Sympathetic neural adaptations to exercise training in humans: insights from microneurography

The deleterious effects of maternal protein deprivation on the brainstem are minimized with moderate physical activity by offspring during early life

Maternal protein malnutrition during developmental periods might impair the redox state and the brain's excitatory/inhibitory neural network, increasing central sympathetic tone. Conversely, moderate physical exercise at an early age reduces the risk of chronic diseases. Thus, we hypothesized that a moderate training protocol could reduce the harmful effects of a low-protein maternal diet on the brainstem of young male offspring. We used a rat model of maternal protein restriction during the gestational and lactation period followed by an offspring's continuous treadmill exercise. Pregnant rats were divided into two groups according to the protein content in the diet: normoprotein (NP), receiving 17% of casein, and low protein (LP), receiving 8% of casein until the end of lactation. At 30 days of age, the male offspring were further subdivided into sedentary (NP-Sed and LP-Sed) or exercised (NP-Ex and LP-Ex) groups. Treadmill exercise was performed as follows: 4 weeks, 5 days/week, 60 min/day at 50% of maximal running capacity. The trained animals performed a treadmill exercise at 50% of the maximal running capacity, 60 min/day, 5 days/week, for 4 weeks. Our results indicate that a low-protein diet promotes deficits in the antioxidant system and a likely mitochondrial uncoupling. On the other hand, physical exercise restores the redox balance, which leads to decreased oxidative stress caused by the diet. In addition, it also promotes benefits to GABAergic inhibitory signaling. We conclude that regular moderate physical exercise performed in youthhood protects the brainstem against changes induced by maternal protein restriction.

Anticipatory Heart Rate Before Moderate- and Vigorous-Intensity Exercise Among Sedentary and Physically Active Young Adult Males

Background Anticipatory heart rate (HR) refers to an increase in HR that occurs in anticipation of a future event or activity. The anticipatory heart rate (HR) response before exercise is an important physiological indicator of exercise readiness. This study aimed to compare the anticipatory HR changes between sedentary and physically active young adult males during moderate- and vigorous-intensity exercise. Understanding these anticipatory heart rate patterns can provide insights into the physiological adaptations and cardiovascular health of individuals with varying physical activity levels. Materials and methods A total of 60 young adult males, comprising sedentary (n = 30) and physically active individuals (n = 30), participated in this study. A brisk walking for a distance of 50 m was considered moderate intensity and one minute of spot jogging at maximum effort with verbal encouragement was considered vigorous intensity exercise. The HR was recorded at baseline, just before the exercise, and on each minute up to 5 minutes after the exercise. Results The study involved 30 physically active young adult males (mean age 20.23 ± 1.43 years) and 30 sedentary adult males (mean age 20.07 ± 1.17 years). In physically active young adults, the resting HR was 76.4±10.89 bpm and just before starting moderate-intensity exercise, it was 78.83±12.98 bpm, paired t-test P = 0.22. The HR just before vigorous-intensity exercise was 80.83±11.18 bpm (paired t-test P = 0.03). In sedentary young adults, the resting HR was 82.23±12.69 bpm and just before starting moderate-intensity exercise, it was 90.13±18.69 bpm, paired t-test P = 0.0008. The HR just before vigorous-intensity exercise was 91.7±15.04 bpm (paired t-test P <0.0001). Conclusion Physically active young adults did not exhibit a significant increase in anticipatory HR before moderate-intensity exercise. However, sedentary individuals exhibit a significant anticipatory HR response. Before vigorous-intensity exercise, both exhibited significant increments in HR. The result highlights the importance of considering the anticipatory HR response as a potential marker of cardiovascular health and exercise readiness.

Recording sympathetic nerve activity in conscious humans and other mammals: guidelines and the road to standardization

Over the past several decades, studies of the sympathetic nervous system in humans, sheep, rabbits, rats, and mice have substantially increased mechanistic understanding of cardiovascular function and dysfunction. Recently, interest in sympathetic neural mechanisms contributing to blood pressure control has grown, in part because of the development of devices or surgical procedures that treat hypertension by manipulating sympathetic outflow. Studies in animal models have provided important insights into physiological and pathophysiological mechanisms that are not accessible in human studies. Across species and among laboratories, various approaches have been developed to record, quantify, analyze, and interpret sympathetic nerve activity (SNA). In general, SNA demonstrates "bursting" behavior, where groups of action potentials are synchronized and linked to the cardiac cycle via the arterial baroreflex. In humans, it is common to quantify SNA as bursts per minute or bursts per 100 heart beats. This type of quantification can be done in other species but is only commonly reported in sheep, which have heart rates similar to humans. In rabbits, rats, and mice, SNA is often recorded relative to a maximal level elicited in the laboratory to control for differences in electrode position among animals or on different study days. SNA in humans can also be presented as total activity, where normalization to the largest burst is a common approach. The goal of the present paper is to put together a summary of "best practices" in several of the most common experimental models and to discuss opportunities and challenges relative to the optimal measurement of SNA across species.Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/guidelines-for-measuring-sympathetic-nerve-activity/.

Endurance training attenuates the increase in peripheral chemoreflex sensitivity with intermittent hypoxia

Patients with heart failure and sleep apnea have greater chemoreflex sensitivity, presumably due to intermittent hypoxia (IH), and this is predictive of mortality. We hypothesized that endurance training would attenuate the effect of IH on peripheral chemoreflex sensitivity in healthy humans. Fifteen young healthy subjects (9 female, 26 ± 1 yr) participated. Between visits, 11 subjects underwent 8 wk of endurance training that included running four times/wk at 80% predicted maximum heart rate and interval training, and four control subjects did not change activity. Chemoreflex sensitivity (the slope of ventilation responses to serial oxygen desaturations), blood pressure, heart rate, and muscle sympathetic nerve activity (MSNA) were assessed before and after 30 min of IH. Endurance training decreased resting systolic blood pressure (119 ± 3 to 113 ± 3 mmHg; P = 0.027) and heart rate (67 ± 3 to 61 ± 2 beats/min; P = 0.004) but did not alter respiratory parameters at rest (P > 0.2). Endurance training attenuated the IH-induced increase in chemoreflex sensitivity (pretraining: Δ 0.045 ± 0.026 vs. posttraining: Δ -0.028 ± 0.040 l·min^-1·% O₂ desaturation^-1; P = 0.045). Furthermore, IH increased mean blood pressure and MSNA burst rate before training (P < 0.05), but IH did not alter these measures after training (P > 0.2). All measurements were similar in the control subjects at both visits (P > 0.05). Endurance training attenuates chemoreflex sensitization to IH, which may partially explain the beneficial effects of exercise training in patients with cardiovascular disease.

