Muscle blood flow and oxygen uptake in recovery from exercise

Part II. Common questions and misconceptions about creatine supplementation: what does the scientific evidence really show?

Creatine monohydrate supplementation (CrM) is a safe and effective intervention for improving certain aspects of sport, exercise performance, and health across the lifespan. Despite its evidence-based pedigree, several questions and misconceptions about CrM remain. To initially address some of these concerns, our group published a narrative review in 2021 discussing the scientific evidence as to whether CrM leads to water retention and fat accumulation, is a steroid, causes hair loss, dehydration or muscle cramping, adversely affects renal and liver function, and if CrM is safe and/or effective for children, adolescents, biological females, and older adults. As a follow-up, the purpose of this paper is to evaluate additional questions and misconceptions about CrM. These include but are not limited to: 1. Can CrM provide muscle benefits without exercise? 2. Does the timing of CrM really matter? 3. Does the addition of other compounds with CrM enhance its effectiveness? 4. Does CrM and caffeine oppose each other? 5. Does CrM increase the rates of muscle protein synthesis or breakdown? 6. Is CrM an anti-inflammatory intervention? 7. Can CrM increase recovery following injury, surgery, and/or immobilization? 8. Does CrM cause cancer? 9. Will CrM increase urine production? 10. Does CrM influence blood pressure? 11. Is CrM safe to consume during pregnancy? 12. Does CrM enhance performance in adolescents? 13. Does CrM adversely affect male fertility? 14. Does the brain require a higher dose of CrM than skeletal muscle? 15. Can CrM attenuate symptoms of sleep deprivation? 16. Will CrM reduce the severity of and/or improve recovery from traumatic brain injury? Similar to our 2021 paper, an international team of creatine research experts was formed to perform a narrative review of the literature regarding CrM to formulate evidence-based responses to the aforementioned misconceptions involving CrM.

Caffeine Augments Sustained Hyperemia in Previously Exercised Leg in Male Competitive Long-Distance Runners

Blood flow in exercised skeletal muscle remains elevated postexercise, potentially playing a critical role in recovery and adaptation process. However, effective and easily implementable interventions that augment this response have yet to be identified. In this study, we investigated whether caffeine intake enhances sustained postexercise hyperemia in previously active leg in competitive long-distance runners. Ten competitive male long-distance runners participated in baseline measurements, during which cardiorespiratory responses, including leg blood flow measured by ultrasound Doppler equipment, were assessed in a supine position. Subsequently, they ingested either (a) placebo capsules or (b) capsules containing 6 mg/kg caffeine in a randomized, counterbalanced, double-blind, and crossover manner. They then completed 8-min high-intensity treadmill running at 98% of the average running speed of their estimated 10,000-m best time, followed by postexercise measurements assessing the same cardiorespiratory responses. Postexercise leg blood flow was elevated from baseline, and this response was 17% higher in caffeine relative to placebo condition (between-trial difference in average with [95% CI] = 182 [2.33, 363] ml/min, p = .048, d = 1.03). Caffeine ingestion also increased minute ventilation (2.06 [0.603, 3.53] L/min, p = .011, d = 1.17), whereas it reduced end-tidal CO2 partial pressure (-2.09 [-3.68, -0.504] mmHg, p = .015, d = 1.15) postexercise relative to placebo condition. We show that preexercise 6 mg/kg caffeine intake increases postexercise leg blood flow following high-intensity running in male competitive long-distance runners. Our results stimulate future studies to assess if caffeine-induced elevations in leg blood flow accelerates recovery and/or adaptations.

Monitoring skeletal muscle oxygen saturation kinetics during graded exercise testing in NCAA division I female rowers

Purpose

The purpose of this study was to analyze changes in skeletal muscle oxygen saturation (SmO2) kinetics during exercise in female rowers both acutely and longitudinally in relation to blood lactate (BLa). We also aimed to determine the agreement and statistical equivalence between physiological thresholds derived from SmO2 and BLa kinetics.

Methods

Twenty-three female NCAA Division I rowers were tested throughout the 2023-2024 academic year. Of these, 11 athletes completed at least two near-infrared spectroscopy (NIRS)-equipped GXTs, with physiological data analyzed for longitudinal changes. A 7x4-min discontinuous GXT protocol was performed by all athletes. First and second SmO2 breakpoints (SmO2BP1 and SmO2BP2) were estimated via piecewise linear regression modeling, and BLa thresholds (LT1 and LT2) were calculated using ADAPT software. Paired-samples t-tests assessed differences, and equivalence was tested using two one-sided tests (TOST). Agreement was determined using Bland-Altman analysis yielding mean differences (MD) and 95% limits of agreement (LoA). Intraclass correlation coefficients (ICC2,1) were also calculated.

Results

No difference was found between SmO2BP2 and LT2 (MD = -5.76W [95% LoA = -38.52 to 22.25W], p = 0.134), moderate-to-good levels of agreement (ICC2,1 = 0.67 [95% CI: 0.36-0.85], p < 0.001), and no statistical equivalence (p = 0.117). This was not the case for SmO2BP1 and LT1, with NIRS significantly underestimating LT1 (MD = -8.14W [95% LoA = -38.90 to 27.37W], p = 0.026), poor-to-moderate agreement (ICC2,1 = 0.24 [95% CI: -0.13-0.58], p = 0.10), and no statistical equivalence (p = 0.487). Additionally, SmO2 recovery kinetics (SmO2resat) during 1-min rest intervals increased in response to graded increases in exercise intensity (p < 0.001, η2 p = 0.71), with higher intensities appearing to blunt this effect (step 6 - step 7: MD = -0.16%⋅s-1, p = 0.69). No statistically significant changes were observed in LT's or SmO2BP's throughout the 2023-2024 season.

Conclusion

In female collegiate rowers, NIRS may be a tool that compliments BLa testing when determining the second lactate threshold (i.e., LT2). However, significant inter-individual variablility exists between SmO2BP2 and LT2 paired with a lack of statistical equivalence suggest the two are not interchangeable. While not a standalone replacement, if used in combination with traditional BLa testing methods NIRS may be a complimentary tool that helps inform individual athlete training zone prescription.

Cardiovascular responses to leg-press exercises during head-down tilt

Introduction

Physical exercise and gravitational load affect the activity of the cardiovascular system. How these factors interact with one another is still poorly understood. Here we investigate how the cardiovascular system responds to leg-press exercise during head-down tilt, a posture that reduces orthostatic stress, limits gravitational pooling, and increases central blood volume.

Methods

Seventeen healthy participants performed leg-press exercise during head-down tilt at different combinations of resistive force, contraction frequency, and exercise duration (30 and 60 s), leading to different exercise power. Systolic (sBP), diastolic (dBP), mean arterial pressure (MAP), pulse pressure (PP) and heart rate (HR) were measured continuously. Cardiovascular responses were evaluated by comparing the values of these signals during exercise recovery to baseline. Mixed models were used to evaluate the effect of exercise power and of individual exercise parameter on the cardiovascular responses.

Results

Immediately after the exercise, we observed a clear undershoot in sBP (Δ = -7.78 ± 1.19 mmHg), dBP (Δ = -10.37 ± 0.84 mmHg), and MAP (Δ = -8.85 ± 0.85 mmHg), an overshoot in PP (Δ = 7.93 ± 1.13 mmHg), and elevated values of HR (Δ = 33.5 ± 0.94 bpm) compared to baseline (p < 0.0001). However, all parameters returned to similar baseline values 2 min following the exercise (p > 0.05). The responses of dBP, MAP and HR were significantly modulated by exercise power (correlation coefficients: rdBP = -0.34, rMAP = -0.25, rHR = 0.52, p < 0.001). All signals' responses were modulated by contraction frequency (p < 0.05), increasing the undershoot in sBP (Δ = -1.87 ± 0.98 mmHg), dBP (Δ = -4.85 ± 1.01 and Δ = -3.45 ± 0.98 mmHg for low and high resistive force respectively) and MAP (Δ = -3.31 ± 0.75 mmHg), and increasing the overshoot in PP (Δ = 2.57 ± 1.06 mmHg) as well as the value of HR (Δ = 16.8 ± 2.04 and Δ = 10.8 ± 2.01 bpm for low and high resistive force respectively). Resistive force affected only dBP (Δ = -4.96 ± 1.41 mmHg, p < 0.0001), MAP (Δ = -2.97 ± 1.07 mmHg, p < 0.05) and HR (Δ = 6.81 ± 2.81 bpm, p < 0.0001; Δ = 15.72 ± 2.86 bpm, p < 0.0001; Δ = 15.72 ± 2.86 bpm, p < 0.05, depending on the values of resistive force and contraction frequency), and exercise duration affected only HR (Δ = 9.64 ± 2.01 bpm, p < 0.0001).

