Effects of a pre-workout supplement on hyperemia following leg extension resistance exercise to failure with different resistance loads

Background We sought to determine if a pre-workout supplement (PWS), containing multiple ingredients thought to enhance blood flow, increases hyperemia associated with resistance training compared to placebo (PBO). Given the potential interaction with training loads/time-under-tension, we evaluated the hyperemic response at two different loads to failure. Methods Thirty males participated in this double-blinded study. At visit 1, participants were randomly assigned to consume PWS (Reckless™) or PBO (maltodextrin and glycine) and performed four sets of leg extensions to failure at 30% or 80% of their 1-RM 45-min thereafter. 1-wk. later (visit 2), participants consumed the same supplement as before, but exercised at the alternate load. Heart rate (HR), blood pressure (BP), femoral artery blood flow, and plasma nitrate/nitrite (NOx) were assessed at baseline (BL), 45-min post-PWS/PBO consumption (PRE), and 5-min following the last set of leg extensions (POST). Vastus lateralis near infrared spectroscopy (NIRS) was employed during leg extension exercise. Repeated measures ANOVAs were performed with time, supplement, and load as independent variables and Bonferroni correction applied for multiple post-hoc comparisons. Data are reported as mean ± SD. Results With the 30% training load compared to 80%, significantly more repetitions were performed (p < 0.05), but there was no difference in total volume load (p > 0.05). NIRS derived minimum oxygenated hemoglobin (O2Hb) was lower in the 80% load condition compared to 30% for all rest intervals between sets of exercise (p < 0.0167). HR and BP did not vary as a function of supplement or load. Femoral artery blood flow at POST was higher independent of exercise load and treatment. However, a time*supplement*load interaction was observed revealing greater femoral artery blood flow with PWS compared to PBO at POST in the 80% (+56.8%; p = 0.006) but not 30% load condition (+12.7%; p = 0.476). Plasma NOx was ~3-fold higher with PWS compared to PBO at PRE and POST (p < 0.001). Conclusions Compared to PBO, the PWS consumed herein augmented hyperemia following multiple sets to failure at 80% of 1-RM, but not 30%. This specificity may be a product of interaction with local perturbations (e.g., reduced tissue oxygenation levels [minimum O2Hb] in the 80% load condition) and/or muscle fiber recruitment.


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Conclusions: Compared to PBO, the PWS consumed herein augmented hyperemia following multiple sets to failure at 80% of 1-RM, but not 30%. This specificity may be a product of interaction with local perturbations (e.g., reduced tissue oxygenation levels [minimum O 2 Hb] in the 80% load condition) and/or muscle fiber recruitment.