Postexercise autonomic function after repeated-sprints training

PURPOSE

We examined the effects of an 8-week repeated-sprint (RS) training protocol on postexercise parasympathetic reactivation (PNSr) in healthy adults.

METHODS

Eighteen male adults (24.3 ± 3.7 years) were assigned to either of two groups. One group (n = 9) performed RS training (EXP, 3 times week(-1), 18 maximal all-out 15-m sprints interspersed with 17 s of passive recovery); the other served as the control group (CON, n = 9). Performance before, during, and after was assessed by measuring RS ability time (S dec) and total sprint time. The subjects were then seated for 10 min immediately after each trial and postexercise HR recovery (HRR), and vagal-related HR variability (HRV) indices were measured.

RESULTS

All subjects demonstrated a decrease in S dec. However, only EXP showed a decrease in total sprint time (-10.5 % of baseline value). Using a qualitative statistical analysis method, we found a likely to almost certain positive effect of RS training on HR. The mean of each HRR and HRV index indicated a greater change in PNSr in EXP than in CON (e.g. with a 78/22/1 % chance to demonstrate a positive/trivial/negative effect on HRR60s after RS training; 74/21/5 % on LN rMSSD5-10min). Large correlations were noted between the changes in S dec [r = 0.59, 90 % CI (0.43)], total sprint time [r = -0.61 (0.42)] and HRR60s.

CONCLUSION

RS training seems to be an effective method to improve postexercise PNSr in healthy adults. Also, HRR60s appears to be a method for evaluating positive adaption to RS training.

Effects of acute and chronic exercise in patients with essential hypertension: benefits and risks

The importance of regular physical activity in essential hypertension has been extensively investigated over the last decades and has emerged as a major modifiable factor contributing to optimal blood pressure control. Aerobic exercise exerts its beneficial effects on the cardiovascular system by promoting traditional cardiovascular risk factor regulation, as well as by favorably regulating sympathetic nervous system (SNS) activity, molecular effects, cardiac, and vascular function. Benefits of resistance exercise need further validation. On the other hand, acute exercise is now an established trigger of acute cardiac events. A number of possible pathophysiological links have been proposed, including SNS, vascular function, coagulation, fibrinolysis, and platelet function. In order to fully interpret this knowledge into clinical practice, we need to better understand the role of exercise intensity and duration in this pathophysiological cascade and in special populations. Further studies in hypertensive patients are also warranted in order to clarify the possibly favorable effect of antihypertensive treatment on exercise-induced effects.

Effect of isometric handgrip exercise training on resting blood pressure in normal healthy adults

INTRODUCTION

The aim of the present study was to study the effect of isometric handgrip (IHG) exercise training on resting blood pressure in normal healthy volunteers.

MATERIALS AND METHODS

Hand grip spring dynamometer was used for IHG exercise training. A total of 30 normal healthy volunteers in the age group of 20-40 y were enrolled for the study. Exercise training protocol consisted of five 3-min bouts of IHG exercise at 30% of maximum voluntary contraction separated by 5 min rest periods. Exercise was performed 3 times/wk for 10 wk. Subject's blood pressure was measured before and after exercise.

RESULT

There was a significant reduction in resting blood pressure following 10 wk of exercise training. Both Systolic and Diastolic blood pressure reduced significantly (p<0.001).

CONCLUSION

IHG exercise training might be a simple, effective, inexpensive and non-pharmacological method in lowering blood pressure.

Aerobic exercise and strength training effects on cardiovascular sympathetic function in healthy adults: a randomized controlled trial

OBJECTIVE

Exercise has widely documented cardioprotective effects, but the mechanisms underlying these effects are not entirely known. Previously, we demonstrated that aerobic but not strength training lowered resting heart rate and increased cardiac vagal regulation, changes that were reversed by sedentary deconditioning. Here, we focus on the sympathetic nervous system and test whether aerobic training lowers levels of cardiovascular sympathetic activity in rest and that deconditioning would reverse this effect.

METHODS

We conducted a randomized controlled trial contrasting the effects of aerobic (A) versus strength (S) training on indices of cardiac (preejection period, or PEP) and vascular (low-frequency blood pressure variability, or LF BPV) sympathetic regulation in 149 young, healthy, and sedentary adults. Participants were studied before and after conditioning, as well as after 4 weeks of sedentary deconditioning.

RESULTS

As previously reported, aerobic capacity increased in response to conditioning and decreased after deconditioning in the aerobic, but not the strength, training group. Contrary to prediction, there was no differential effect of training on either PEP (A: mean [SD] -0.83 [7.8] milliseconds versus S: 1.47 [6.69] milliseconds) or LF BPV (A: mean [SD] -0.09 [0.93] ln mm Hg(2) versus S: 0.06 [0.79] ln mm Hg(2)) (both p values > .05).

CONCLUSIONS

These findings, from a large randomized controlled trial using an intent-to-treat design, show that moderate aerobic exercise training has no effect on resting state cardiovascular indices of PEP and LF BPV. These results indicate that in healthy, young adults, the cardioprotective effects of exercise training are unlikely to be mediated by changes in resting sympathetic activity.

TRIAL REGISTRATION

Clinicaltrials.gov identifier: NCT00358137.

Selective enhancement of glutamate-mediated pressor responses after GABA(A) receptor blockade in the RVLM of sedentary versus spontaneous wheel running rats

Overactivity of the sympathetic nervous system (SNS) is a hallmark of many cardiovascular diseases. It is also well-known that physical inactivity independently contributes to cardiovascular diseases, likely in part via increased SNS activity. Recent work from our laboratory has demonstrated increased SNS responses in sedentary animals following either direct activation or disinhibition of the rostral ventrolateral medulla (RVLM), an integral cardiovascular brainstem region. These data led us to hypothesize that the interaction between excitation and inhibition of the RVLM is altered in sedentary versus physically active animals. To test this hypothesis, we recorded mean arterial pressure (MAP) and lumbar sympathetic nerve activity (LSNA) in Inactin anesthetized rats that were housed for 8-12 weeks with or without access to a running wheel. Pressor responses to direct activation of the RVLM with glutamate were similar between groups under intact conditions. However, blockade of γ-aminobutyric acid (GABA)(A) receptors with bicuculline selectively enhanced pressor responses to glutamate in sedentary animals. Interestingly, LSNA responses to glutamate were not enhanced in sedentary versus active animals in the presence or absence of tonic GABAergic tone. These results suggest that sedentary compared to active conditions enhance GABAergic inhibition of glutamate-sensitive neurons in the RVLM that are involved in blood pressure regulation, and by mechanisms that do not involve LSNA. We also speculate that regular physical activity has differential effects on SNS activity to specific vascular beds and may reduce the risk of developing cardiovascular diseases via changes occurring in the RVLM.