Conclusion

Leg exercises caused only immediate cardiovascular responses, potentially due to facilitated venous return by the head-down tilt position. The modulation of dBP, MAP and HR responses by exercise power and that of all signals by contraction frequency may help optimizing exercise prescription in conditions of limited orthostatic stress.

Muscle reoxygenation is slower after higher cycling intensity, and is faster and more reliable in locomotor than in accessory muscle sites

Introduction

Wearable near-infrared spectroscopy (NIRS) can be used during dynamic exercise to reflect the balance of muscle oxygen delivery and uptake. This study describes the behaviour and reliability of postexercise reoxygenation with NIRS as a function of exercise intensity at four muscle sites during an incremental cycling test. We discuss physiological components of faster and slower reoxygenation kinetics in the context of sport science and clinical applications. We hypothesised that reoxygenation would be slower at higher intensity, and that locomotor muscles would be faster than accessory muscles. We quantified test-retest reliability and agreement for each site.

Methods

Twenty-one trained cyclists performed two trials of an incremental cycling protocol with 5-min work stages and 1-min rest between stages. NIRS was recorded from the locomotor vastus lateralis and rectus femoris muscles, and accessory lumbar paraspinal and lateral deltoid muscles. Reoxygenation time course was analysed as the half-recovery time (HRT) from the end of work to half of the peak reoxygenation amplitude during rest. Coefficient of variability (CV) between participants, standard error of the measurement (SEM) within participants, and intraclass correlation coefficient (ICC) for test-retest reliability were evaluated at 50%, 75%, and 100% peak workloads. A linear mixed-effects model was used to compare differences between workloads and muscle sites.

Results

HRT was slower with increasing workload in the VL, RF, and PS, but not DL. VL had the fastest reoxygenation (lowest HRT) across muscle sites at all workloads (HRT = 8, 12, 17 s at 50%, 75%, 100% workload, respectively). VL also had the greatest reliability and agreement. HRT was sequentially slower between muscle sites in the order of VL < RF < PS < DL, and reliability was lower than for the VL.

Discussion

This study highlights the potential for using wearable NIRS on multiple muscle sites during exercise. Reoxygenation kinetics differ between local muscle sites with increasing intensity. Moderate-to-good reliability in the VL support its increasing use in sport science and clinical applications. Lower reliability in other muscle sites suggest they are not appropriate to be used alone, but may add information when combined to better reflect systemic intensity and fatigue during exercise at different intensities.

Differential changes in blood flow and oxygen utilization in active muscles between voluntary exercise and electrical muscle stimulation in young adults

The physiological effects on blood flow and oxygen utilization in active muscles during and after involuntary contraction triggered by electrical muscle stimulation (EMS) remain unclear, particularly compared with those elicited by voluntary (VOL) contractions. Therefore, we used diffuse correlation and near-infrared spectroscopy (DCS-NIRS) to compare changes in local muscle blood flow and oxygen consumption during and after these two types of muscle contractions in humans. Overall, 24 healthy young adults participated in the study, and data were successfully obtained from 17 of them. Intermittent (2-s contraction, 2-s relaxation) isometric ankle dorsiflexion with a target tension of 20% of maximal VOL contraction was performed by EMS or VOL for 2 min, followed by a 6-min recovery period. DCS-NIRS probes were placed on the tibialis anterior muscle, and relative changes in local tissue blood flow index (rBFI), oxygen extraction fraction (rOEF), and metabolic rate of oxygen (rMRO2) were continuously derived. EMS induced more significant increases in rOEF and rMRO2 than VOL exercise but a comparable increase in rBFI. After EMS, rBFI and rMRO2 decreased more slowly than after VOL and remained significantly higher until the end of the recovery period. We concluded that EMS augments oxygen consumption in contracting muscles by enhancing oxygen extraction while increasing oxygen delivery at a rate similar to the VOL exercise. Under the conditions examined in this study, EMS demonstrated a more pronounced and/or prolonged enhancement in local muscle perfusion and aerobic metabolism compared with VOL exercise in healthy participants.NEW & NOTEWORTHY This is the first study to visualize continuous changes in blood flow and oxygen utilization within contracted muscles during and after electrical muscle stimulation (EMS) using combined diffuse correlation and near-infrared spectroscopy. We found that initiating EMS increases blood flow at a rate comparable to that during voluntary (VOL) exercise but enhances oxygen extraction, resulting in higher oxygen consumption. Furthermore, EMS increased postexercise muscle perfusion and oxygen consumption compared with that after VOL exercise.

Do Sports Compression Garments Alter Measures of Peripheral Blood Flow? A Systematic Review with Meta-Analysis

BACKGROUND

One of the proposed mechanisms underlying the benefits of sports compression garments may be alterations in peripheral blood flow.

OBJECTIVE

We aimed to determine if sports compression garments alter measures of peripheral blood flow at rest, as well as during, immediately after and in recovery from a physiological challenge (i.e. exercise or an orthostatic challenge).

METHODS

We conducted a systematic literature search of databases including Scopus, SPORTDiscus and PubMed/MEDLINE. The criteria for inclusion of studies were: (1) original papers in English and a peer-reviewed journal; (2) assessed effect of compression garments on a measure of peripheral blood flow at rest and/or before, during or after a physiological challenge; (3) participants were healthy and without cardiovascular or metabolic disorders; and (4) a study population including athletes and physically active or healthy participants. The PEDro scale was used to assess the methodological quality of the included studies. A random-effects meta-analysis model was used. Changes in blood flow were quantified by standardised mean difference (SMD) [± 95% confidence interval (CI)].

RESULTS

Of the 899 articles identified, 22 studies were included for the meta-analysis. The results indicated sports compression garments improve overall peripheral blood flow (SMD = 0.32, 95% CI 0.13, 0.51, p = 0.001), venous blood flow (SMD = 0.37, 95% CI 0.14, 0.60, p = 0.002) and arterial blood flow (SMD = 0.30, 95% CI 0.01, 0.59, p = 0.04). At rest, sports compression garments did not improve peripheral blood flow (SMD = 0.18, 95% CI - 0.02, 0.39, p = 0.08). However, subgroup analyses revealed sports compression garments enhance venous (SMD = 0.31 95% CI 0.02, 0.60, p = 0.03), but not arterial (SMD = 0.12, 95% CI - 0.16, 0.40, p = 0.16), blood flow. During a physiological challenge, peripheral blood flow was improved (SMD = 0.44, 95% CI 0.19, 0.69, p = 0.0007), with subgroup analyses revealing sports compression garments enhance venous (SMD = 0.48, 95% CI 0.11, 0.85, p = 0.01) and arterial blood flow (SMD = 0.44, 95% CI 0.03, 0.86, p = 0.04). At immediately after a physiological challenge, there were no changes in peripheral blood flow (SMD = - 0.04, 95% CI - 0.43, 0.34, p = 0.82) or subgroup analyses of venous (SMD = - 0.41, 95% CI - 1.32, 0.47, p = 0.35) and arterial (SMD = 0.12, 95% CI - 0.26, 0.51, p = 0.53) blood flow. In recovery, sports compression garments did not improve peripheral blood flow (SMD = 0.25, 95% CI - 0.45, 0.95, p = 0.49). The subgroup analyses showed enhanced venous (SMD = 0.67, 95% CI 0.17, 1.17, p = 0.009), but not arterial blood flow (SMD = 0.02, 95% CI - 1.06, 1.09, p = 0.98).

CONCLUSIONS

Use of sports compression garments enhances venous blood flow at rest, during and in recovery from, but not immediately after, a physiological challenge. Compression-induced changes in arterial blood flow were only evident during a physiological challenge.

The Relationship between VO2 and Muscle Deoxygenation Kinetics and Upper Body Repeated Sprint Performance in Trained Judokas and Healthy Individuals

The present study sought to investigate if faster upper body oxygen uptake (VO₂) and hemoglobin/myoglobin deoxygenation ([HHb]) kinetics during heavy intensity exercise were associated with a greater upper body repeated-sprint ability (RSA) performance in a group of judokas and in a group of individuals of heterogenous fitness level. Eight judokas (JT) and seven untrained healthy participants (UT) completed an incremental step test, two heavy intensity square-wave transitions and an upper body RSA test consisting of four 15 s sprints, with 45 s rest, from which the experimental data were obtained. In the JT group, VO₂ kinetics, [HHb] kinetics and the parameters determined in the incremental test were not associated with RSA. However, when the two groups were combined, the amplitude of the primary phase VO₂ and [HHb] were positively associated with the accumulated work in the four sprints (ΣWork). Additionally, maximal aerobic power (MAP), peak VO₂ and the first ventilatory threshold (VT₁) showed a positive correlation with ΣWork and an inverse correlation with the decrease in peak power output (Dec-PPO) between the first and fourth sprints. Faster VO₂ and [HHb] kinetics do not seem to be associated with an increased upper body RSA in JT. However, other variables of aerobic fitness seem to be associated with an increased upper body RSA performance in a group of individuals with heterogeneous fitness level.

