Anthony V. Incognito

Acute beetroot juice supplementation on sympathetic nerve activity: a randomized, double-blind, placebo-controlled proof-of-concept study

Acute dietary nitrate ([Formula: see text]) supplementation reduces resting blood pressure in healthy normotensives. This response has been attributed to increased nitric oxide bioavailability and peripheral vasodilation, although nitric oxide also tonically inhibits central sympathetic outflow. We hypothesized that acute dietary [Formula: see text] supplementation using beetroot (BR) juice would reduce blood pressure and muscle sympathetic nerve activity (MSNA) at rest and during exercise. Fourteen participants (7 men and 7 women, age: 25 ± 10 yr) underwent blood pressure and MSNA measurements before and after (165-180 min) ingestion of 70ml high-[Formula: see text] (~6.4 mmol [Formula: see text]) BR or [Formula: see text]-depleted BR placebo (PL; ~0.0055 mmol [Formula: see text]) in a double-blind, randomized, crossover design. Blood pressure and MSNA were also collected during 2 min of static handgrip (30% maximal voluntary contraction). The changes in resting MSNA burst frequency (-3 ± 5 vs. 3 ± 4 bursts/min, P = 0.001) and burst incidence (-4 ± 7 vs. 4 ± 5 bursts/100 heart beats, P = 0.002) were lower after BR versus PL, whereas systolic blood pressure (-1 ± 5 vs. 2 ± 5 mmHg, P = 0.30) and diastolic blood pressure (4 ± 5 vs. 5 ± 7 mmHg, P = 0.68) as well as spontaneous arterial sympathetic baroreflex sensitivity (P = 0.95) were not different. During static handgrip, the change in MSNA burst incidence (1 ± 8 vs. 8 ± 9 bursts/100 heart beats, P = 0.04) was lower after BR versus PL, whereas MSNA burst frequency (6 ± 6 vs. 11 ± 10 bursts/min, P = 0.11) as well as systolic blood pressure (11 ± 7 vs. 12 ± 8 mmHg, P = 0.94) and diastolic blood pressure (11 ± 4 vs. 11 ± 4 mmHg, P = 0.60) were not different. Collectively, these data provide proof of principle that acute BR supplementation can decrease central sympathetic outflow at rest and during exercise. Dietary [Formula: see text] supplementation may represent a novel intervention to target exaggerated sympathetic outflow in clinical populations.NEW & NOTEWORTHY The hemodynamic benefits of dietary nitrate supplementation have been attributed to nitric oxide-mediated peripheral vasodilation. Here, we provide proof of concept that acute dietary nitrate supplementation using beetroot juice can decrease muscle sympathetic outflow at rest and during exercise in a normotensive population. These results have applications for targeting central sympathetic overactivation in disease.

Validity and reliability of measuring resting muscle sympathetic nerve activity using short sampling durations in healthy humans

Resting muscle sympathetic nerve activity (MSNA) demonstrates high intraindividual reproducibility when sampled over 5-30 min epochs, although shorter sampling durations are commonly used before and during a stress to quantify sympathetic responsiveness. The purpose of the present study was to examine the intratest validity and reliability of MSNA sampled over 2 and 1 min and 30 and 15 s epoch durations. We retrospectively analyzed 68 resting fibular nerve microneurographic recordings obtained from 53 young, healthy participants (37 men; 23 ± 6 yr of age). From a stable 7-min resting baseline, MSNA (burst frequency and incidence, normalized mean burst amplitude, total burst area) was compared among each epoch duration and a standard 5-min control. Bland-Altman plots were used to determine agreement and bias. Three sequential MSNA measurements were collected using each sampling duration to calculate absolute and relative reliability (coefficients of variation and intraclass correlation coefficients). MSNA values were similar among each sampling duration and the 5-min control (all P > 0.05), highly correlated (r = 0.69-0.93; all P < 0.001), and demonstrated no evidence of fixed bias (all P > 0.05). A consistent proportional bias (P < 0.05) was present for MSNA burst frequency (all sampling durations) and incidence (1 min and 30 and 15 s), such that participants with low and high average MSNA underestimated and overestimated the true value, respectively. Reliability decreased progressively using the 30- and 15-s sampling durations. In conclusion, short 2 and 1 min and 30 s sampling durations can provide valid and reliable measures of MSNA, although increased sample size may be required for epochs ≤30 s, due to poorer reliability.