Effects of the intensity of leg isometric training on the vasculature of trained and untrained limbs and resting blood pressure in middle-aged men

The purpose of this study was to establish whether changes in resting blood pressure and the vasculature of trained and untrained limbs are dependent on training intensity, following isometric-leg training. Thirty middle-aged males undertook an 8 week training programme (4 × 2 min bilateral-leg isometric contractions 3 times per week). Two groups trained at either high (HI; 14%MVC) or low (LO; 8%MVC) intensity a third group (CON) acted as controls. All parameters were measured at baseline, 4-weeks and post-training. Resting SBP (−10.8 ± 7.9 mmHg), MAP (−4.7 ± 6.8 mmHg) and HR (−4.8 ± 5.9 b·min⁻¹) fell significantly in the HI group post-training with concomitant significant increases in resting femoral mean artery diameter (FMAD; 1.0 ± 0.4 mm), femoral mean blood velocity (FMBV; 0.68 ± 0.83 cm·s⁻¹), resting femoral artery blood flow (FABF; 82.06 ± 31.92 ml·min⁻¹) and resting femoral vascular conductance (FVC, 45%). No significant changes occurred in any brachial artery measure nor in any parameters measured in the LO or CON groups. These findings show that training-induced reductions in resting blood pressure after isometric-leg training in healthy middle-aged men are associated with concomitant adaptations in the local vasculature, that appear to be dependent on training intensity and take place in the later stages of training.

Impact of exercise training on arterial wall thickness in humans

Thickening of the carotid artery wall has been adopted as a surrogate marker of pre-clinical atherosclerosis, which is strongly related to increased cardiovascular risk. The cardioprotective effects of exercise training, including direct effects on vascular function and lumen dimension, have been consistently reported in asymptomatic subjects and those with cardiovascular risk factors and diseases. In the present review, we summarize evidence pertaining to the impact of exercise and physical activity on arterial wall remodelling of the carotid artery and peripheral arteries in the upper and lower limbs. We consider the potential role of exercise intensity, duration and modality in the context of putative mechanisms involved in wall remodelling, including haemodynamic forces. Finally, we discuss the impact of exercise training in terms of primary prevention of wall thickening in healthy subjects and remodelling of arteries in subjects with existing cardiovascular disease and risk factors.

(In)activity-dependent alterations in resting and reflex control of splanchnic sympathetic nerve activity

The negative effects of sympathetic overactivity on long-term cardiovascular health are becoming increasingly clear. Moreover, recent work done in animal models of cardiovascular disease suggests that sympathetic tone to the splanchnic vasculature may play an important role in the development and maintenance of these disease states. Work from our laboratory and others led us to hypothesize that a lack of chronic physical activity increases resting and reflex-mediated splanchnic sympathetic nerve activity, possibly through changes occurring in a key brain stem center involved in sympathetic regulation, the rostral ventrolateral medulla (RVLM). To address this hypothesis, we recorded mean arterial pressure (MAP) and splanchnic sympathetic nerve activity (SSNA) in a group of active and sedentary animals that had been housed for 10-13 wk with or without running wheels, respectively. In experiments performed under Inactin anesthesia, we tested responses to RVLM microinjections of glutamate, responses to baroreceptor unloading, and vascular reactivity, the latter of which was performed under conditions of autonomic blockade. Sedentary animals exhibited enhanced resting SSNA and MAP, augmented increases in SSNA to RVLM activation and baroreceptor unloading, and enhanced vascular reactivity to α(1)-receptor mediated vasoconstriction. Our results suggest that a sedentary lifestyle increases the risk of cardiovascular disease by augmenting resting and reflex-mediated sympathetic output to the splanchnic circulation and also by increasing vascular sensitivity to adrenergic stimulation. We speculate that regular physical exercise offsets or reverses the progression of these disease processes via similar or disparate mechanisms and warrant further examination into physical (in)activity-induced sympathetic nervous system plasticity.

The effect of 4 weeks of aerobic exercise on vascular and baroreflex function of young men with a family history of hypertension

The effect of short-term aerobic exercise on vascular function of young individuals with a family history of hypertension was investigated. Thirty young men with a family history of hypertension were randomly assigned to either an exercise (n=15) or control (n=15) group. Exercise subjects performed 30 min of supervised cycle training at 65% of their maximal oxygen uptake (VO(2max)), three times per week for 4 weeks. Control subjects were asked to maintain their normal levels of physical activity. Peak leg and forearm blood flow were assessed using plethysmography and was determined as the highest blood flow following 5 min of reactive hyperemia. Cardiopulmonary baroreceptor (CPBR) sensitivity was measured using lower body negative pressure (LBNP) for 5 min at -20 mm Hg. CPBR was determined by calculating change of stroke volume and forearm vascular resistance at baseline and during LBNP. Carotid baroreceptor (CBR) sensitivity was assessed using neck suction at -20, -40, -60 and -80 mm Hg pressures, and was determined from RR interval divided by systolic blood pressure. Augmentation index (AIx), a measure of arterial stiffness, was assessed using applanation tonometry and was calculated as the ratio of augmented pressure and pulse pressure. The major findings were that the exercise group showed increase in leg vasodilation, reduction in AIx and increase in VO(2max) compared with the control group (P<0.05). However, there was no significant change for CPBR and CBR. A short-term moderate-intensity aerobic exercise intervention in young men with a family history of hypertension significantly reduced arterial stiffness and increased aerobic fitness.