Timing of Creatine Supplementation around Exercise: A Real Concern?

Creatine has been considered an effective ergogenic aid for several decades; it can help athletes engaged in a variety of sports and obtain performance gains. Creatine supplementation increases muscle creatine stores; several factors have been identified that may modify the intramuscular increase and subsequent performance benefits, including baseline muscle Cr content, type II muscle fibre content and size, habitual dietary intake of Cr, aging, and exercise. Timing of creatine supplementation in relation to exercise has recently been proposed as an important consideration to optimise muscle loading and performance gains, although current consensus is lacking regarding the ideal ingestion time. Research has shifted towards comparing creatine supplementation strategies pre-, during-, or post-exercise. Emerging evidence suggests greater benefits when creatine is consumed after exercise compared to pre-exercise, although methodological limitations currently preclude solid conclusions. Furthermore, physiological and mechanistic data are lacking, in regard to claims that the timing of creatine supplementation around exercise moderates gains in muscle creatine and exercise performance. This review discusses novel scientific evidence on the timing of creatine intake, the possible mechanisms that may be involved, and whether the timing of creatine supplementation around exercise is truly a real concern.

Reduced post-exercise muscle microvascular perfusion with compression is offset by increased muscle oxygen extraction: Assessment by contrast-enhanced ultrasound

The microvasculature is important for both health and exercise tolerance in a range of populations. However, methodological limitations have meant changes in microvascular blood flow are rarely assessed in humans during interventions designed to affect skeletal muscle blood flow such as the wearing of compression garments. The aim of this study is, for the first time, to use contrast-enhanced ultrasound to directly measure the effects of compression on muscle microvascular blood flow alongside measures of femoral artery blood flow and muscle oxygenation following intense exercise in healthy adults. It was hypothesized that both muscle microvascular and femoral artery blood flows would be augmented with compression garments as compared with a control condition. Ten recreationally active participants completed two repeated-sprint exercise sessions, with and without lower-limb compression tights. Muscle microvascular blood flow, femoral arterial blood flow (2D and Doppler ultrasound), muscle oxygenation (near-infrared spectroscopy), cycling performance, and venous blood samples were measured/taken throughout exercise and the 1-hour post-exercise recovery period. Compared with control, compression reduced muscle microvascular blood volume and attenuated the exercise-induced increase in microvascular velocity and flow immediately after exercise and 1 hour post-exercise. Compression increased femoral artery diameter and augmented the exercise-induced increase in femoral arterial blood flow during exercise. Markers of blood oxygen extraction in muscle were increased with compression during and after exercise. Compression had no effect on blood lactate, glucose, or exercise performance. We provide new evidence that lower-limb compression attenuates the exercise-induced increase in skeletal muscle microvascular blood flow following exercise, despite a divergent increase in femoral artery blood flow. Decreased muscle microvascular perfusion is offset by increased muscle oxygen extraction, a potential mechanism allowing for the maintenance of exercise performance.

Spectral Analysis of Muscle Hemodynamic Responses in Post-Exercise Recovery Based on Near-Infrared Spectroscopy

Spectral analysis of blood flow or blood volume oscillations can help to understand the regulatory mechanisms of microcirculation. This study aimed to explore the relationship between muscle hemodynamic response in the recovery period and exercise quantity. Fifteen healthy subjects were required to perform two sessions of submaximal plantarflexion exercise. The blood volume fluctuations in the gastrocnemius lateralis were recorded in three rest phases (before and after two exercise sessions) using near-infrared spectroscopy. Wavelet transform was used to analyze the total wavelet energy of the concerned frequency range (0.005–2 Hz), which were further divided into six frequency intervals corresponding to six vascular regulators. Wavelet amplitude and energy of each frequency interval were analyzed. Results showed that the total energy raised after each exercise session with a significant difference between rest phases 1 and 3. The wavelet amplitudes showed significant increases in frequency intervals I, III, IV, and V from phase 1 to 3 and in intervals III and IV from phase 2 to 3. The wavelet energy showed similar changes with the wavelet amplitude. The results demonstrate that local microvascular regulators contribute greatly to the blood volume oscillations, the activity levels of which are related to the exercise quantity.

Histamine, mast cell tryptase and post-exercise hypotension in healthy and collapsed marathon runners

PURPOSE

Heat stress exacerbates post-exercise hypotension (PEH) and cardiovascular disturbances from elevated body temperature may contribute to exertion-related incapacity. Mast cell degranulation and muscle mass are possible modifiers, though these hypotheses lack practical evidence. This study had three aims: (1) to characterise pre-post-responses in histamine and mast cell tryptase (MCT), (2) to investigate relationships between whole body muscle mass (WBMM) and changes in blood pressure post-marathon, (3) to identify any differences in incapacitated runners.

METHODS

24 recreational runners were recruited and successfully completed the 2019 Brighton Marathon (COMPLETION). WBMM was measured at baseline. A further eight participants were recruited from incapacitated runners (COLLAPSE). Histamine, MCT, blood pressure, heart rate, body temperature and echocardiographic measures were taken before and after exercise (COMPLETION) and upon incapacitation (COLLAPSE).

RESULTS

In completion, MCT increased by nearly 50% from baseline (p = 0.0049), whereas histamine and body temperature did not vary (p > 0.946). Systolic (SBP), diastolic (DBP) and mean (MAP) arterial blood pressures and systemic vascular resistance (SVR) declined (p < 0.019). WBMM negatively correlated with Δ SBP (r = - 0.43, p = 0.046). For collapse versus completion, there were significant elevations in MCT (1.77 ± 0.25 μg/L vs 1.18 ± 0.43 μg/L, p = 0.001) and body temperature (39.8 ± 1.3 °C vs 36.2 ± 0.8 °C, p < 0.0001) with a non-significant rise in histamine (9.6 ± 17.9 μg/L vs 13.7 ± 33.9 μg/L, p = 0.107) and significantly lower MAP, DBP and SVR (p < 0.033).

CONCLUSION

These data support the hypothesis that mast cell degranulation is a vasodilatory mechanism underlying PEH and exercise associated collapse. The magnitude of PEH is inversely proportional to the muscle mass and enhanced by concomitant body heating.

Exercise-Induced Changes in Bioactive Lipids Might Serve as Potential Predictors of Post-Exercise Hypotension. A Pilot Study in Healthy Volunteers

Post-exercise hypotension (PEH) is the phenomenon of lowered blood pressure after a single bout of exercise. Only a fraction of people develops PEH but its occurrence correlates well with long-term effects of sports on blood pressure. Therefore, PEH has been suggested as a suitable predictor for the effectivity of exercise as therapy in hypertension. Local vascular bioactive lipids might play a potential role in this context. We performed a cross-over clinical pilot study with 18 healthy volunteers to investigate the occurrence of PEH after a single short-term endurance exercise. Furthermore, we investigated the plasma lipid profile with focus on arachidonic acid (AA)-derived metabolites as potential biomarkers of PEH. A single bout of ergometer cycling induced a significant PEH in healthy volunteers with the expected high inter-individual variability. Targeted lipid spectrum analysis revealed significant upregulation of several lipids in the direct post-exercise phase. Among these changes, only 15- hydroxyeicosatetranoic acid (HETE) correlated significantly with the extent of PEH but in an AA-independent manner, suggesting that 15-HETE might act as specific PEH-marker. Our data indicate that specific lipid modulation might facilitate the identification of patients who will benefit from exercise activity in hypertension therapy. However, larger trials including hypertonic patients are necessary to verify the clinical value of this hypothesis.

Influence of sprint exercise on aortic pulse wave velocity and femoral artery shear patterns

PURPOSE

Aortic stiffness may affect shear patterns in the peripheral vasculature. This study examined if sprint exercise, which typically increases aortic stiffness is associated with increased peripheral retrograde blood flow and impaired microvascular function.

METHODS

Twenty participants (10 women; age: 27 ± 5 years) underwent arterial stiffness, shear rate, and microvascular function assessment at three time points: baseline; following time control; ~ 2 min post a 30-s cycle ergometer sprint against 7.0% body mass. Aortic stiffness was assessed using carotid-femoral pulse wave velocity (cfPWV). Superficial femoral artery (SFA) diameter and blood velocity were assessed using Doppler-ultrasound and were used to calculate shear rates and resistance index (RI). SFA wave reflections were obtained via wave intensity analysis. Vastus medialis microvascular function was measured as tissue saturation index reactivity pre-post exercise via near-infrared spectroscopy.