Background
The phenomenon of local hyperemia in response to muscle contraction is a product of various factors including, but not limited to, metabolic signals (e.g., pCO 2 /pO 2 , lactate, K + , adenosine), endothelium-derived factors (e.g., nitric oxide [NO], prostacyclin, and endothelium derived hyperpolarizing factor [EDHF]), and erythrocyte ATP release [1]. With respect to metabolic signals, increased metabolic flux/demand associated with a given exercise is associated with elevations in local pCO 2 , lactate, and adenosine levels which, collectively, potentiate vasodilation through arteriole smooth muscle cell relaxation [2]. Moreover, K + -efflux from working skeletal muscle under conditions of high action potential frequency can hyperpolarize smooth muscle cells and promote vasodilation [1,3]. Endothelium-derived factors that interact with vascular smooth muscle cells and likely mediate hyperemia are released in response to blood flow shear stress as well as mechanical deformation/compression of the arterioles [4,5]. Finally, mechanical deformation and/or decreased oxygen saturation levels during exercise may elicit erythrocyte ATP release promoting vasodilation [6,7]. The relative contribution of these vasodilatory mediators, in sum and over the time course of the response, has been difficult to quantify due to heterogeneity in metabolic perturbations throughout the skeletal muscle as well as heterogeneity in the vascular phenotype throughout the skeletal muscle arterioles [8,9]. Regardless, it is likely that at the local tissue level, exercise-induced hyperemia involves a complex interaction of many or all of these factors.
While the formulation of 'pre-workout' supplements (PWS) varies markedly from product to product, many of these formulas contain a variety of ingredients, blended together, which may act to enhance blood flow in response to resistance training stimuli (i.e., exercise induced hyperemia). PWS "blends", particularly those marketed for increased blood flow (e.g., N.O.-Xplode®, Nitraflex®, etc.), often include, but are not limited to, creatine, L-arginine, and L-citrulline. Creatine supplementation with resistance training has been shown to significantly increase peripheral blood flow with resistance training, but it is unclear if the mechanism is due to changes in NO synthesis or total body water [10]. Larginine, which can be used to form NO in a NO synthase (NOS) catalyzed reaction [11], supplementation has been shown to increase muscle blood volume following resistance training stimuli [12]. Moreover, supplementation with L-citrulline, a precursor to L-arginine, has been shown to dose-dependently increase plasma arginine levels and NO-dependent signaling in healthy men and women [13]. Collectively, many of the blood flow potentiating ingredients including in a PWS are included in an effort to increase NO synthesis via provision of additional substrate for NOS catalyzed NO formation (i.e., feed forward).
Recently, the PWS Reckless™ (Maxium Human Performance [MHP] LLC, West Caldwell, NJ USA) was designed and includes not only the aforementioned ingredients, but also additional ingredients which may potentiate blood flow via alternative mechanisms. Indeed, the Reckless™ formula also includes the less common PWS ingredients L-norvaline, ancient peat and apple fruit extract (elevATP®), and Spectra™. L-norvaline acts as an arginase inhibitor and has been shown to augment NO production in animal models [14,15]. Thus, L-norvaline may act synergistically with L-argininie and L-citruline by increasing bioavailability of L-arginine for NO formation. Spectra™, a "full-spectrum" antioxidant product comprised of fruit, vegetable and herb extracts, may also act in a synergistic fashion with other ingredients by reducing NO scavenging by reactive oxygen species and improving NO bioavailability [16]. Finally, ancient peat and apple fruit extract has been shown to enhance circulating ATP levels [17,18] which may augment blood flow in a NOS independent manner [19,20]. Indeed, although many PWS marketed to increase the blood flow response to resistance training focus on NO modulation, the contribution of NO to resistance exercise induced hyperemia is not absolute and likely partially includes ATP-sensitive potassium channels [21] which may interact with ingredient(s) found in a PWS (e.g., elevATP®).
While many studies have demonstrated the effects of individual ingredients, investigations of formulations with many potentially synergistic ingredients are sparse. Notably, many PWS formulations, including Reckless™, also include additional ingredients targeting such things as increases in perceived energy (e.g., caffeine), strength/ performance (e.g., caffeine, β-alanine), and focus (e.g., whole coffee fruit extract). Considerable attention has been given to the NO potentiating effects of PWS, but, other ingredients, in theory, may accentuate factors mediating hyperemia from the exercise stimulus as a result of greater effort, training loads, etc.
To our knowledge, little is known regarding the effectiveness of PWS in different training conditions (varying time under tension, load, etc.). With varying training conditions there are likely differences in local perturbations of the factors contributing to exercise-induced hyperemia (e.g., metabolic signals, redox status, endothelium-derived factors, ATP concentration, etc.). Moreover, relative muscle activation varies markedly with high (80% of 1-repetition maximum [1-RM]) vs. low (30% of 1RM) training loads [22]. Thus, variations in training factors such as time-under-tension/training load may also interact differentially with PWS ingredients secondary to resultant differences in local perturbations and relative muscle fiber recruitment.
While the ergogenic value of improved local blood flow in response to resistance training is controversial, it may help to facilitate muscle repair [23][24][25][26][27]. Moreover, the post-exercise blood flow response which contributes to tissue volumizing (i.e., the "muscle pump"), is often sought by recreational and competitive resistance trainees alike. Herein, we sought to determine the effect of a multi-ingredient PWS on leg extensor resistance training-mediated hyperemia with different relative training loads. Training loads were pre-determined as low (30% of 1-RM) and high (80% of 1-RM) loads. Resistance exercise bouts included four sets to repetition failure under both conditions. Four sets to failure were selected to 1) ensure marked differences in time-undertension between the load conditions and 2) to improve generalizability to practical settings as larger training adaptations occur with a greater number of sets [28]. We hypothesized that 1) greater time under tension would be associated with a greater exercise-induced hyperemia (femoral artery blood flow) and 2) PWS would be associated with increased circulating concentrations of NO metabolites and an accentuated exercise-induced hyperemic response. In addition, we sought to explore a potential interaction between PWS and exercise load.

