption (V' O2peak ) in patients with PAH, several peripheral abnormalities have been described over the recent decade as key determinants in exercise intolerance, including impaired skeletal muscle (SKM) morphology, convective O2 transport, capillarity and metabolism indicating that peripheral abnormalities play a greater role in limiting exercise capacity than previously thought. More recently, cerebrovascular alterations potentially contributing to exercise intolerance in patients with PAH were also documented. Currently, only cardiopulmonary rehabilitation has been shown to efficiently improve the peripheral components of exercise intolerance in patients with PAH. However, more extensive studies are needed to identify targeted interventions that would ultimately improve patients' exercise tolerance and QoL. The present review offers a broad and comprehensive analysis of the present literature about the complex mechanisms and their interactions limiting exercise in patients and suggests several gaps in knowledge that need to be addressed in the future for a better understanding of exercise intolerance in patients with PAH.

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.

The capillary fascicle in skeletal muscle: Structural and functional physiology of RBC distribution in capillary networks

KEY POINTS

The capillary module, consisting of parallel capillaries from arteriole to venule, is classically considered as the building block of complex capillary networks. In skeletal muscle, this structure fails to address how blood flow is regulated along the entire length of the synchronously contracting muscle fibres. Using intravital video microscopy of resting extensor digitorum longus muscle in rats, we demonstrated the capillary fascicle as a series of interconnected modules forming continuous columns that align naturally with the dimensions of the muscle fascicle. We observed structural heterogeneity for module topology, and functional heterogeneity in space and time for capillary-red blood cell (RBC) haemodynamics within a module and between modules. We found that module RBC haemodynamics were independent of module resistance, providing direct evidence for microvascular flow regulation at the level of the capillary module. The capillary fascicle is an updated paradigm for characterizing blood flow and RBC distribution in skeletal muscle capillary networks.

ABSTRACT

Capillary networks are the fundamental site of oxygen exchange in the microcirculation. The capillary module (CM), consisting of parallel capillaries from terminal arteriole (TA) to post-capillary venule (PCV), is classically considered as the building block of complex capillary networks. In skeletal muscle, this structure fails to address how blood flow is regulated along the entire length of the synchronously contracting muscle fibres, requiring co-ordination from numerous modules. It has previously been recognized that TAs and PCVs interact with multiple CMs, creating interconnected networks. Using label-free intravital video microscopy of resting extensor digitorum longus muscle in rats, we found that these networks form continuous columns of linked CMs spanning thousands of microns, herein denoted as the capillary fascicle (CF); this structure aligns naturally with the dimensions of the muscle fascicle. We measured capillary-red blood cell (RBC) haemodynamics and module topology (n = 9 networks, 327 modules, 1491 capillary segments). The average module had length 481 μm, width 157 μm and 9.51 parallel capillaries. We observed structural heterogeneity for CM topology, and functional heterogeneity in space and time for capillary-RBC haemodynamics within a module and between modules. There was no correlation between capillary RBC velocity and lineal density. A passive inverse relationship between module length and haemodynamics was remarkably absent, providing direct evidence for microvascular flow regulation at the level of the CM. In summary, the CF is an updated paradigm for characterizing RBC distribution in skeletal muscle, and strengthens the theory of capillary networks as major contributors to the signal that regulates capillary perfusion.

Association between [Formula: see text]O2 kinetics and [Formula: see text]O2max in groups differing in fitness status

PURPOSE

This study evaluated (i) the relationship between oxygen uptake ([Formula: see text]O₂) kinetics and maximal [Formula: see text]O₂ ([Formula: see text]O_2max) within groups differing in fitness status, and (ii) the adjustment of [Formula: see text]O₂ kinetics compared to that of central [cardiac output (Q̇), heart rate (HR)] and peripheral (deoxyhemoglobin over [Formula: see text]O₂ ratio ([HHb]/[Formula: see text]O₂)] O₂ delivery, during step-transitions to moderate-intensity exercise.

METHODS

Thirty-six young healthy male participants (18 untrained; 18 trained) performed a ramp-incremental test to exhaustion and 3 step-transitions to moderate-intensity exercise. Q̇ and HR kinetics were measured in 18 participants (9 untrained; 9 trained).