RESULTS

cfPWV increased by + 0.8 ± 0.7 m·s^-1 following exercise (p < 0.001). Retrograde shear was reduced following exercise compared with time control (- 4.9 ± 3.8 s^-1; p < 0.001), while tissue saturation index was increased post-exercise from baseline (+ 2.3 ± 4.6%; p = 0.04). Reductions in SFA wave reflections (- 1.70 ± 1.96 aU) and RI (- 0.17 ± 0.13 aU) were also noted following exercise (p < 0.001).

CONCLUSION

These data suggest sprint exercise-mediated changes in peripheral shear patterns and microvascular function in the exercised vasculature occur independent from increases in aortic stiffness. Exercise-induced reductions in SFA retrograde shear may be related to decreased wave reflections and peripheral vascular resistance.

Investigating the effect of fatigue on muscle microvasculature blood flow during intermittent isometric contraction

Localized muscle fatigue (LMF) decreases muscular strength, while affects the performance and potentially increases the risk of musculoskeletal disorders (MSD). An important mechanism in recovering from muscle fatigue is blood flow (BF). The BF response to muscle contraction and fatigue is highly dynamic and difficult to predict, as it depends on both metabolic demand and intramuscular pressure. The aim of this study was to measure both fatigue and BF during intermittent exertion of the first dorsal interosseous (FDI) muscle, in order to better characterize the relationship between BF and LMF during muscle contraction and rest. This study utilized Diffuse Correlation Spectroscopy (DCS) for BF measurement within the microvasculature of the FDI muscle. Exertion levels (EL) for intermittent fatiguing contraction were set to 20%, 30%, and 40% of an individual's maximum voluntary contraction (MVC). Our results showed that as an individual fatigued, relative BF rates increased, on average, by ~66% during exertion periods and ~330% during rest periods. Differences between exerting and resting BF increased over time for every EL (p<0.04), increasing by up to 11 times the baseline BF. At the same levels of muscle capacity (%MVC), resting BF was also found to increase with EL consistently. Our findings highlight BF dependence on both EL and history of muscle contraction. These results imply a variable recovery rate based on both the current state of contraction, (i.e., exertion vs. rest), and the muscle contraction history. The outcome of our study may facilitate the estimation of BF, thus, the muscle recovery rate, which can be implemented in the fatigue models to improve the prediction of muscle capacity to generate force/power.

Anthocyanin-Rich New Zealand Blackcurrant Extract Supports the Maintenance of Forearm Blood-Flow During Prolonged Sedentary Sitting

Objectives: We examined the acute effects of anthocyanin-rich New Zealand blackcurrant extract and a placebo on hemodynamics during 120 min of sedentary sitting in healthy males. Additionally, we investigated whether changes in resting hemodynamics altered repeated isometric hand-grip exercise performance and post exercise forearm blood flow (FBF). Methods: Ten healthy males completed two trials during which they ingested either blackcurrant extract (1.87 mg total anthocyanins/kg bodyweight) or placebo powder. Heart rate, blood pressure and forearm blood flow were measured, and venous blood was sampled, prior to and 30, 60, 90 and 120 min-post ingestion. Participants remained seated for the duration of each trial. At 120 min post-ingestion participants completed as many repetitions of isometric hand-grip contractions as possible. Results: Heart rate, blood pressure and mean arterial pressure changed over time (all p < 0.001) but did not differ between treatments. A treatment x time interaction for FBF (p = 0.025) and forearm vascular resistance (FVR) (p = 0.002) was found. No difference in the number of isometric hand-grip contractions was observed between treatments (p = 0.68) nor was there any treatment x time interaction in post-exercise FBF (p = 0.997). Plasma endothelin-1 (p = 0.023) and nitrate (p = 0.047) changed over time but did not differ between treatments (both p > 0.1). Plasma nitrite did not change over time (p = 0.732) or differ between treatments (p = 0.373). Conclusion: This study demonstrated that acute ingestion of a single dose of blackcurrant extract maintained FBF and FVR during an extended period of sitting; however, this did not influence exercise performance during hand-grip exercise.

Near-infrared spectroscopy-derived total haemoglobin as an indicator of changes in muscle blood flow during exercise-induced hyperaemia

Blood flow changes in response to exercise have been attributed, among other factors, to the effect of vasodilators factors on the microvasculature, suggesting a close relationship between small blood vessels and conducting arteries. The main purpose of this study was to determine the relationship between the changes in near infrared spectroscopy (NIRS)-derived total haemoglobin ([tHb]) and muscle oxygen saturation (SmO₂) signals and femoral artery blood flow in response to resistance exercise at fast- and slow-velocity muscle contraction. The study randomised crossover design included twelve participants. NIRS and blood flow measurements were continuously monitored before, during, and 5 min after the exercise protocol. There was a significant correlation between [tHb] reperfusion slope ([tHb]_slope) and peak blood flow (BF_peak) after slow- and fast-velocity muscle contraction (r = 0.83, p = 0.0008 and r = 0.72, p = 0.0080, respectively). No significant correlation existed between the SmO₂ reperfusion slope (SmO_{2_slope}) and BF_peak after both slow- and fast-velocity muscle contraction exercise (r = -0.46, p = 0.1253 and r = 0.33, p = 0.2841, respectively). This study demonstrated a strong relationship between the NIRS-derived [tHb] and Doppler ultrasound BF during the recovery period of dynamic resistance exercise at both slow- and fast-velocity contraction.

Effects of External Counterpulsation on Postexercise Recovery in Elite Rugby League Players

PURPOSE

External counterpulsation (ECP) has previously been used to treat cardiac patients via compression of the lower extremities during diastole to increase venous return and coronary perfusion. However, the effects of ECP on exercise performance and markers of recovery in elite athletes are largely unknown.

METHODS

On 2 separate occasions, 48 h apart, 7 elite National Rugby League players performed an identical 60-min field-based conditioning session followed by a 30-min period of either regular ECP treatment or placebo. Power measures during repeated cycle bouts and countermovement jump height and contraction time derivatives were measured at rest and 5 h postexercise. Saliva samples and venous blood samples were taken at rest, postexercise, and 5 h postexercise to assess stress, inflammation, and muscle damage.

RESULTS

After ECP treatment, cycling peak power output (P = .028; 11%) and accumulated peak power (P = .027; 14%) increased compared with the placebo condition. Postexercise plasma interleukin 1 receptor antagonist only increased after ECP (P = .024; 84%), and concentrations of plasma interleukin 1 receptor antagonist tended to be higher (P = .093; 76%) 5 h postexercise. Furthermore, testosterone-to-cortisol ratio was increased above baseline and placebo 5 h postexercise (P = .017-.029; 24-77%). The ratio of postexercise salivary α-amylase to immunoglobulin A decreased after treatment (P = .013; 50%) compared with the placebo control.

CONCLUSIONS

Exercise performance and hormonal indicators of stress were improved and inflammation markers were reduced following acute ECP.

Skeletal muscle interstitial Po2 kinetics during recovery from contractions

The oxygen partial pressure in the interstitial space (Po_2 is) drives O₂ into the myocyte via diffusion, thus supporting oxidative phosphorylation. Although crucial for metabolic recovery and the capacity to perform repetitive tasks, the time course of skeletal muscle Po_2 is during recovery from contractions remains unknown. We tested the hypothesis that Po_2 is would recover to resting values and display considerable on-off asymmetry (fast on-, slow off-kinetics), reflective of asymmetric capillary hemodynamics. Microvascular Po₂ (Po_2 mv) was also evaluated to test the hypothesis that a significant transcapillary gradient (ΔPo₂ = Po_2 mv - Po_2 is) would be sustained during recovery. Po_2 mv and Po_2 is (expressed in mmHg) were determined via phosphorescence quenching in the exposed rat spinotrapezius muscle during and after submaximal twitch contractions (n = 12). Po_2 is rose exponentially (P < 0.05) from end-contraction (11.1 ± 5.1), such that the end-recovery value (17.9 ± 7.9) was not different from resting Po_2 is (18.5 ± 8.1; P > 0.05). Po_2 is off-kinetics were slower than on-kinetics (mean response time: 53.1 ± 38.3 versus 18.5 ± 7.3 s; P < 0.05). A significant transcapillary ΔPo₂ observed at end-contraction (16.6 ± 7.4) was maintained throughout recovery (end-recovery: 18.8 ± 9.6; P > 0.05). Consistent with our hypotheses, muscle Po_2 is recovered to resting values with slower off-kinetics compared with the on-transient in line with the on-off asymmetry for capillary hemodynamics. Maintenance of a substantial transcapillary ΔPo₂ during recovery supports that the microvascular-interstitium interface provides considerable resistance to O₂ transport. As dictated by Fick's law (V̇o₂ = Do₂ × ΔPo₂), modulation of O₂ flux (V̇o₂) during recovery must be achieved via corresponding changes in effective diffusing capacity (Do₂; mainly capillary red blood cell hemodynamics and distribution) in the face of unaltered ΔPo₂.NEW & NOTEWORTHY Capillary blood-myocyte O₂ flux (V̇o₂) is determined by effective diffusing capacity (Do₂; mainly erythrocyte hemodynamics and distribution) and microvascular-interstitial Po₂ gradients (ΔPo₂ = Po_2 mv - Po_2 is). We show that Po_2 is demonstrates on-off asymmetry consistent with Po_2 mv and erythrocyte kinetics during metabolic transitions. A substantial transcapillary ΔPo₂ was preserved during recovery from contractions, indicative of considerable resistance to O₂ diffusion at the microvascular-interstitium interface. This reveals that effective Do₂ declines in step with V̇o₂ during recovery, as per Fick's law.