Exercise and vascular adaptation in asymptomatic humans

Beneficial effects of exercise training on the vasculature have been consistently reported in subjects with cardiovascular risk factors or disease, whereas studies in apparently healthy subjects have been less uniform. In this review, we examine evidence pertaining to the impact of exercise training on conduit and resistance vessel function and structure in asymptomatic subjects. Studies of arterial function in vivo have mainly focused on the endothelial nitric oxide dilator system, which has generally been shown to improve following training. Some evidence suggests that the magnitude of benefit depends upon the intensity or volume of training and the relative impact of exercise on upregulation of dilator pathways versus effects of inflammation and/or oxidation. Favourable effects of training on autonomic balance, baroreflex function and brainstem modulation of sympathetic control have been reported, but there is also evidence that basal vasoconstrictor tone increases as a result of training such that improvements in intrinsic vasodilator function and arterial remodelling are counterbalanced at rest. Studies of compliance suggest increases in both the arterial and the venous sides of the circulation, particularly in older subjects. In terms of mechanisms, shear stress appears to be a key signal to improvement in vascular function, whilst increases in pulse pressure and associated haemodynamics during bouts of exercise may transduce vascular adaptation, even in vascular beds which are distant from the active muscle. Different exercise modalities are associated with idiosyncratic patterns of blood flow and shear stress, and this may have some impact on the magnitude of exercise training effects on arterial function and remodelling. Other studies support the theory that that there may be different time course effects of training on specific vasodilator and constrictor pathways. A new era of understanding of the direct impacts of exercise and training on the vasculature is evolving, and future studies will benefit greatly from technological advances which allow direct characterization of arterial function and structure.

Physical (in)activity-dependent alterations at the rostral ventrolateral medulla: influence on sympathetic nervous system regulation

A sedentary lifestyle is a major risk factor for cardiovascular disease, and rates of inactivity and cardiovascular disease are highly prevalent in our society. Cardiovascular disease is often associated with overactivity of the sympathetic nervous system, which has both direct and indirect effects on multiple organ systems. Although it has been known for some time that exercise positively affects the brain in terms of memory and cognition, only recently have changes in how the brain regulates the cardiovascular system been examined in terms of physical activity and inactivity. This brief review will discuss the evidence for physical activity-dependent neuroplasticity related to control of sympathetic outflow. It will focus particularly on recent studies from our laboratory and others that have examined changes that occur in the rostral ventrolateral medulla (RVLM), considered one of the primary brain regions involved in the regulation and generation of sympathetic nervous system activity.

Impact of inactivity and exercise on the vasculature in humans

The effects of inactivity and exercise training on established and novel cardiovascular risk factors are relatively modest and do not account for the impact of inactivity and exercise on vascular risk. We examine evidence that inactivity and exercise have direct effects on both vasculature function and structure in humans. Physical deconditioning is associated with enhanced vasoconstrictor tone and has profound and rapid effects on arterial remodelling in both large and smaller arteries. Evidence for an effect of deconditioning on vasodilator function is less consistent. Studies of the impact of exercise training suggest that both functional and structural remodelling adaptations occur and that the magnitude and time-course of these changes depends upon training duration and intensity and the vessel beds involved. Inactivity and exercise have direct "vascular deconditioning and conditioning" effects which likely modify cardiovascular risk.

Muscle sympathetic nerve activity is related to a surrogate marker of endothelial function in healthy individuals

BACKGROUND

Evidence from animal studies indicates the importance of an interaction between the sympathetic nervous system and the endothelium for cardiovascular regulation. However the interaction between these two systems remains largely unexplored in humans. The aim of this study was to investigate whether directly recorded sympathetic vasoconstrictor outflow is related to a surrogate marker of endothelial function in healthy individuals.

METHODS AND RESULTS

In 10 healthy normotensive subjects (3 f/7 m), (age 37+/-11 yrs), (BMI 24+/-3 kg/m(2)) direct recordings of sympathetic action potentials to the muscle vascular bed (MSNA) were performed and endothelial function estimated with the Reactive Hyperaemia- Peripheral Arterial Tonometry (RH-PAT) technique. Blood samples were taken and time spent on leisure-time physical activities was estimated. In all subjects the rate between resting flow and the maximum flow, the Reactive Hyperemic index (RH-PAT index), was within the normal range (1.9-3.3) and MSNA was as expected for age and gender (13-44 burst/minute). RH-PAT index was inversely related to MSNA (r = -0.8, p = 0.005). RH-PAT index and MSNA were reciprocally related to time (h/week) spent on physical activity (p = 0.005 and p = 0.006 respectively) and platelet concentration (PLT) (p = 0.02 and p = 0.004 respectively).

CONCLUSIONS

Our results show that sympathetic nerve activity is related to a surrogate marker of endothelial function in healthy normotensive individuals, indicating that sympathetic outflow may be modulated by changes in endothelial function. In this study time spent on physical activity is identified as a predictor of sympathetic nerve activity and endothelial function in a group of healthy individuals. The results are of importance in understanding mechanisms underlying sympathetic activation in conditions associated with endothelial dysfunction and emphasise the importance of a daily exercise routine for maintenance of cardiovascular health.

Low-frequency electroacupuncture and physical exercise decrease high muscle sympathetic nerve activity in polycystic ovary syndrome

We have recently shown that polycystic ovary syndrome (PCOS) is associated with high muscle sympathetic nerve activity (MSNA). Animal studies support the concept that low-frequency electroacupuncture (EA) and physical exercise, via stimulation of ergoreceptors and somatic afferents in the muscles, may modulate the activity of the sympathetic nervous system. The aim of the present study was to investigate the effect of these interventions on sympathetic nerve activity in women with PCOS. In a randomized controlled trial, 20 women with PCOS were randomly allocated to one of three groups: low-frequency EA (n = 9), physical exercise (n = 5), or untreated control (n = 6) during 16 wk. Direct recordings of multiunit efferent postganglionic MSNA in a muscle fascicle of the peroneal nerve before and following 16 wk of treatment. Biometric, hemodynamic, endocrine, and metabolic parameters were measured. Low-frequency EA (P = 0.036) and physical exercise (P = 0.030) decreased MSNA burst frequency compared with the untreated control group. The low-frequency EA group reduced sagittal diameter (P = 0.001), while the physical exercise group reduced body weight (P = 0.004) and body mass index (P = 0.004) compared with the untreated control group. Sagittal diameter was related to MSNA burst frequency (Rs = 0.58, P < 0.005) in the EA group. No correlation was found for body mass index and MSNA in the exercise group. There were no differences between the groups in hemodynamic, endocrine, and metabolic variables. For the first time we demonstrate that low-frequency EA and physical exercise lowers high sympathetic nerve activity in women with PCOS. Thus, treatment with low-frequency EA or physical exercise with the aim to reduce MSNA may be of importance for women with PCOS.