Muscle sympathetic nerve responses to passive and active one-legged cycling: insights into the contributions of central command

The contribution of central command to the peripheral vasoconstrictor response during exercise has been investigated using primarily handgrip exercise. The purpose of the present study was to compare muscle sympathetic nerve activity (MSNA) responses during passive (involuntary) and active (voluntary) zero-load cycling to gain insights into the effects of central command on sympathetic outflow during dynamic exercise. Hemodynamic measurements and contralateral leg MSNA (microneurography) data were collected in 18 young healthy participants at rest and during 2 min of passive and active zero-load one-legged cycling. Arterial baroreflex control of MSNA burst occurrence and burst area were calculated separately in the time domain. Blood pressure and stroke volume increased during exercise ( P < 0.0001) but were not different between passive and active cycling ( P > 0.05). In contrast, heart rate, cardiac output, and total vascular conductance were greater during the first and second minute of active cycling ( P < 0.001). MSNA burst frequency and incidence decreased during passive and active cycling ( P < 0.0001), but no differences were detected between exercise modes ( P > 0.05). Reductions in total MSNA were attenuated during the first ( P < 0.0001) and second ( P = 0.0004) minute of active compared with passive cycling, in concert with increased MSNA burst amplitude ( P = 0.02 and P = 0.005, respectively). The sensitivity of arterial baroreflex control of MSNA burst occurrence was lower during active than passive cycling ( P = 0.01), while control of MSNA burst strength was unchanged ( P > 0.05). These results suggest that central feedforward mechanisms are involved primarily in modulating the strength, but not the occurrence, of a sympathetic burst during low-intensity dynamic leg exercise. NEW & NOTEWORTHY Muscle sympathetic nerve activity burst frequency decreased equally during passive and active cycling, but reductions in total muscle sympathetic nerve activity were attenuated during active cycling. These results suggest that central command primarily regulates the strength, not the occurrence, of a muscle sympathetic burst during low-intensity dynamic leg exercise.

Ischemic preconditioning does not alter muscle sympathetic responses to static handgrip and metaboreflex activation in young healthy men

Ischemic preconditioning (IPC) has been hypothesized to elicit ergogenic effects by reducing feedback from metabolically sensitive group III/IV muscle afferents during exercise. If so, reflex efferent neural outflow should be attenuated. We investigated the effects of IPC on muscle sympathetic nerve activity (MSNA) during static handgrip (SHG) and used post‐exercise circulatory occlusion (PECO) to isolate for the muscle metaboreflex. Thirty‐seven healthy men (age: 24 ± 5 years [mean ± SD]) were randomized to receive sham (n = 16) or IPC (n = 21) interventions. Blood pressure, heart rate, and MSNA (microneurography; sham n = 11 and IPC n = 18) were collected at rest and during 2 min of SHG (30% maximal voluntary contraction) and 3 min of PECO before (PRE) and after (POST) sham or IPC treatment (3 × 5 min 20 mmHg or 200 mmHg unilateral upper arm cuff inflation). Resting mean arterial pressure was higher following sham (79 ± 7 vs. 83 ± 6 mmHg, P < 0.01) but not IPC (81 ± 6 vs. 82 ± 6 mmHg, P > 0.05), while resting MSNA burst frequency was unchanged (P > 0.05) with sham (18 ± 7 vs. 19 ± 9 bursts/min) or IPC (17 ± 7 vs. 19 ± 7 bursts/min). Mean arterial pressure, heart rate, stroke volume, cardiac output, and total vascular conductance responses during SHG and PECO were comparable PRE and POST following sham and IPC (All P > 0.05). Similarly, MSNA burst frequency, burst incidence, and total MSNA responses during SHG and PECO were comparable PRE and POST with sham and IPC (All P > 0.05). These findings demonstrate that IPC does not reduce hemodynamic responses or central sympathetic outflow directed toward the skeletal muscle during activation of the muscle metaboreflex using static exercise or subsequent PECO.

Commentaries on Viewpoint: Could small-diameter muscle afferents be responsible for the ergogenic effect of limb ischemic preconditioning?