RESULTS

No significant correlation between τ̇[Formula: see text]O₂ and [Formula: see text]O_2max was found in trained participants (r = 0.29; p > 0.05) whereas a significant negative correlation was found in untrained (r = - 0.58; p < 0.05) and all participants (r = - 0.82; p < 0.05). τQ̇ (18.8 ± 5.5 s) and τHR (20.1 ± 6.2 s) were significantly greater than τ[Formula: see text]O₂ (13.9 ± 2.7 s) for trained (p < 0.05). No differences were found between τQ̇ (22.8 ± 8.45 s), τHR (21.2 ± 8.3 s) and τ[Formula: see text]O₂ (28.9 ± 5.7 s) for untrained (p > 0.05). τQ̇ demonstrated a significant strong positive correlation with τHR in trained (r = 0.76; p < 0.05) but not untrained (r = 0.61; p > 0.05). A significant overshoot in the [HHb]/[Formula: see text]O₂ ratio was found in the untrained groups (p < 0.05) but not in the trained groups (p > 0.05) CONCLUSION: The results indicated that when comparing participants of different fitness status (i) there is a point at which greater V̇O_2max values are not accompanied by faster [Formula: see text]O₂ kinetics; (ii) central delivery of O₂ does not seem to limit the kinetics of [Formula: see text]O₂; and (iii) O₂ delivery within the active tissues might contribute to the slower [Formula: see text]O₂ kinetics response in untrained participants.

August Krogh: Muscle capillary function and oxygen delivery

The capillary bed constitutes the obligatory pathway for almost all oxygen (O₂) and substrate molecules as they pass from blood to individual cells. As the largest organ, by mass, skeletal muscle contains a prodigious surface area of capillaries that have a critical role in metabolic homeostasis and must support energetic requirements that increase as much as 100-fold from rest to maximal exercise. In 1919 Krogh's 3 papers, published in the Journal of Physiology, brilliantly conflated measurements of muscle capillary function at rest and during contractions with Agner K. Erlang's mathematical model of O₂ diffusion. These papers single-handedly changed the perception of capillaries from passive vessels serving at the mercy of their upstream arterioles into actively contracting vessels that were recruited during exercise to elevate blood-myocyte O₂ flux. Although seminal features of Krogh's model have not withstood the test of time and subsequent technological developments, Krogh is credited with helping found the field of muscle microcirculation and appreciating the role of the capillary bed and muscle O₂ diffusing capacity in facilitating blood-myocyte O₂ flux. Today, thanks in large part to Krogh, it is recognized that comprehending the role of the microcirculation, as it supports perfusive and diffusive O₂ conductances, is fundamental to understanding skeletal muscle plasticity with exercise training and resolving the mechanistic bases by which major pathologies including heart failure and diabetes cripple exercise tolerance and cerebrovascular dysfunction predicates impaired executive function.

Whole body passive heating versus dynamic lower body exercise: a comparison of peripheral hemodynamic profiles

Passive heating has emerged as a therapeutic intervention for the treatment and prevention of cardiovascular disease. Like exercise, heating increases peripheral artery blood flow and shear rate, which is thought to be a primary mechanism underpinning endothelium-mediated vascular adaptation. However, few studies have compared the increase in arterial blood flow and shear rate between dynamic exercise and passive heating. In a fixed crossover design study, 15 moderately trained healthy participants (25.6 ± 3.4 yr) (5 female) underwent 30 min of whole body passive heating (42°C bath), followed on a separate day by 30 min of semi-recumbent stepping exercise performed at two workloads corresponding to the increase in cardiac output (Qc) (Δ3.72 L·min^-1) and heart rate (HR) (Δ40 beats/min) recorded at the end of passive heating. At the same Qc (Δ3.72 L·min^-1 vs. 3.78 L·min^-1), femoral artery blood flow (1,599 mL/min vs. 1,947 mL/min) (P = 0.596) and shear rate (162 s^-1 vs. 192 s^-1) (P = 0.471) measured by ultrasonography were similar between passive heating and stepping exercise. However, for the same HR_MATCHED intensity, femoral blood flow (1,599 mL·min^-1 vs. 2,588 mL·min^-1) and shear rate (161 s^-1 vs. 271 s^-1) were significantly greater during exercise, compared with heating (both P = <0.001). The results indicate that, for moderately trained individuals, passive heating increases common femoral artery blood flow and shear rate similar to low-intensity continuous dynamic exercise (29% V̇o_2max); however, exercise performed at a higher intensity (53% V̇o_2max) results in significantly larger shear rates toward the active skeletal muscle.NEW & NOTEWORTHY Passive heating and exercise increase blood flow through arteries, generating a frictional force, termed shear rate, which is associated with positive vascular health. Few studies have compared the increase in arterial blood flow and shear rate elicited by passive heating with that elicited by dynamic continuous exercise. We found that 30 min of whole body passive hot-water immersion (42°C bath) increased femoral artery blood flow and shear rate equivalent to exercising at a moderate intensity (∼57% HR_max).