Central sympathetic overactivity: maladies and mechanisms

There is growing evidence to suggest that many disease states are accompanied by chronic elevations in sympathetic nerve activity. The present review will specifically focus on central sympathetic overactivity and highlight three main areas of interest: 1) the pathological consequences of excessive sympathetic nerve activity; 2) the potential role of centrally derived nitric oxide in the genesis of neural dysregulation in disease; and 3) the promise of several novel therapeutic strategies targeting central sympathetic overactivity. The findings from both animal and human studies will be discussed and integrated in an attempt to provide a concise update on current work and ideas in these important areas.

No Effect of Training State on Ambulatory Measures of Cardiac Autonomic Control

We examined the effect of training state on cardiac autonomic control in a naturalistic setting. Twenty-four vigorous exercisers were compared to age- and sex-matched sedentary controls. The regular exercisers were subjected to a 6-week training program after which they were randomized to 2 weeks of continued training or 2 weeks of detraining. Cardiac autonomic control was measured over a 24-h period by ambulatory recording, using the preejection period (PEP) and respiratory sinus arrhythmia (RSA). Nonexercising controls had a significantly higher ambulatory heart rate (HR) compared to the regular exercisers but comparable 24-h levels of PEP and RSA. In regular exercisers, 2 weeks of detraining did not significantly change the 24-h levels of HR, PEP, or RSA. We conclude that the bradycardia in healthy regular exercisers is the result of a lower intrinsic heart rate rather than a shift in cardiac autonomic balance from sympathetic to vagal control.

Exercise training and sympathetic nervous system activity: evidence for physical activity dependent neural plasticity

1. It has been generally accepted that regular physical activity is associated with beneficial effects on the cardiovascular system. In fact, the idea that exercise maintains cardiovascular health is evident by the direct links between a sedentary lifestyle and the risk of cardiovascular and other disease states. 2. Cardiovascular diseases, such as hypertension and heart failure, are often associated with sympathetic nervous system (SNS) overactivity. Conversely, exercise has been shown to reduce hypertension and decrease elevated SNS activity. In addition, there is evidence that exercise may reduce resting blood pressure and sympathetic outflow in normal individuals. 3. Although somewhat controversial in humans, evidence from animal studies also indicates that exercise training reduces baroreflex-mediated and other forms of sympathoexcitation in normal individuals. Collectively, these data are consistent with the hypothesis that physical activity may decrease, and physical inactivity may increase, the incidence of cardiovascular disease via alterations in SNS activity. Despite the important clinical implications of this possibility, the mechanisms by which exercise alters control of SNS activity remain to be fully elucidated. 4. Recent evidence suggests that central nervous system (CNS) plasticity occurs under a variety of conditions, including varying levels of physical activity. The purpose of the present brief review is to provide evidence that changes within the CNS contribute importantly to altered regulation of the SNS observed following exercise training. The primary hypothesis is that physical activity versus inactivity produces plasticity within neural networks that regulate SNS activity. This hypothesis is supported by published and preliminary data that suggest that exercise training may reduce sympathoexcitation by reducing activation of neurons within cardiovascular regions of the brain. These mechanisms are likely to be important in disease states of sympathetic overactivity and in normal healthy individuals whose risk of cardiovascular disease is reduced by leading an active versus sedentary lifestyle.

Regulation of cardiac function during a cold pressor test in athletes and untrained subjects

By using (dP/dt)/P of carotid artery pulse, a non-invasive index of cardiac contractility, we examined the regulatory mechanism of cardiac function during a cold pressor test in athletes and untrained subjects. Twenty-four healthy subjects (9 athletes, 8 untrained subjects, and 7 hyperreactors of 4 athletes and 3 untrained subjects with a rise of 15 mmHg or greater in systolic and/or diastolic blood pressure) underwent the cold pressor test according to Hines and Brown (Am Heart J 11:1-9, 1936): immersion of the right hand in 4 degrees C water for 1 min. Although mean blood pressure increased during the cold stress in all the groups, cardiac function differed. In athletes, heart rate and cardiac contractility caused cardiac output to increase while total peripheral resistance (TPR) did not change. In untrained subjects, however, heart rate and cardiac contractility tended to decrease cardiac output and thus TPR increased. In hyperreactors, heart rate and cardiac contractility increased during cold stress, and also TPR increased. After the end of the test, heart rate and cardiac contractility decreased only in untrained group. The findings that during a cold pressor test heart rate and cardiac contractility are enhanced in athletes but depressed in untrained subjects indicate that the state of physical training influences cardiac sympathetic neural reactivity to cold stress, except for hyperreactors.

Parasympathetic reactivation after repeated sprint exercise

The purpose of this study was to examine the effects of muscular power engagement, anaerobic participation, aerobic power level, and energy expenditure on postexercise parasympathetic reactivation. We compared the response of heart rate (HR) after repeated sprinting with that of exercise sessions of comparable net energy expenditure and anaerobic energy contribution. Fifteen moderately trained athletes performed 1) 18 maximal all-out 15-m sprints interspersed with 17 s of passive recovery (RS), 2) a moderate isocaloric continuous exercise session (MC) at a level of mean oxygen uptake similar to that of the RS trial, and 3) a high-intensity intermittent exercise session (HI) conducted at a level of anaerobic energy expenditure similar to that of the RS trial. Subjects were immediately seated after the exercise trials, and beat-to-beat HR was recorded for 10 min. Parasympathetic reactivation was evaluated through 1) immediate postexercise HR recovery, 2) the time course of the root mean square for the successive R-R interval difference between successive 30-s segments (RMSSD(30s)) and 3) HR variability vagal-related indexes calculated for the last 5-min stationary period of recovery. RMSSD(30s) increased during the 10-min period after the MC trial, whereas RMSSD(30s) remained depressed after both the RS and HI trials. Parasympathetic reactivation indexes were similar for the RS and HI trials but lower than for the MC trial (P < 0.001). When data of the three exercise trials were considered together, only anaerobic contribution was related to HR trial-derived indexes. Parasympathetic reactivation is highly impaired after RS exercise and appears to be mainly related to anaerobic process participation.