TO THE EDITOR: Cruz and colleagues (3) suggested that the ergogenic effect of ischemic preconditioning (IP) is in part caused by a reduced activity of sensory muscle afferents (SMA). This is an intriguing hypothesis that also further highlights some important implications of SMA for endurance performance. However, given the complex and integrative role of SMA, some points should be considered. First, unlike IP, spinal blockade of SMA did not provide any ergogenic effect on healthy subjects (1, 2), albeit the last most probably has a stronger suppression of SMA activity. Second, blockade of SMA demonstrated that perception of effort (RPE) is independent of SMA activity (4) and therefore changes in RPE after IP, should not be caused by a reduced activity of SMA. Finally, the ergogenic effect of IP might be also caused by a placebo effect. Indeed, the inability to effectively perform a sham-control IP treatment still remains. The placebo effect mainly relies on the assumption that participant believes that the intervention will alter results. For IP treatment, sham procedure commonly involves a very low cuff pressure that does not induce the same sensation experienced during IP treatment. Therefore participant expectancy about the treatment is unpredictable and might explain the improvement in performance and/or an altered pacing strategy (3, 5). Accordingly, future experiments should deserve more attention to reduce this confounding variable. In conclusion, future studies are required to confirm this hypothesis and more research is needed to understand the physiological mechanisms responsible for the ergogenic effect of IP on exercise performance.

Interindividual variability in muscle sympathetic responses to static handgrip in young men: evidence for sympathetic responder types?

Negative and positive muscle sympathetic nerve activity (MSNA) responders have been observed during mental stress. We hypothesized that similar MSNA response patterns could be identified during the first minute of static handgrip and contribute to the interindividual variability throughout exercise. Supine measurements of multiunit MSNA (microneurography) and continuous blood pressure (Finometer) were recorded in 29 young healthy men during the first (HG1) and second (HG2) minute of static handgrip (30% maximal voluntary contraction) and subsequent postexercise circulatory occlusion (PECO). Responders were identified on the basis of differences from the typical error of baseline total MSNA: 7 negative, 12 positive, and 10 nonresponse patterns. Positive responders demonstrated larger total MSNA responses during HG1 ( P < 0.01) and HG2 ( P < 0.0001); however, the increases in blood pressure throughout handgrip exercise were similar between all groups, as were the changes in heart rate, stroke volume, cardiac output, total vascular conductance, and respiration (all P > 0.05). Comparing negative and positive responders, total MSNA responses were similar during PECO ( P = 0.17) but opposite from HG2 to PECO (∆40 ± 46 vs. ∆-21 ± 62%, P = 0.04). Negative responders also had a shorter time-to-peak diastolic blood pressure during HG1 (20 ± 20 vs. 44 ± 14 s, P < 0.001). Total MSNA responses during HG1 were associated with responses to PECO ( r = 0.39, P < 0.05), the change from HG2 to PECO ( r = -0.49, P < 0.01), and diastolic blood pressure time to peak ( r = 0.50, P < 0.01). Overall, MSNA response patterns during the first minute of static handgrip contribute to interindividual variability and appear to be influenced by differences in central command, muscle metaboreflex activation, and rate of loading of the arterial baroreflex.

Cutaneous mechanoreceptor feedback from the hand and foot can modulate muscle sympathetic nerve activity

Stimulation of high threshold mechanical nociceptors on the skin can modulate efferent sympathetic outflow. Whether low threshold mechanoreceptors from glabrous skin are similarly capable of modulating autonomic outflow is unclear. Therefore, the purpose of this study was to examine the effects of cutaneous afferent feedback from the hand palm and foot sole on efferent muscle sympathetic nerve activity (MSNA). Fifteen healthy young participants (9 male; 25 ± 3 years [range: 22–29]) underwent microneurographic recording of multi-unit MSNA from the right fibular nerve during 2 min of baseline and 2 min of mechanical vibration (150 Hz, 220 μm peak-to-peak) applied to the left hand or foot. Each participant completed three trials of both hand and foot stimulation, each separated by 5 min. MSNA burst frequency decreased similarly during the 2 min of both hand (20.8 ± 8.9 vs. 19.3 ± 8.6 bursts/minute [Δ −8%], p = 0.035) and foot (21.0 ± 8.3 vs. 19.5 ± 8.3 bursts/minute [Δ −8%], p = 0.048) vibration but did not alter normalized mean burst amplitude or area (All p > 0.05). Larger reductions in burst frequency were observed during the first 10 s (onset) of both hand (20.8 ± 8.9 vs. 17.0 ± 10.4 [Δ −25%], p < 0.001) and foot (21.0 ± 8.3 vs. 18.3 ± 9.4 [Δ −16%], p = 0.035) vibration, in parallel with decreases in normalized mean burst amplitude (hand: 0.45 ± 0.06 vs. 0.36 ± 0.14% [Δ −19%], p = 0.03; foot: 0.47 ± 0.07 vs. 0.34 ± 0.19% [Δ −27%], p = 0.02) and normalized mean burst area (hand: 0.42 ± 0.05 vs. 0.32 ± 0.12% [Δ −25%], p = 0.003; foot: 0.47 ± 0.05 vs. 0.34 ± 0.16% [Δ −28%], p = 0.01). These results demonstrate that tactile feedback from the hands and feet can influence efferent sympathetic outflow to skeletal muscle.