Cardiovascular responses to rhythmic handgrip exercise in heart failure with preserved ejection fraction

Although the contribution of noncardiac complications to the pathophysiology of heart failure with preserved ejection fraction (HFpEF) have been increasingly recognized, disease-related changes in peripheral vascular control remain poorly understood. We utilized small muscle mass handgrip exercise to concomitantly evaluate exercising muscle blood flow and conduit vessel endothelium-dependent vasodilation in individuals with HFpEF (n = 25) compared with hypertensive controls (HTN) (n = 25). Heart rate (HR), stroke volume (SV), cardiac output (CO), mean arterial pressure (MAP), brachial artery blood velocity, and brachial artery diameter were assessed during progressive intermittent handgrip (HG) exercise [15-30-45% maximal voluntary contraction (MVC)]. Forearm blood flow (FBF) and vascular conductance (FVC) were determined to quantify the peripheral hemodynamic response to HG exercise, and changes in brachial artery diameter were evaluated to assess endothelium-dependent vasodilation. HR, SV, and CO were not different between groups across exercise intensities. However, although FBF was not different between groups at the lowest exercise intensity, FBF was significantly lower (20-40%) in individuals with HFpEF at the two higher exercise intensities (30% MVC: 229 ± 8 versus 274 ± 23 ml/min; 45% MVC: 283 ± 17 versus 399 ± 34 ml/min, HFpEF versus HTN). FVC was not different between groups at 15 and 30% MVC but was ∼20% lower in HFpEF at the highest exercise intensity. Brachial artery diameter increased across exercise intensities in both HFpEF and HTN, with no difference between groups. These findings demonstrate an attenuation in muscle blood flow during exercise in HFpEF in the absence of disease-related changes in central hemodynamics or endothelial function.NEW & NOTEWORTHY The current study identified, for the first time, an attenuation in exercising muscle blood flow during handgrip exercise in individuals with heart failure with preserved ejection fraction (HFpEF) compared with overweight individuals with hypertension, two of the most common comorbidities associated with HFpEF. These decrements in exercise hyperemia cannot be attributed to disease-related changes in central hemodynamics or endothelial function, providing additional evidence for disease-related vascular dysregulation, which may be a predominant contributor to exercise intolerance in individuals with HFpEF.

August Krogh's theory of muscle microvascular control and oxygen delivery: a paradigm shift based on new data

August Krogh twice won the prestigious international Steegen Prize, for nitrogen metabolism (1906) and overturning the concept of active transport of gases across the pulmonary epithelium (1910). Despite this, at the beginning of 1920, the consummate experimentalist was relatively unknown worldwide and even among his own University of Copenhagen faculty. But, in early 1919, he had submitted three papers to Dr Langley, then editor of The Journal of Physiology in England. These papers coalesced anatomical observations of skeletal muscle capillary numbers with O₂ diffusion theory to propose a novel active role for capillaries that explained the prodigious increase in blood-muscle O₂ flux from rest to exercise. Despite his own appraisal of the first two papers as "rather dull" to his friend, the eminent Cambridge respiratory physiologist, Joseph Barcroft, Krogh believed that the third one, dealing with O₂ supply and capillary regulation, was"interesting". These papers, which won Krogh an unopposed Nobel Prize for Physiology or Medicine in 1920, form the foundation for this review. They single-handedly transformed the role of capillaries from passive conduit and exchange vessels, functioning at the mercy of their upstream arterioles, into independent contractile units that were predominantly closed at rest and opened actively during muscle contractions in a process he termed 'capillary recruitment'. Herein we examine Krogh's findings and some of the experimental difficulties he faced. In particular, the boundary conditions selected for his model (e.g. heavily anaesthetized animals, negligible intramyocyte O₂ partial pressure, binary open-closed capillary function) have not withstood the test of time. Subsequently, we update the reader with intervening discoveries that underpin our current understanding of muscle microcirculatory control and place a retrospectroscope on Krogh's discoveries. The perspective is presented that the imprimatur of the Nobel Prize, in this instance, may have led scientists to discount compelling evidence. Much as he and Marie Krogh demonstrated that active transport of gases across the blood-gas barrier was unnecessary in the lung, capillaries in skeletal muscle do not open and close spontaneously or actively, nor is this necessary to account for the increase in blood-muscle O₂ flux during exercise. Thus, a contemporary model of capillary function features most muscle capillaries supporting blood flow at rest, and, rather than capillaries actively vasodilating from rest to exercise, increased blood-myocyte O₂ flux occurs predominantly via elevating red blood cell and plasma flux in already flowing capillaries. Krogh is lauded for his brilliance as an experimentalist and for raising scientific questions that led to fertile avenues of investigation, including the study of microvascular function.