Isometric handgrip training does not improve flow-mediated dilation in subjects with normal blood pressure

Isometric HG (handgrip) training lowers resting arterial BP (blood pressure), yet the mechanisms are elusive. In the present study, we investigated improved systemic endothelial function as a mechanism of arterial BP modification following isometric HG training in normotensive individuals. This study employed a within-subject repeated measures design primarily to assess improvements in BA FMD (brachial artery flow-mediated dilation; an index of endothelium-dependent vasodilation), with the non-exercising limb acting as an internal control. Eleven subjects performed four 2-min unilateral isometric HG contractions at 30% of maximal effort, three times per week for 8 weeks. Pre-, mid- and post-training resting ABP and BA FMD (exercised arm and non-exercised arm) were measured via automated brachial oscillometry and ultrasound respectively. BA FMD (normalized to the peak shear rate experienced in response to the reactive hyperaemic stimulus) remained unchanged [exercised arm, 0.029±0.003 to 0.026±0.003 to 0.029±0.004%/s−1 (pre- to mid- to post-training respectively); non-exercised arm, 0.023±0.003 to 0.023±0.003 to 0.024±0.003%/s−1 (pre- to mid- to post-training respectively); P=0.22]. In conclusion, improved systemic endothelial function is unlikely to be responsible for lowering arterial BP in this population.

Acute sympathetic vasoconstriction at rest and during dynamic exercise in cyclists and sedentary humans

The impact of exercise training on sympathetic activation is not well understood, especially across untrained and trained limbs in athletes. Therefore, in eight sedentary subjects (maximal oxygen consumption = 40 ± 2 ml·kg−1·min−1) and eight competitive cyclists (maximal oxygen consumption= 64 ± 2 ml·kg−1·min−1), we evaluated heart rate, blood pressure, blood flow, vascular conductance, and vascular resistance in the leg and arm during acute sympathetic stimulation [cold pressor test (CPT)]. The CPT was also performed during dynamic leg (knee extensor) or arm (handgrip) exercise at 50% of maximal work rate (WRmax) with measurements in the exercising limb. At rest, the CPT decreased vascular conductance similarly in the leg and arm of sedentary subjects (−33 ± 8% leg, −38 ± 6% arm) and cyclists (−34 ± 4% leg, −31 ± 9% arm), and during exercise CPT-induced vasoconstriction was blunted (i.e., sympatholysis) in both the leg and arm of both groups. However, the magnitude of sympatholysis was significantly different between the arm and leg of the sedentary group (−47 ± 11% arm, −25 ± 8% leg), and it was less in the arm of cyclists (−28 ± 11%) than sedentary controls. Taken together, these data provide evidence that sympathetically mediated vasoconstriction is expressed equally and globally at rest in both sedentary and trained individuals, with a differential pattern of vasoconstriction during acute exercise according to limb and exercise training status.

Different Types of Circulatory Responses to Mental Tasks

The present study investigated the circulatory responses to two mental tasks. Forty males and females performed a mental subtraction task and a color-word task. During each task, the systolic and diastolic blood pressure, mean arterial pressure, heart rate, stroke volume, cardiac output, and total peripheral resistance were measured as cardiovascular indices for a 5-min baseline, a 5-min task period, and a 10-min recovery period. As for the results, three hemodynamic reactivity patterns were verified: Pattern C, characterized by increased cardiac output and decreased total peripheral resistance; Pattern M, characterized by a moderate increase in both cardiac output and total peripheral resistance; and Pattern V, characterized by increased total peripheral resistance and decreased cardiac output. Also, four response types were found among all subjects: Type 1: cardiovascular responses showed the cardiac pattern for both tasks; Type 2: cardiovascular responses changed between the cardiac pattern and the mixed pattern with a change of tasks; Type 3: cardiovascular responses showed the mixed pattern for both tasks; Type 4: cardiovascular responses changed between the mixed pattern and the vascular pattern with a change of tasks. The comparison between types showed that Type 3 and Type 4 had an elevation in their blood pressure by an increased total peripheral resistance. On the other hand, Type 1 and Type 2 tended to have an increased blood pressure by a rise in their cardiac output. And Type 3 and Type 4 showed higher blood pressure and higher scores on the Type A behavior pattern questionnaire. In conclusion, at least four types of circulation response to the mental tasks existed, with Type 3 and Type 4 having higher blood pressure responses and tending to have an elevated blood pressure by a rise in their total peripheral resistance.

Effects of adaptive exercise on apoptosis in cells of rat renal tubuli

Regular exercise is known to improve physiological and functional capacity of many organs due to adaptive processes. We have previously shown that acute exercise in untrained rats results in apoptosis of renal tubular cells and that the apoptotic process seems to be associated with stimulation of angiotensin II, AT1 and AT2 receptors. In this study, we examined the influence of regular training on apoptosis and the role of angiotensin II receptors and antioxidant enzymes in mediating the adaptive response in renal tubular cells. We measured apoptosis, expression of AT1 and AT2 receptors, level of lipid peroxidation (TBARS) and activities of antioxidant enzymes, SOD, GPx and CAT in kidneys of sedentary rats that were exposed to acute exercise and rats that were trained for 8 weeks. In untrained animals, the acute exercise resulted in increased apoptosis and increased expression of AT1 and AT2 receptors in renal tubular cells, while in the rats exposed to the 8-week regular training, there were no changes in apoptosis nor AT1 and AT2 receptor expression as compared to the sedentary animals. The TBARS levels were significantly increased in acutely exercised rats, while in rats pre-exposed to the training they remained unchanged. The acute exercise, as well as regular training, did not change SOD, CAT or GPx activities. These findings suggested that the acute exercise-induced apoptosis in renal tubules could involve action of AT1 and AT2 receptors as well as oxidative stress, while the regular training was shown to prevent apoptosis in renal tubular cells via modulated expression of AT1 and AT2 receptors.

Effects of training resumption on conduit arterial diameter in elite rowers

BACKGROUND

Exercise training is a known stimulus for arteriogenesis, but it is unclear whether elite athletes, who exhibit increased conduit vessel diameter at rest, experience further structural vascular adaptations as a result of intense exercise training.

METHODS

Cross-sectional comparisons were performed between elite rowers (N = 17), following a respite from training, and eight untrained age- and gender-matched controls to assess the effects of long-term exercise on vessel structure. To determine the impact of the resumption of intensive exercise training on conduit artery structure, measures were repeated following 3 and 6 months of training in the athletes; the controls remained inactive. Conduit vessel structure was assessed, using high-resolution B-mode ultrasound, as brachial artery diameter at rest (BADr) and in response to 5-min (BAD5) and 10-min (BAD10) periods of forearm cuff ischemia. Shear rate profiles were also analyzed following cuff deflation at all time points.