Muscle sympathetic outflow during exercise: a tale of two limbs

Central sympathetic outflow to different skeletal muscle vascular beds (muscle sympathetic nerve activity; MSNA) during exercise is believed to be largely uniform (Ray et al. 1992), leaving the redistribution of blood flow to be regulated by local metabolic factors, including the disruption of neurally mediated vasoconstriction (i.e. functional sympatholysis). However, the assessment of MSNA in humans using microneurographic techniques is limited by the requirement to minimize limb movement and muscle activation. As a result, the majority of prior work has measured MSNA from an inactive limb (e.g. Ray et al. 1992). More recently, advancements in microneurographic signal denoising and sympathetic action potential identification have permitted the examination of the MSNA response in the active limb during exercise (Boulton et al. 2014). These results now provide the capacity to investigate whether sympathetic outflow can be controlled differentially between active and non-active limbs. In a recent publication in The Journal of Physiology, Boulton and colleagues (2018) have answered this question, studying the MSNA responses during low-intensity ipsilateral and contralateral isometric dorsiflexion of the foot. Contractions were performed unilaterally for 4 min at 10% of maximal volitional effort in three randomized conditions: (1) freely perfused contraction and recovery (termed no ischaemia), (2) freely perfused contraction and circulatory occluded recovery (termed post-exercise ischaemia) and (3) circulatory occluded contraction and recovery (termed continuous ischaemia). Circulatory occlusions were achieved by pneumatic pressure cuff inflation around the upper thigh. MSNA was assessed by identification of negative deflections from the raw neurogram, indicative of sympathetic action potentials, and quantified as spike frequency (spikes min−1). This method differs from conventional burst analysis of the integrated neurogram but is advantageous when recording from the active limb as it allows discrimination of positive deflecting neural activity related to motor activation or muscle spindle and Golgi tendon organ afferent feedback, all of which are active during contraction. The results demonstrated that MSNA to the non-active leg increased progressively throughout the contraction, becoming significantly elevated from baseline after 2 min, and remained elevated during ischaemia, while MSNA in the active leg increased rapidly (within the first minute of contraction), plateaued during contraction and returned to baseline immediately following contraction, even during ischaemia. As a result, the authors concluded that central command (input from higher order brain regions) was responsible for control of MSNA to the contracting muscles, while activation of metabolically sensitive afferents (muscle metaboreflex) was responsible for MSNA responses in the non-contracting leg. These results provide strong evidence for the existence of limb-specific differential control of sympathetic outflow and bring forth novel hypotheses for skeletal muscle blood flow regulation and the integrated control of the sympathetic nervous system during exercise. The strongest evidence that MSNA to the active limb is influenced predominantly by central command was the rapid increase from baseline in the active limb. Noteworthy, however, was the MSNA activation in the non-active limb after the first minute of non-ischaemic exercise, as well as an accentuated response during ischaemic exercise, indicating chemosensitive group III/IV afferent firing in all conditions (though no measurable onset of fatigue given consistent electromyographic recordings). Group III/IV afferent activation has been shown to elicit an inhibitory influence on central motor output (Sidhu et al. 2017), requiring central command to progressively increase over time during non-ischaemic exercise and during ischaemic exercise to maintain consistent force output. The observations of a plateau in MSNA after the first minute of contraction in the active limb and the similarities in MSNA magnitude between non-ischaemic and ischaemic exercise do not parallel this expected difference. Perhaps this was related to the use of low-intensity static dorsiflexion. Whether similar responses are present at higher intensities under stronger muscle metaboreflex activation or during other modes of exercise is unclear. For example, a change in heart rate would indicate a vagal withdrawal (a hallmark of central command), yet this was absent during the exercise protocol. How these results compare to other modes of exercise is an important issue. Static leg extension at 10% MVC was observed to elicit a reduction in MSNA in the non-active limb in the first minute of exercise and no change from baseline in the second minute of exercise (Ray et al. 1992). The progressive increase in non-active limb MSNA in the present study, however, more closely resembles the MSNA responses observed during static handgrip exercise (Ray et al. 1992). Additionally, our laboratory recently compared passive versus unloaded (minimal metaboreflex activation) one-legged cycling to probe the contributions of central command and demonstrated increases in multi-unit MSNA burst amplitude, but not occurrence, in the non-active leg during the first and second minute in the dynamic exercise condition (Doherty et al. 2017). Given that the authors established in a previous investigation that their measure of MSNA using spike frequency was most closely representative of MSNA burst amplitude, as opposed to burst frequency or total MSNA, the absence of a rapid increase in MSNA in the non-active limb during a static contraction is in contrast to our findings. Whether this reflects differences in the sympathetic response to static and dynamic exercise or muscle mass recruited is unclear. The most important consideration of the current findings is that the assumption of MSNA measured from the non-active limb during exercise as reflective of active limb sympathetic outflow can no longer be accepted. An emphasis should now be placed on describing the contribution of