Cardiovascular responses to leg muscle loading during head-down tilt at rest and after dynamic exercises

The physiological processes underlying hemodynamic homeostasis can be modulated by muscle activity and gravitational loading. The effects of leg muscle activity on cardiovascular regulation have been observed during orthostatic stress. Here, we evaluated such effects during head-down tilt (HDT). In this posture, the gravitational gradient along the body is different than in upright position, leading to increased central blood volume and reduced venous pooling. We compared the cardiovascular signals obtained with and without leg muscle loading during HDT in healthy human subjects, both at rest and during recovery from leg-press exercises using a robotic device. Further, we compared such cardiovascular responses to those obtained during upright position. Loading leg muscles during HDT at rest led to significantly higher values of arterial blood pressure than without muscle loading, and restored systolic values to those observed during upright posture. Maintaining muscle loading post-exercise altered the short-term cardiovascular responses, but not the values of the signals five minutes after the exercise. These results suggest that leg muscle activity modulates cardiovascular regulation during HDT. This modulation should therefore be considered when interpreting cardiovascular responses to conditions that affect both gravity loading and muscle activity, for example bed rest or microgravity.

Continuous and Accumulated Bouts of Cycling Matched by Intensity and Energy Expenditure Elicit Similar Acute Blood Pressure Reductions in Prehypertensive Men

Fonseca, GF, Farinatti, PTV, Midgley, AW, Ferreira, A, de Paula, T, Monteiro, WD, and Cunha, FA. Continuous and accumulated bouts of cycling matched by intensity and energy expenditure elicit similar acute blood pressure reductions in prehypertensive men. J Strength Cond Res 32(3): 857-866, 2018-This study investigated differences in postexercise hypotension (PEH) after continuous vs. accumulated isocaloric bouts of cycling. Ten prehypertensive men, aged 23-34 years, performed 2 bouts of cycling at 75% oxygen uptake reserve, with total energy expenditures of 400 kcal per bout. One exercise bout was performed continuously (CONTIN) and the other as 2 smaller bouts each expending 200 kcal (INTER1 and INTER2). Systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), and cardiac autonomic control were monitored in a supine position for 10 minutes before and 60 minutes after each exercise bout, and during a control session. Compared with control, blood pressure was significantly reduced after CONTIN (SBP: [INCREMENT] - 3.4 mm Hg, p < 0.001; MAP: [INCREMENT] - 2.5 mm Hg, p = 0.001), INTER1 (SBP: [INCREMENT] - 2.2 mm Hg, p = 0.045), and INTER2 (SBP: [INCREMENT] - 4.4 mm Hg, p < 0.001; DBP: [INCREMENT] - 2.7 mm Hg, p = 0.045; MAP: [INCREMENT] - 3.3 mm Hg, p = 0.001). The PEH was similar in CONTIN and INTER2, whereas INTER2 elicited greater PEH than INTER1 (SBP and MAP: [INCREMENT] - 2.0 and [INCREMENT] - 1.8 mm Hg, respectively, p ≤ 0.05). Increases in sympathovagal balance from baseline were inversely related to changes in SBP and DBP after CONTIN and INTER2 (r = -0.64 to -0.71; p = 0.021-0.047). These findings indicate that similar amounts of PEH are observed when exercise is performed as a single 400-kcal exercise bout or 2 × 200-kcal bouts and that the exercise recovery pattern of cardiac autonomic activity may be important in eliciting PEH.

Oxygen availability and skeletal muscle oxidative capacity in patients with peripheral artery disease: implications from in vivo and in vitro assessments

Evidence suggests that the peak skeletal muscle mitochondrial ATP synthesis rate ( V_max) in patients with peripheral artery disease (PAD) may be attenuated due to disease-related impairments in O₂ supply. However, in vitro assessments suggest intrinsic deficits in mitochondrial respiration despite ample O₂ availability. To address this conundrum, Doppler ultrasound, near-infrared spectroscopy, phosphorus magnetic resonance spectroscopy, and high-resolution respirometry were combined to assess convective O₂ delivery, tissue oxygenation, V_max, and skeletal muscle mitochondrial capacity (complex I + II, state 3 respiration), respectively, in the gastrocnemius muscle of 10 patients with early stage PAD and 11 physical activity-matched healthy control (HC) subjects. All participants were studied in free-flow control conditions (FF) and with reactive hyperemia (RH) induced by a period of brief ischemia during the last 30 s of submaximal plantar flexion exercise. Patients with PAD repeated the FF and RH trials under hyperoxic conditions (FF + 100% O₂ and RH + 100% O₂). Compared with HC subjects, patients with PAD exhibited attenuated O₂ delivery at the same absolute work rate and attenuated tissue reoxygenation and V_max after relative intensity-matched exercise. Compared with the FF condition, only RH + 100% O₂ significantly increased convective O₂ delivery (~44%), tissue reoxygenation (~54%), and V_max (~60%) in patients with PAD ( P < 0.05), such that V_max was now not different from HC subjects. Furthermore, there was no evidence of an intrinsic mitochondrial deficit in PAD, as assessed in vitro with adequate O₂. Thus, in combination, this comprehensive in vivo and in vitro investigation implicates O₂ supply as the predominant factor limiting mitochondrial oxidative capacity in early stage PAD. NEW & NOTEWORTHY Currently, there is little accord as to the role of O₂ availability and mitochondrial function in the skeletal muscle dysfunction associated with peripheral artery disease. This is the first study to comprehensively use both in vivo and in vitro approaches to document that the skeletal muscle dysfunction associated with early stage peripheral artery disease is predominantly a consequence of limited O₂ supply and not the impact of an intrinsic mitochondrial defect in this pathology.

Triggered intravoxel incoherent motion MRI for the assessment of calf muscle perfusion during isometric intermittent exercise

The main aim of this paper was to propose triggered intravoxel incoherent motion (IVIM) imaging sequences for the evaluation of perfusion changes in calf muscles before, during and after isometric intermittent exercise. Twelve healthy volunteers were involved in the study. The subjects were asked to perform intermittent isometric plantar flexions inside the MRI bore. MRI of the calf muscles was performed on a 3.0 T scanner and diffusion-weighted (DW) images were obtained using eight different b values (0 to 500 s/mm² ). Acquisitions were performed at rest, during exercise and in the subsequent recovery phase. A motion-triggered echo-planar imaging DW sequence was implemented to avoid movement artifacts. Image quality was evaluated using the average edge strength (AES) as a quantitative metric to assess the motion artifact effect. IVIM parameters (diffusion D, perfusion fraction f and pseudo-diffusion D*) were estimated using a segmented fitting approach and evaluated in gastrocnemius and soleus muscles. No differences were observed in quality of IVIM images between resting state and triggered exercise, whereas the non-triggered images acquired during exercise had a significantly lower value of AES (reduction of more than 20%). The isometric intermittent plantar-flexion exercise induced an increase of all IVIM parameters (D by 10%; f by 90%; D* by 124%; fD* by 260%), in agreement with the increased muscle perfusion occurring during exercise. Finally, IVIM parameters reverted to the resting values within 3 min during the recovery phase. In conclusion, the IVIM approach, if properly adapted using motion-triggered sequences, seems to be a promising method to investigate muscle perfusion during isometric exercise.