RESULTS

At entry, all measures of BAD were greater (all P < 0.05) in the athletes relative to controls (athletes vs controls; BADr 4.47 +/- 0.10 vs 3.84 +/- 0.22 mm; BAD5 4.70 +/- 0.10 vs 4.05 +/- 0.36 mm, and BAD10 4.93 +/- 0.10 vs 4.07 +/- 0.25 mm). Resumption of exercise training caused a further increase in brachial artery diameters in the athletes at 3 months (BADr, 4.71 +/- 0.10 mm, P < 0.01; BAD5 4.94 +/- 0.10 mm, P < 0.05; BAD10 5.12 +/- 0.10 mm, P < 0.001), which were maintained, but not further increased, after 6 months of training.

CONCLUSIONS

Athletes exhibit enhanced conduit artery diameters at rest and in response to vasodilator stimuli. Despite this long-term training effect on arterial structure, resumption of training further enhances diameter, an effect that occurs within 3 months.

Sympathetic neural regulation in endurance-trained humans: fitness vs. fatness

We tested the hypothesis that muscle sympathetic nerve activity (MSNA) would be higher in endurance-trained (ET) compared with sedentary (Sed) men with similar levels of total body and abdominal adiposity. We further hypothesized that sympathetic baroreflex gain would be augmented in ET compared with Sed men independent of the level of adiposity. To address this, we measured MSNA (via microneurography), sympathetic and vagal baroreflex responses (the modified Oxford technique), body composition (dual-energy X-ray absorptiometry), and waist circumference (Gulick tape) in Sed ( n = 22) and ET men ( n = 8). The ET men were also compared with a subgroup of Sed men ( n = 6) with similar levels of total body and abdominal adiposity. Basal MSNA was greater in the ET compared with Sed men with similar levels of total body and abdominal adiposity (28 ± 2.0 vs. 21 ± 2.0 bursts/min; P < 0.05) but similar to the larger group of Sed men ( n = 22) with higher total body and abdominal adiposity (vs. 26 ± 3 bursts/min; P > 0.05). In contrast to our hypothesis, sympathetic baroreflex gain was lower in the ET compared with Sed men (−6.4 ± 0.8 vs. −8.4 ± 0.4 arbitrary integrative units·beat−1·mmHg−1; P < 0.05) regardless of the level of adiposity. Taken together, the results of the present study suggest that MSNA is higher in ET compared with Sed men with similar levels of total body and abdominal adiposity. In addition, sympathetic baroreflex gain is lower in ET compared with Sed men. That sympathetic baroreflex gain was lower in ET compared with Sed men regardless of the level of adiposity suggests an influence of the ET state per se.

American College of Sports Medicine position stand. Exercise and hypertension

Hypertension (HTN), one of the most common medical disorders, is associated with an increased incidence of all-cause and cardiovascular disease (CVD) mortality. Lifestyle modifications are advocated for the prevention, treatment, and control of HTN, with exercise being an integral component. Exercise programs that primarily involve endurance activities prevent the development of HTN and lower blood pressure (BP) in adults with normal BP and those with HTN. The BP lowering effects of exercise are most pronounced in people with HTN who engage in endurance exercise with BP decreasing approximately 5-7 mm HG after an isolated exercise session (acute) or following exercise training (chronic). Moreover, BP is reduced for up to 22 h after an endurance exercise bout (e.g.postexercise hypotension), with greatest decreases among those with highest baseline BP. The proposed mechanisms for the BP lowering effects of exercise include neurohumoral, vascular, and structural adaptations. Decreases in catecholamines and total peripheral resistance, improved insulin sensitivity, and alterations in vasodilators and vasoconstrictors are some of the postulated explanations for the antihypertensive effects of exercise. Emerging data suggest genetic links to the BP reductions associated with acute and chronic exercise. Nonetheless, definitive conclusions regarding the mechanisms for the BP reductions following endurance exercise cannot be made at this time. Individuals with controlled HTN and no CVD or renal complications may participated in an exercise program or competitive athletics, but should be evaluated, treated and monitored closely. Preliminary peak or symptom-limited exercise testing may be warranted, especially for men over 45 and women over 55 yr planning a vigorous exercise program (i.e. > or = 60% VO2R, oxygen uptake reserve). In the interim, while formal evaluation and management are taking place, it is reasonable for the majority of patients to begin moderate intensity exercise (40-<60% VO2R) such as walking. When pharmacological therapy is indicated in physically active people it should be, ideally: a) lower BP at rest and during exertion; b) decrease total peripheral resistance; and, c) not adversely affect exercise capacity. For these reasons, angiotensin converting enzyme (ACE) inhibitors (or angiotensin II receptor blockers in case of ACE inhibitor intolerance) and calcium channel blockers are currently the drugs of choice for recreational exercisers and athletes who have HTN. Exercise remains a cornerstone therapy for the primary prevention, treatment, and control of HTN. The optimal training frequency, intensity, time, and type (FITT) need to be better defined to optimize the BP lowering capacities of exercise, particularly in children, women, older adults, and certain ethnic groups. based upon the current evidence, the following exercise prescription is recommended for those with high BP: Frequency: on most, preferably all, days of the week. Intensity: moderate-intensity (40-<60% VO2R). Time: > or = 30 min of continuous or accumulated physical activity per day. Type: primarily endurance physical activity supplemented by resistance exercise.

Measurements of vascular function using strain-gauge plethysmography: technical considerations, standardization, and physiological findings

The main purpose of the present study was to examine the relationships between measures of fitness [estimated peak oxygen consumption (V̇o2 peak) and handgrip strength] and forearm vascular function in 55 young (22.6 ± 3.5 yr) adults. In addition, the present study considered methodological and technical aspects regarding the examination of the venous system using mercury in-Silastic strain-gauge plethysmography (MSGP). Forearm venous capacitance and outflow were examined using five different [7, 14, 21, 28, and 35 mmHg < diastolic blood pressure (DBP)] venous occlusion pressures and after a 5- and 10-min period of venous occlusion. A pressure of 7 mmHg < DBP and a period of 10 min venous occlusion produced the greatest ( P < 0.05) venous capacitance and outflow, without altering arterial indexes. Reproducibility of forearm arterial and venous indexes were evaluated at rest and after 5 min of upper arm arterial occlusion at 240 mmHg on three different occasions within 10 days with the interclass correlation coefficient ranging from 0.70 and 0.94. Estimated V̇o2 peak correlated with postocclusion arterial inflow ( r = 0.54, P = 0.012) and resting venous outflow ( r = 0.56, P = 0.016). Finally, handgrip strength was associated with venous capacitance ( r = 0.57, P = 0.007) and outflow ( r = 0.67, P = 0.001). These results indicate that the examination of forearm vascular function using MSGP is reproducible. Moreover, the data show the importance of careful consideration of the selection of venous occlusion pressure and period when implementing these measures in longitudinal trials. Finally, the associations between fitness and venous measures suggest a link between venous function and exercise performance.