Pharmacological assessment of the arterial baroreflex in a young healthy obese male with extremely low baseline muscle sympathetic nerve activity

Multi-unit microneurographic recordings of muscle sympathetic nerve activity (MSNA) in humans have shown sympathetic impulses to present as pulse-synchronous bursts under baroreflex control, increasing in response to spontaneous diastolic blood pressure (DBP) reductions and inhibited during elevations [1]. Studies commonly report large interindividual variability in baseline MSNA (42–84 bursts/100 heartbeats in upright sitting), which may in part be exacerbated by the supine posture (range increased to 10–83 bursts/100 heartbeats) [1]. The increase of low-end MSNA values when supine could be due to cardiopulmonary baroreceptor inhibition of MSNA via variable elevations in central venous pressure [1, 2] and/or from significant reductions in the magnitude of spontaneous blood pressure fluctuations [1, 3], which would induce a more tonic loading of the arterial baroreflex. Theoretically, if strong inhibitory mechanisms maintain baseline tonicity, we would expect some individuals to possess absent baseline MSNA. This has yet to be reported, potentially due to the mislabeling of individuals as failed microneurographic recordings. In this case report, testing followed written informed consent and study procedures were approved by the University of Brasilia institutional research committee in accordance with the Declaration of Helsinki. A young (32 years), obese (height 180 cm; weight 110 kg; BMI 34 kg/m2), unmedicated male, with no previously diagnosed medical conditions, was assessed. Using the microneurography technique in the right common fibular nerve, we observed near-absent baseline MSNA. Usually, microneurographic experiments are terminated when MSNA bursts are absent at baseline, as this often indicates distant microelectrode placement to active postganglionic muscle sympathetic efferent fibres. Indeed, we changed the microelectrode recording site multiple times during the 1 h search, repeatably finding nerves directed towards skeletal muscle (indicative by auditory feedback during tapping/palpation of the tibialis anterior/ peroneal muscles and absent auditory feedback from light stroking of skin on the dorsal foot/lower shank) with no detectable spontaneous bursts before suspecting absent baseline activity. We began the experimental protocol after observing large bursts during an apnea following a maximal expiratory effort (to evoke increases in MSNA) and no response to unexpected clapping (a startle maneuver which evokes skin sympathetic nerve activity). The protocol involved a 10 min baseline to ensure stability of the recording site, followed by an additional 10 min * Lauro C. Vianna lcvianna@unb.br

Chronic heat exposure for health and exercise performance – cardiovascular research heats up