Anterior vena caval oxygen profiles in a deep-diving California sea lion: arteriovenous shunts, a central venous oxygen store and oxygenation during lung collapse

Deep-diving California sea lions (Zalophus californianus) can maintain arterial hemoglobin saturation (S_O2 ) above 90% despite lung collapse (lack of gas exchange) and extremely low posterior vena caval S_O2  in the middle of the dive. We investigated anterior vena caval P_O2 and S_O2  during dives of an adult female sea lion to investigate two hypotheses: (1) posterior vena caval S_O2  is not representative of the entire venous oxygen store and (2) a well-oxygenated (arterialized) central venous oxygen reservoir might account for maintenance of arterial S_O2  during lung collapse. During deep dives, initial anterior vena caval S_O2  was elevated at 83.6±8.4% (n=102), presumably owing to arteriovenous shunting. It remained high until the bottom phase of the dive and then decreased during ascent, whereas previously determined posterior vena caval S_O2  declined during descent and then often increased during ascent. These divergent patterns confirmed that posterior vena caval S_O2  was not representative of the entire venous oxygen store. Prior to and early during descent of deep dives, the high S_O2  values of both the anterior and posterior venae cavae may enhance arterialization of a central venous oxygen store. However, anterior vena caval S_O2  values at depths beyond lung collapse reached levels as low as 40%, making it unlikely that even a completely arterialized central venous oxygen store could account for maintenance of high arterial S_O2 These findings suggest that maintenance of high arterial S_O2  during deep dives is due to persistence of some gas exchange at depths beyond presumed lung collapse.

CORP: Ultrasound assessment of vascular function with the passive leg movement technique

As dysfunction of the vascular system is an early, modifiable step in the progression of many cardiovascular diseases, there is demand for methods to monitor the health of the vascular system noninvasively in clinical and research settings. Validated by very good agreement with more technical assessments of vascular function, like intra-arterial drug infusions and flow-mediated dilation, the passive leg movement (PLM) technique has emerged as a powerful, yet relatively simple, test of peripheral vascular function. In the PLM technique, the change in leg blood flow elicited by the passive movement of the leg through a 90° range of motion is quantified with Doppler ultrasound. This relatively easy-to-learn test has proven to be ≤80% dependent on nitric oxide bioavailability and is especially adept at determining peripheral vascular function across the spectrum of cardiovascular health. Indeed, multiple reports have documented that individuals with decreased cardiovascular health such as the elderly and those with heart failure tend to exhibit a substantially blunted PLM-induced hyperemic response (~50 and ~85% reduction, respectively) compared with populations with good cardiovascular health such as young individuals. As specific guidelines have not yet been put forth, the purpose of this Cores of Reproducibility in Physiology (CORP) article is to provide a comprehensive reference for the assessment and interpretation of vascular function with PLM with the aim to increase reproducibility and consistency among studies and facilitate the use of PLM as a research tool with clinical relevance.

Combined effects of aerobic exercise and l-arginine ingestion on blood pressure in normotensive postmenopausal women: A crossover study

After menopause the incidence of cardiovascular diseases increases in women. A decrease in nitric oxide (NO) bioavailability has been pointed out to play a major role in this phenomenon. Since it is believed that l-arginine administration could improve NO bioavailability, the aim of this study was to examine the effects of acute l-arginine administration associated with aerobic exercise on blood pressure (BP), redox state and inflammatory biomarkers in normotensive postmenopausal women (NPW). Sixteen volunteers (57±6yr) were subjected to four experimental sessions (crossover design): arginine+exercise (A-E); arginine (ARG); exercise+placebo (EXE); control (CON). Each session was initiated with either 9g of l-arginine ingestion (ARG or A-E days), placebo (EXE day), or nothing (CON day). The participants performed 30min of aerobic exercise (A-E and EXE days) or sitting rest (CON and ARG days). Blood samples were collected before each session and 45min after the intervention. Office BP and ambulatory blood pressure monitoring (ABPM) were evaluated. NO/cGMP pathway, redox state and inflammatory biomarkers were measured. Systolic BP decreased during the 24-hour in A-E and EXE sessions. However, diastolic BP reduced only in A-E session. No changes were found in the biomarkers concentrations. In conclusion, the association was effective in lowering diastolic BP in NPW. Additionally, physical exercise alone promoted a long lasting effect on systolic BP measured by ABPM in this population, although this beneficial effect was not associated with changes in the cardio-inflammatory biomarkers. Possibly, other factors such as neural influences could be mediating this effect.

Recovery mismatch between myocardial blood flow and cardiac workload after physical exercise: a positron emission tomography study

AIMS

We studied the interrelation between oxygen consumption and myocardial blood flow (MBF) during recovery. MBF is directly dependent on oxygen consumption. The latter is linearly related to the heart rate-blood pressure product (RPP, bpm × mmHg), an index reflecting external cardiac work. In the immediate post-exercise period, cardiac output decreases considerably. This is expected to be paralleled by a rapid fall in oxygen demand, rendering ischaemia unlikely. Thus, the phenomenon of ST-segment depression during recovery remains unexplained.

METHODS AND RESULTS

(15)O-labelled water and positron emission tomography were used to measure MBF in 14 young healthy volunteers (mean age 27 ± 3 years) during the following study conditions: (i) at rest, (ii) during a steady submaximal supine bicycle exercise stress within the scanner, and (iii) during recovery immediately after cessation of exercise. During recovery, RPP decreased by 43% (18 768 ± 1337 vs. 11 652 ± 3224, P < 0.001). In contrast, the associated decrease in MBF (2.52 ± 0.52 vs. 1.93 ± 0.50 mL/min/g, P < 0.001) and perfusion reserve (2.68 ± 0.51 vs. 2.03 ± 0.42, P < 0.001) was significantly less pronounced (-24%, P < 0.01), indicating a relative delay in MBF recovery compared with cardiac work load.

CONCLUSION

The mismatch between a rapid decrease in cardiac workload but preserved hyperaemic response early after cessation of physical exercise suggests an uncoupling of cardiac work and MBF during recovery.

Insights from venous oxygen profiles: oxygen utilization and management in diving California sea lions

The management and depletion of O2 stores underlie the aerobic dive capacities of marine mammals. The California sea lion (Zalophus californianus) presumably optimizes O2 store management during all dives, but approaches its physiological limits during deep dives to greater than 300 m depth. Blood O2 comprises the largest component of total body O2 stores in adult sea lions. Therefore, we investigated venous blood O2 depletion during dives of California sea lions during maternal foraging trips to sea by: (1) recording venous partial pressure of O2 (P(O2)) profiles during dives, (2) characterizing the O2-hemoglobin (Hb) dissociation curve of sea lion Hb and (3) converting the P(O2) profiles into percent Hb saturation (S(O2)) profiles using the dissociation curve. The O2-Hb dissociation curve was typical of other pinnipeds (P50=28±2 mmHg at pH 7.4). In 43% of dives, initial venous S(O2) values were greater than 78% (estimated resting venous S(O2)), indicative of arterialization of venous blood. Blood O2 was far from depleted during routine shallow dives, with minimum venous S(O2) values routinely greater than 50%. However, in deep dives greater than 4 min in duration, venous S(O2) reached minimum values below 5% prior to the end of the dive, but then increased during the last 30-60 s of ascent. These deep dive profiles were consistent with transient venous blood O2 depletion followed by partial restoration of venous O2 through pulmonary gas exchange and peripheral blood flow during ascent. These differences in venous O2 profiles between shallow and deep dives of sea lions reflect distinct strategies of O2 store management and suggest that underlying cardiovascular responses will also differ.

Blood pressure regulation X: what happens when the muscle pump is lost? Post-exercise hypotension and syncope

Syncope which occurs suddenly in the setting of recovery from exercise, known as post-exercise syncope, represents a failure of integrative physiology during recovery from exercise. We estimate that between 50 and 80% of healthy individuals will develop pre-syncopal signs and symptoms if subjected to a 15-min head-up tilt following exercise. Post-exercise syncope is most often neurally mediated syncope during recovery from exercise, with a combination of factors associated with post-exercise hypotension and loss of the muscle pump contributing to the onset of the event. One can consider the initiating reduction in blood pressure as the tip of the proverbial iceberg. What is needed is a clear model of what lies under the surface; a model that puts the observational variations in context and provides a rational framework for developing strategic physical or pharmacological countermeasures to ultimately protect cerebral perfusion and avert loss of consciousness. This review summarizes the current mechanistic understanding of post-exercise syncope and attempts to categorize the variation of the physiological processes that arise in multiple exercise settings. Newer investigations into the basic integrative physiology of recovery from exercise provide insight into the mechanisms and potential interventions that could be developed as countermeasures against post-exercise syncope. While physical counter maneuvers designed to engage the muscle pump and augment venous return are often found to be beneficial in preventing a significant drop in blood pressure after exercise, countermeasures that target the respiratory pump and pharmacological countermeasures based on the involvement of histamine receptors show promise.