Blood pressure and muscle sympathetic nerve activity during cold pressor stress: fitness and gender

We examined putative autonomic and hemodynamic mechanisms that might explain our prior finding that cardiorespiratory fitness mitigates blood pressure responses by normotensive women during the hand cold pressor test. We report that fitness level was inversely related to increases in systolic and diastolic blood pressures and muscle sympathetic nerve activity (MSNA) during the cold pressor among women but not men. The pattern of responses among fitter women was consistent with decreased central sympathetic outflow resulting in reduced stroke volume or dampened peripheral resistance in vascular beds other than calf skeletal muscle. Fitter men and women had slightly larger increases in blood pressure during mental arithmetic, but otherwise fitness was not directly related to stress responses. The results further encourage consideration of cardiorespiratory fitness as a modifying covariate when the hand cold pressor test is used as a predictor of future hypertension among women.

Effects of dynamic resistance training on heart rate variability in healthy older women

Twenty healthy women aged between 65 and 74 years, trained three times a week, for 16 weeks, on a cycle ergometer, to determine the effects of dynamic resistance training on heart rate variability (HRV). Subjects were allocated to two training groups, high (HI, n=10) and low (LO, n=10) intensity. The HI group performed eight sets of 8 revolutions at 80% of the maximum resistance to complete 2 pedal revolutions (2RM); the LO group performed eight sets of 16 pedal revolutions at 40% of 2RM. Subjects were tested twice before, as control period (-4 weeks and 0 weeks) and once after training (16 weeks) for HRV, maximum voluntary contraction (MVC) of knee extensors and peak power (P(p)) of lower limbs by jumping on a force platform. HRV was measured using time and frequency domain parameters. Two-way ANOVA for repeated measures was performed on all variables (P<0.05). Results showed no differences between training groups. Following training HRV was not modified, while MVC and P(p) significantly increased. The two proposed forms of dynamic resistance training were appropriate to improve muscle power and strength in elderly females without affecting HRV. More research should verify the effects of an isometric and more prolonged training stimulus on HRV in older subjects.

Training reduces autonomic cardiovascular responses to both exercise-dependent and -independent stimuli in humans

Training attenuates the sympathetic pressor response to dynamic exercise. However, it is uncertain how training alters other patterns of cardiovascular autonomic activation. Therefore, we have quantified circulatory responses to a series of standard autonomic tests in highly fit and unfit subjects and examined the effects of a short-term training programme on these responses. Subjects were defined as either unfit (n = 8) or fit (n = 8) on the basis of training history and a maximal fitness test (VO2peak 54 +/- 2.3 cf. 68 +/- 2.8 (ml min(-1)) kg(-1), means + S.E.M., P < 0.05). On a separate day, the blood pressure, heart rate and forearm vascular conductance responses to a sustained handgrip to fatigue, 2 min mental arithmetic and 2 min of cold exposure were measured. All stimuli were associated with elevated blood pressures and heart rates, but these responses were significantly attenuated in the trained group. In the untrained subjects, forearm vascular conductance increased during exercise (from 0.032 +/- 0.004 to 0.05 +/- 0.007 (ml min(-1)) 100 ml(-1) mm Hg(-1), P < 0.05) and during mental arithmetic (from 0.028 +/- 0.003 to 0.04 +/- 0.006 (ml min(-1)) 100 ml(-1) mm Hg(-1) , p < 0.05), but trained subjects showed no rise in conductance during either test. All untrained subjects undertook a moderate intensity 5-week training programme, which significantly increased VO2peak (54 +/- 2.3 to 57 +/- 2 (ml min(-1)) kg(-1), p < 0.05). Qualitatively similar blunting of pressor, tachycardic and vasodilator responses were seen in this group post-training. These results demonstrate that the blunting of sympathetic vasomotor activation that follows training is not restricted to reflexes associated with exercise, and does not depend on training being prolonged or intense.

Differential effects of training on the control of skeletal muscle perfusion

Endurance and high-intensity sprint training have been shown to alter skeletal muscle blood flow and factors that govern muscle perfusion under various conditions. Neither endurance nor sprint training alter skeletal muscle perfusion at rest but can result in an increase in muscle blood flow during the anticipation of exercise. The magnitude of the anticipatory increases in muscle blood flow is dependent on the intensity and duration of the prior training bouts and results from elevations in mean arterial pressure and decreases in vascular resistance in skeletal muscle. The decrements in skeletal muscle vascular resistance appear to be mediated through increases in muscle sympathetic cholinergic nerve activity or decreases in muscle sympathetic adrenergic nerve activity. During submaximal exercise, total muscle blood flow is either unchanged or slightly lower. However, a redistribution of muscle blood flow may occur following aerobic training, resulting in an enhanced perfusion of high-oxidative skeletal muscles and less flow going to low-oxidative muscles. The increased perfusion of the high-oxidative muscles may result from various factors including: a) increased recruitment of high-oxidative motor units, b) increased local release of metabolic vasodilator substances, c) qualitative changes in the metabolic substances released, d) decreased muscle sympathetic nerve activity, e) diminished sensitivity of the arterial vasculature to norepinephrine or other vasoconstrictor agents, f) enhanced endothelium-mediated dilation in the resistance vasculature, and g) an increased effectiveness of the skeletal muscle pump. Conversely, the decreases in blood flow to low-oxidative muscles may result from an enhanced autoregulatory responsiveness of the resistance vasculature. Endurance and sprint training increase muscle perfusion during exercise at VO2max: this primarily appears to be the result of an enhanced pumping capacity of the heart to increase in maximal cardiac output. Many of the training-induced alterations in muscle blood flow and vascular structure are localized in the muscles that are most active during the training bouts. Therefore, differences in muscle recruitment patterns that occur with low-intensity endurance exercise and high-intensity sprint exercise may account for differences observed between these two training regimens.