Flow-mediated dilatation (FMD) is a method used to measure endothelialdependent vasodilatation, quantified by the change in conduit artery luminal diameter during post-occlusive hyperaemia. The hyperaemic response is predominately governed by nitric oxide (NO)-mediated vasodilatation secondary to endothelial shear stress; therefore, FMD is commonly used as an index of endothelial function and indirect measure of NO bioavailability. Additionally, FMD has been negatively associated with risk of future cardiovascular (CV) events in healthy adults (Shechter et al. 2014), which justifies investigation into strategies aimed at enhancing endothelial function. In a recent article published in The Journal of Physiology, Brunt and colleagues (2016b) examined the effects of chronic heat exposure on CV health. Young, sedentary but otherwise healthy participants underwent 36 hot or thermoneutral water immersion sessions over an 8-week period with CV measures (blood pressure, carotid artery wall thickness, dynamic arterial compliance, pulse wave velocity, brachial artery FMD and endothelial-independent vasodilatation) assessed at baseline and 2 week intervals thereafter. Of importance, the authors demonstrated for the first time that chronic exposure to an elevated core temperature (38.5°C) resulted in increased brachial artery FMD and reductions in arterial stiffness, carotid artery wall thickness and blood pressure. As stated in the article, this work built upon a 30 year prospective study which found a negative association between sauna use and incidence of CV disease. The increases in FMD of > 5% at study completion were both statistically and clinically relevant (a 2% increase in FMD corresponds to a 15% reduction in CV disease risk), and were higher than previously reported increases observed after 6 weeks of sprint interval training or endurance training ( 1% and 2% increases in popliteal artery FMD, respectively) (Rakobowchuk et al. 2008). Therefore, repeated heat exposure provides an impressive strategy for improving vascular function and reducing risk of cardiovascular disease in an already healthy cardiovascular system. Further, repeated heat exposure may have a greater effect on brachial artery FMD in diseased or at risk (e.g. pre-hypertension) populations than in healthy participants due to a large degree of endothelial dysfunction associated with CV disease; however, this warrants further investigation. Additionally, the use of sauna or hot tubs may be a safer or more accessible treatment strategy compared to regular exercise in these diseased populations. Though Brunt and colleagues (2016b) did not specifically aim to investigate changes in NO bioavailability, changes in endothelial-dependent vasodilatation but not endothelial-independent vasodilatation suggests that the changes in CV measures were due to increased local vasodilators, particularly NO. In a following paper, Brunt and colleagues confirmed that chronic heat exposure improves microcirculation via increased NO bioavailability, as an NO synthase inhibitor was capable of abolishing the effect (Brunt et al. 2016a). Research using nitrate supplementation as an ergogenic aid also revolves around the supplement’s ability to influence endothelial-dependent vasodilatation. The use of exogenous nitrate supplementation to increase NO bioavailability for the specific purpose of aiding sport performance is a hotly debated topic, and current literature has been criticized for a lack of standardized supplementation protocols and small or inappropriate sample populations used to evaluate the ergogenic effects. This obfuscates firm conclusions on the topic (Allen, 2015). Based on our own data and early reports, our research group believes that improvements in endothelial function in elite athletes will be small (even negligible) with increased NO bioavailability, but it is possible that performance may still be meaningfully increased. Similarly, in regards to its ability to increase plasma nitrite concentrations, ischaemic preconditioning has also been discussed as a potential ergogenic aid with conflicting empirical support (Incognito et al. 2016). A potential mechanism of ischaemic preconditioning is the ability to increase skeletal muscle blood flow during exercise by preserving NO bioavailability, which has been shown through measures of brachial artery FMD after ischaemic stressors and strenuous exercise (Incognito et al. 2016). The ability of repeated heat exposure to increase resting brachial artery FMD (Brunt et al. 2016b) and therefore NO bioavailability, may present as an alternative strategy for improving skeletal muscle blood flow during exercise in thermoneutral environments. However, whether resting levels of brachial artery FMD are associated with improved muscle blood flow during exercise remains unknown, as do the influence of additionally layered factors such as sex, training status, diet and fitness. The use of heat exposure has classically been used in athletics to acclimate individuals prior to competing in hot environments, which promotes more effective regulation of core temperature, increases sweat rates and increases plasma retention. As Brunt et al. (2016b) noted, the minimal effective dose of passive heat exposure for altering CV or exercise performance measures has not been determined. A CrossTalk debate between Minson & Cotter and Nybo & Lundby (Minson & Cotter, 2016; Nybo & Lundby, 2016) has discussed the effects of heat acclimation on exercise performance in cool and hot environmental conditions, yet there remains considerable equipoise within the literature as well as variability in heat acclimation protocols between studies. Nonetheless, the increased NO bioavailability found by Brunt et al. (2016a) suggests another potential mechanism by which heat acclimation may improve a number of CV risk factors and alter exercise performance. We suggest that future studies investigate the effects of chronic heat exposure on skeletal muscle blood flow during exercise to provide insight of the potential integration of heat exposure to exercises strategies for CV disease and in athletic training regimes as an ergogenic aid.