Effect of H1- and H2-histamine receptor blockade on postexercise insulin sensitivity

Following a bout of dynamic exercise, humans experience sustained postexercise vasodilatation in the previously exercised skeletal muscle which is mediated by activation of histamine (H1 and H2) receptors. Skeletal muscle glucose uptake is also enhanced following dynamic exercise. Our aim was to determine if blunting the vasodilatation during recovery from exercise would have an adverse effect on blood glucose regulation. Thus, we tested the hypothesis that insulin sensitivity following exercise would be reduced with H1- and H2-receptor blockade versus control (no blockade). We studied 20 healthy young subjects (12 exercise; eight nonexercise sham) on randomized control and H1- and H2-receptor blockade (fexofenadine and ranitidine) days. Following 60 min of upright cycling at 60% VO2 peak or nonexercise sham, subjects consumed an oral glucose tolerance beverage (1.0 g/kg). Blood glucose was determined from "arterialized" blood samples (heated hand vein). Postexercise whole-body insulin sensitivity (Matsuda insulin sensitivity index) was reduced 25% with H1- and H2-receptor blockade (P < 0.05), whereas insulin sensitivity was not affected by histamine receptor blockade in the sham trials. These results indicate that insulin sensitivity following exercise is blunted by H1- and H2-receptor blockade and suggest that postexercise H1- and H2-receptor-mediated skeletal muscle vasodilatation benefits glucose regulation in healthy humans.

Contribution of non-endothelium-dependent substances to exercise hyperaemia: are they O(2) dependent?

This review considers the contributions to exercise hyperaemia of substances released into the interstitial fluid, with emphasis on whether they are endothelium dependent or O(2) dependent. The early phase of exercise hyperaemia is attributable to K(+) released from contracting muscle fibres and acting extraluminally on arterioles. Hyperpolarization of vascular smooth muscle and endothelial cells induced by K(+) may also facilitate the maintained phase, for example by facilitating conduction of dilator signals upstream. ATP is released into the interstitium from muscle fibres, at least in part through cystic fibrosis transmembrane conductance regulator-associated channels, following the fall in intracellular H(+). ATP is metabolized by ectonucleotidases to adenosine, which dilates arterioles via A(2A) receptors, in a nitric oxide-independent manner. Evidence is presented that the rise in arterial achieved by breathing 40% O(2) attenuates efflux of H(+) and lactate, thereby decreasing the contribution that adenosine makes to exercise hyperaemia; efflux of inorganic phosphate and its contribution may likewise be attenuated. Prostaglandins (PGs), PGE(2) and PGI(2), also accumulate in the interstitium during exercise, and breathing 40% O(2) abolished the contribution of PGs to exercise hyperaemia. This suggests that PGE(2) released from muscle fibres and PGI(2) released from capillaries and venular endothelium by a fall in their local act extraluminally to dilate arterioles. Although modest hyperoxia attenuates exercise hyperaemia by improving O(2) supply, limiting the release of O(2)-dependent adenosine and PGs, higher O(2) concentrations may have adverse effects. Evidence is presented that breathing 100% O(2) limits exercise hyperaemia by generating O(2)(-), which inactivates nitric oxide and decreases PG synthesis.

Postexercise hypotension and sustained postexercise vasodilatation: what happens after we exercise?

A single bout of aerobic exercise produces a postexercise hypotension associated with a sustained postexercise vasodilatation of the previously exercised muscle. Work over the last few years has determined key pathways for the obligatory components of postexercise hypotension and sustained postexercise vasodilatation and points the way to possible benefits that may result from these robust responses. During the exercise recovery period, the combination of centrally mediated decreases in sympathetic nerve activity with a reduced signal transduction from sympathetic nerve activation into vasoconstriction, as well as local vasodilator mechanisms, contributes to the fall in arterial blood pressure seen after exercise. Important findings from recent studies include the recognition that skeletal muscle afferents may play a primary role in postexercise resetting of the baroreflex via discrete receptor changes within the nucleus tractus solitarii and that sustained postexercise vasodilatation of the previously active skeletal muscle is primarily the result of histamine H(1) and H(2) receptor activation. Future research directions include further exploration of the potential benefits of these changes in the longer term adaptations associated with exercise training, as well as investigation of how the recovery from exercise may provide windows of opportunity for targeted interventions in patients with hypertension and diabetes.

Local histamine H(1-) and H(2)-receptor blockade reduces postexercise skeletal muscle interstitial glucose concentrations in humans

Elevated blood flow can potentially influence skeletal muscle glucose uptake, but the impact of postexercise hyperemia on glucose availability to skeletal muscle remains unknown. Because postexercise hyperemia is mediated by histamine H(1)- and H(2)-receptors, we tested the hypothesis that postexercise interstitial glucose concentrations would be lower in the presence of combined H1- and H2-receptor blockade. To this end, 4 microdialysis probes were inserted into the vastus lateralis muscle of 14 healthy subjects (21-27 years old) immediately after 60 min of either upright cycling at 60% peak oxygen uptake (exercise, n = 7) or quiet rest (sham, n = 7). Microdialysis probes were perfused with a modified Ringer's solution containing 3 mmol L(-1) glucose, 5 mmol L(-1) ethanol, and [6-3H] glucose (200 disintegrations·min-1 microL(-1)). Two sites (blockade) received both H1- and H2-receptor antagonists (1 mmol L(-1) pyrilamine and 3 mmol L-1 cimetidine) and 2 sites (control) did not receive antagonists. Ethanol outflow/inflow ratios (an inverse surrogate of local blood flow) were higher in blockade sites than in control sites following exercise (p < 0.05), whereas blockade had no effect on ethanol outflow/inflow ratios following sham (p = 0.80). Consistent with our hypothesis, during 3 of the 5 dialysate collection periods, interstitial glucose concentrations were lower in blockade sites vs. control sites following exercise (p < 0.05), whereas blockade had no effect on interstitial glucose concentrations following sham (p = 0.79). These findings indicate that local H1- and H2-receptor activation modulates skeletal muscle interstitial glucose levels during recovery from exercise in humans and suggest that the availability of glucose to skeletal muscle is enhanced by postexercise hyperemia.

Acute effects of blood flow restriction on muscle activity and endurance during fatiguing dynamic knee extensions at low load

The purpose of this study was to investigate muscle activity and endurance during fatiguing low-intensity dynamic knee extension exercise with and without blood flow restriction. Eleven healthy subjects with strength training experience performed 3 sets of unilateral knee extensions with no relaxation between repetitions to concentric torque failure at 30% of the 1 repetition maximum. One leg was randomized to exercise with cuff occlusion and the other leg to exercise without occlusion. The muscle activity in the quadriceps was recorded with electromyography (EMG). Ratings of perceived exertion (RPE) and acute pain were collected immediately, and delayed onset muscle soreness (DOMS) was rated before and at 24, 48, and 72 hours after exercise. The results demonstrated high EMG levels in both experimental conditions, but there were no significant differences regarding maximal muscle activity, except for a higher EMG in the eccentric phase in set 3 for the nonoccluded condition (p = 0.005). Significantly more repetitions were performed with the nonoccluded leg in every set (p < 0.05). The RPE and acute pain ratings were similar, but DOMS was higher in the nonoccluded leg (p < 0.05). We conclude that blood flow restriction during low-intensity dynamic knee extension decreases the endurance but does not increase the maximum muscle activity compared with training without restriction when both regimes are performed to failure. The high levels of muscle activity suggest that performing low-load dynamic knee extensions in a no-relaxation manner may be a useful method in knee rehabilitation settings when large forces are contraindicated. However, similarly to fatiguing blood flow restricted exercise, this method is associated with ischemic muscle pain, and thus its applications may be limited to highly motivated individuals.

Postnatal development of muscle biochemistry in nursing harbor seal (Phoca vitulina) pups: limitations to diving behavior?

Adult marine mammal muscles rely upon a suite of adaptations for sustained aerobic metabolism in the absence of freely available oxygen (O(2)). Although the importance of these adaptations for supporting aerobic diving patterns of adults is well understood, little is known about postnatal muscle development in young marine mammals. However, the typical pattern of vertebrate muscle development, and reduced tissue O(2) stores and diving ability of young marine mammals suggest that the physiological properties of harbor seal (Phoca vitulina) pup muscle will differ from those of adults. We examined myoglobin (Mb) concentration, and the activities of citrate synthase (CS), beta-hydroxyacyl coA dehydrogenase (HOAD), and lactate dehydrogenase (LDH) in muscle biopsies from harbor seal pups throughout the nursing period, and compared these biochemical parameters to those of adults. Pups had reduced O(2) carrying capacity ([Mb] 28-41% lower than adults) and reduced metabolically scaled catabolic enzyme activities (LDH/RMR 20-58% and CS/RMR 29-89% lower than adults), indicating that harbor seal pup muscles are biochemically immature at birth and weaning. This suggests that pup muscles do not have the ability to support either the aerobic or anaerobic performance of adult seals. This immaturity may contribute to the lower diving capacity and behavior in younger pups. In addition, the trends in myoglobin concentration and enzyme activity seen in this study appear to be developmental and/or exercise-driven responses that together work to produce the hypoxic endurance phenotype seen in adults, rather than allometric effects due to body size.