Indices of leg resistance artery function are independently related to cycling V̇O2 max

PURPOSE

While maximum blood flow influences one's maximum rate of oxygen consumption (V̇O2 max), with so many indices of vascular function, it is still unclear if vascular function is related to V̇O2 max in healthy, young adults. The purpose of this study was to determine if several common vascular tests of conduit artery and resistance artery function provide similar information about vascular function and the relationship between vascular function and V̇O2 max.

METHODS

Twenty-two healthy adults completed multiple assessments of leg vascular function, including flow-mediated dilation (FMD), reactive hyperemia (RH), passive leg movement (PLM), and rapid onset vasodilation (ROV). V̇O2 max was assessed with a graded exercise test on a cycle ergometer.

RESULTS

Indices associated with resistance artery function (e.g., peak flow during RH, PLM, and ROV) were generally related to each other (r = 0.47-77, p < .05), while indices derived from FMD were unrelated to other tests (p < .05). Absolute V̇O2 max (r = 0.57-0.73, p < .05) and mass-specific V̇O2 max (r = 0.41-0.46, p < .05) were related to indices of resistance artery function, even when controlling for factors like body mass and sex. FMD was only related to mass-specific V̇O2 max after statistically controlling for baseline artery diameter (r = 0.44, p < .05).

CONCLUSION

Indices of leg resistance artery function (e.g., peak flow during RH, PLM, and ROV) relate well to each other and account for ~30% of the variance in V̇O2 max not accounted for by other factors, like body mass and sex. Vascular interventions should focus on improving indices of resistance artery function, not conduit artery function, when seeking to improve exercise capacity.

Training with blood flow restriction increases femoral artery diameter and thigh oxygen delivery during knee-extensor exercise in recreationally trained men

KEY POINTS

Endurance-type training with blood flow restriction (BFR) increases maximum oxygen uptake ( V ̇ O 2 max ) and exercise endurance of humans. However, the physiological mechanisms behind this phenomenon remain uncertain. In the present study, we show that BFR-interval training reduces the peripheral resistance to oxygen transport during dynamic, submaximal exercise in recreationally-trained men, mainly by increasing convective oxygen delivery to contracting muscles. Accordingly, BFR-training increased oxygen uptake by, and concomitantly reduced net lactate release from, the contracting muscles during relative-intensity-matched exercise, at the same time as invoking a similar increase in diffusional oxygen conductance compared to the training control. Only BFR-training increased resting femoral artery diameter, whereas increases in oxygen transport and uptake were dissociated from changes in the skeletal muscle content of mitochondrial electron-transport proteins. Thus, physically trained men benefit from BFR-interval training by increasing leg convective oxygen transport and reducing lactate release, thereby improving the potential for increasing the percentage of V ̇ O 2 max that can be sustained throughout exercise.

ABSTRACT

In the present study, we investigated the effect of training with blood flow restriction (BFR) on thigh oxygen transport and uptake, and lactate release, during exercise. Ten recreationally-trained men (50 ± 5 mL kg^-1  min^-1 ) completed 6 weeks of interval cycling with one leg under BFR (BFR-leg; pressure: ∼180 mmHg) and the other leg without BFR (CON-leg). Before and after the training intervention (INT), thigh oxygen delivery, extraction, uptake, diffusion capacity and lactate release were determined during knee-extensor exercise at 25% incremental peak power output (iPPO) (Ex1), followed by exercise to exhaustion at 90% pre-training iPPO (Ex2), by measurement of femoral-artery blood flow and femoral-arterial and -venous blood sampling. A muscle biopsy was obtained from legs before and after INT to determine mitochondrial electron-transport protein content. Femoral-artery diameter was also measured. In the BFR-leg, after INT, oxygen delivery and uptake were higher, and net lactate release was lower, during Ex1 (vs. CON-leg; P < 0.05), with an 11% larger increase in workload (vs. CON-leg; P < 0.05). During Ex2, after INT, oxygen delivery was higher, and oxygen extraction was lower, in the BFR-leg compared to the CON-leg (P < 0.05), resulting in an unaltered oxygen uptake (vs. CON-leg; P > 0.05). In the CON-leg, at both intensities, oxygen delivery, extraction, uptake and lactate release remained unchanged (P > 0.05). Resting femoral artery diameter increased with INT only in the BFR-leg (∼4%; P < 0.05). Oxygen diffusion capacity was similarly raised in legs (P < 0.05). Mitochondrial protein content remained unchanged in legs (P > 0.05). Thus, BFR-interval training enhances oxygen utilization by, and lowers lactate release from, submaximally-exercising muscles of recreationally-trained men mainly by increasing leg convective oxygen transport.