A technique based on laser Doppler flowmetry and photoplethysmography for simultaneously monitoring blood flow at different tissue depths

The aim of this study was to validate a non-invasive optical probe for simultaneous blood flow measurement at different vascular depths combining three photoplethysmography (PPG) channels and laser Doppler flowmeter (LDF). Wavelengths of the PPG were near-infrared 810 nm with source-to-detector separation of 10 and 25 mm, and green 560 nm with source-to-detector separation of 4 mm. The probe is intended for clinical studies of pressure ulcer aetiology. The probe was placed over the trapezius muscle, and depths from the skin to the trapezius muscle were measured using ultrasound and varied between 3.8 and 23 mm in the 11 subjects included. A provocation procedure inducing a local enhancement of blood flow in the trapezius muscle was used. Blood flows at rest and post-exercise were compared. It can be concluded that this probe is useful as a tool for discriminating between blood flows at different vascular tissue depths. The vascular depths reached for the different channels in this study were at least 23 mm for the near-infrared PPG channel (source-to-detector separation 25 mm), 10-15 mm for the near-infrared PPG channel (separation 10 mm), and shallower than 4 mm for both the green PPG channel (separation 4 mm) and LDF.

The effect of AMPD1 genotype on blood flow response to sprint exercise

Inherited deficiency of skeletal muscle myoadenylate deaminase (mAMPD) is a genetic disorder characterized primarily by a 34C>T transition in exon 2 of the AMPD1 gene. mAMPD deficient individuals exhibit alterations in ATP catabolic flow, resulting in greater adenosine accumulation during high intensity exercise that may possibly enhance exercise-induced hyperaemia. This study tested the hypothesis that individuals with diminished mAMPD activity due to mutations in the AMPD1 gene develop a greater and faster blood flow response to high intensity exercise than individuals with two AMPD1 normal alleles (NN). Four 34C>T homozygotes, two compound heterozygotes (34C>T in one allele and a recently identified 404delT mutation in the other AMPD1 allele), collectively termed MM, one 34C>T heterozygote (NM) and eight NN males were studied. They performed a 30 s Wingate cycling test with monitoring of power output and other parameters of exercise performance. Common femoral artery blood flow was measured before and after (up to 25 min) exercise, using ultrasonography. Mean power during Wingate cycling was approximately 10% lower in MM/NM than in NN; p<0.01. Blood flow response to exercise also differed between MM/NM and NN individuals (ANOVA; p<0.001). There was also a difference in peak post-exercise blood flow (p<0.05), and the subsequent fall in blood flow during the recovery phase (T1/2) occurred more than twice as fast in MM/NM compared to NN subjects (7.8+/-1.1 min vs. 16.1+/-1.4 min, p<0.001). These results suggest a better circulatory adaptation to exercise in individuals with diminished mAMPD activity, probably due to an AMPD1 genotype-dependent increase in adenosine formation.

Physiological characteristics of America's Cup sailors

The aim of this study was to assess the physiological profile of America's Cup grinders and mastmen, by measuring energy expenditure during sailing and assessing their aerobic and anaerobic fitness. The study focused on estimating the energy used during grinding activity, by measuring oxygen uptake (VO(2)) during sail setting in real sailing conditions. In the laboratory, using an arm-cranking ergometer, we measured VO(2peak) during an incremental maximal exercise test and total energy expended during the effort and recovery phases of an all-out test that simulated grinding activity, in six grinders and mastmen and ten sailors of the same crew. Total energy used during grinding corresponded to 45% (s = 9) and 51% (s = 5) of that used in the all-out test (234 kJ, s = 21.7) for tacks and gybes, respectively. In both grinding activity and the all-out test, VO(2) increased during and after the effort. The "VO(2) top value" was 53% (s = 8.6), 68% (s = 5.5), and 78% (s = 3.1) of VO(2peak) (4.7 l . min(-1), s = 0.43) in tacks, gybes, and the all-out test, respectively. During fast sequences of grinding activity, the "VO(2) top value" reached 65% (s = 7.1) VO(2peak) in tacks and 91% (s = 3.3) VO(2peak) in gybes. Our results suggest that grinders and mastmen are characterized by a high anaerobic capacity but their performance can be improved by powering aerobic fitness, to increase this energy contribution to all-out efforts and to guarantee fast recovery when grinding activity is repeated with short rest intervals.

Blood flow changes in the trapezius muscle and overlying skin following transcutaneous electrical nerve stimulation

BACKGROUND AND PURPOSE

Various researchers have studied the effects of transcutaneous electrical nerve stimulation (TENS) on hemodynamics. The purpose of this study was to examine the effects of TENS on local blood flow in the trapezius muscle and overlying skin.

SUBJECTS

Thirty-three women who were healthy, aged 25 to 55 years, were randomly assigned to receive 1 of 3 different modes of TENS.

METHODS

Skin and muscle blood flow were monitored noninvasively using a new application of photoplethysmography for 15 minutes of TENS applied at high frequency (80 Hz) and sensory-level intensity and at low frequency (2 Hz) and motor-level intensity and for 15 minutes after stimulation. Subliminal 80-Hz TENS was used as a control. Blood flow was monitored simultaneously on stimulated and nonstimulated shoulders.

RESULTS

Blood flow in the trapezius muscle, but not skin blood flow, increased significantly with motor-level 2-Hz TENS, whereas no increase occurred with sensory-level 80-Hz TENS or subliminal 80-Hz TENS.

DISCUSSION AND CONCLUSION

Muscle contractions induced by motor-level 2-Hz TENS appear to be a prerequisite for increasing blood flow in the trapezius muscle. However, high stimulation intensity may prevent increased blood flow in the overlying skin.

Partitioning locomotor energy use among and within muscles Muscle blood flow as a measure of muscle oxygen consumption

Linking the mechanics and energetics of locomotion in vertebrates has been hampered by a lack of information regarding the energy use of individual skeletal muscles in vivo. Here, we present a review of the available data concerning the relationship between the rates of skeletal muscle blood flow and oxygen consumption(V̇O2). In active muscle, during aerobically supported exercise, there is a linear relationship between these variables, irrespective of the muscle fiber type and intensity of exercise through most of the aerobic exercise range. We conclude that the rate of blood flow is the best available indicator of aerobic metabolic rate in multiple individual muscles or regions of muscles during locomotion. The practical considerations of using the injectable microsphere technique to measure muscle blood flow in this context are discussed.

A novel method to measure regional muscle blood flow continuously using NIRS kinetics information

BACKGROUND

This article introduces a novel method to continuously monitor regional muscle blood flow by using Near Infrared Spectroscopy (NIRS). We demonstrate the feasibility of the new method in two ways: (1) by applying this new method of determining blood flow to experimental NIRS data during exercise and ischemia; and, (2) by simulating muscle oxygenation and blood flow values using these newly developed equations during recovery from exercise and ischemia.

METHODS

Deoxy (Hb) and oxyhemoglobin (HbO2), located in the blood of the skeletal muscle, carry two internal relationships between blood flow and oxygen consumption. One is a mass transfer principle and the other describes a relationship between oxygen consumption and Hb kinetics in a two-compartment model. To monitor blood flow continuously, we transfer these two relationships into two equations and calculate the blood flow with the differential information of HbO2 and Hb. In addition, these equations are used to simulate the relationship between blood flow and reoxygenation kinetics after cuff ischemia and a light exercise. Nine healthy subjects volunteered for the cuff ischemia, light arm exercise and arm exercise with cuff ischemia for the experimental study.

RESULTS

Analysis of experimental data of both cuff ischemia and light exercise using the new equations show greater blood flow (four to six times more than resting values) during recovery, agreeing with previous findings. Further, the simulation and experimental studies of cuff ischemia and light exercise agree with each other.

CONCLUSION

We demonstrate the accuracy of this new method by showing that the blood flow obtained from the method agrees with previous data as well as with simulated data. We conclude that this novel continuous blood flow monitoring method can provide blood flow information non-invasively with NIRS.