Subjects
Thirty (N = 30) apparently healthy males were recruited from the local community through advertisement to participate in this double-blinded study. Inclusion criteria included being aged 18-35 years, BMI between 23 and 30 kg/m 2 , and ≥ 6 months of lower body resistance training history (i.e., resistance training legs at least once per week). Potential participants were excluded if they consumed >300 mg of caffeine per day (in beverages or supplements), consumed creatine monohydrate, betaalanine, or pre-workout supplements in the previous two weeks, or had been regular tobacco users in the last year. To improve generalizability of the study, coffee/caffeine consumption in the prior two weeks was not an absolute exclusion criteria given that 85% of the U.S. population consumes at least 1 caffeinated beverage a day [29]. However, participants were asked to abstain from caffeine consumption (besides that included in the PWS) on the day of each visit. All procedures described herein were approved by the Auburn University Institutional Review Board and conformed to the standards set by the latest revision of the Declaration of Helsinki.

Study design and procedures
After meeting inclusion/exclusion criteria and providing their informed consent, participants reported to the laboratory for 3 separate visits. For each visit, subjects were instructed to arrive at the same time of day in a hydrated state ≥4-h fasted and to avoid all lower body resistance exercise for at least 24-h prior. At visit #1, height and weight was measured (weigh beam scale with height rod; Detecto 439, Deteco Scale, Webb City, MO USA). Thereafter, leg extensor strength was assessed via a 'plate-loaded' training apparatus (Body-Solid®, GCEC340, Forest Park, IL, USA). Finally, after the leg extensor strength testing, participants were randomly assigned to either supplementation Group ' A' or 'B'. Both groups were given canisters, labeled only as ' A' or 'B' for respective groups, containing a soluble powder for consumption. Participants were provided with instructions for consuming 2 scoops (~6.8 g of PWS total; maximum manufacturer recommended dose) of the supplement per day in the morning for the next 6 days. On day 7, participants reported for visit #2 (described below) and consumed the supplement in the lab. Notably, some of the ingredients included in the Reckless™ PWS generally require a loading phase for best results (e.g., creatine). Thus, although the focus of this work was to evaluate the acute effect of the PWS on reactive hyperemia responses to resistance training, one week of 'loading' prior to testing days was included to maximize responses.
For visit #2, participants returned to the lab and baseline (BL) venipuncture (see venipuncture section) and heart rate (HR), blood pressure (BP), and right femoral artery blood flow measures were performed at rest. Thereafter, participants consumed 2 scoops (i.e., 1 serving) of their respective supplement (' A' or 'B') and rested for 45-min. During the 45-min rest time, participants were fitted with a near infrared spectroscopy system (NIRS; Artinis Portamon, MKII, Einsteinweg 17, Netherlands) for measurement during subsequent exercise. At 45-min following ingestion of the respective supplement, pre-exercise (PRE) venipuncture and HR, BP, and femoral artery blood flow measured were again performed. This was followed by completion of 4 sets of leg extensions to failure at a load of either 30% or 80% (counter-balanced) of their pre-determined 1-RM. At exactly 5-min following leg extension exercise (POST) another blood sample was collected along with measures of HR, BP, and femoral artery blood flow. Thereafter, participants were dismissed and instructed to continue consuming their supplement in the morning each day.
Seven days following visit #2, participants reported to the laboratory for visit #3. Visit #3 was performed in exactly the same manner as visit #2 with the lone exception of the intensity of leg extension exercise performed being the intensity not performed at visit #2 (e.g. if 30% at visit #2, then 80% at visit #3 and vice versa).

Leg extensor strength testing (1RM determination)
At visit #1, leg extensor strength was assessed via the 'plate-loaded' training apparatus. Briefly, after participants were positioned on the training apparatus they completed a 10 repetition warm-up set of bilateral knee extensions with a 45lbs (20.5 kg) load followed by a 2-min rest. Thereafter, the load was increased by 25lbs (11.4 kg) and participants were instructed to perform only one repetition followed by 2-min of rest. If participants were successful with the lift, the load was increased another 25lbs followed by another 2-min of rest. This process continued until there was an unsuccessful lift and the load at the last successful lift was recorded as the 1-RM.

Supplementation
Investigators and participants were blinded to supplement assignments until following completion of the study. Procedurally, canisters, provided by MHP, were simply labeled as supplement ' A' or 'B'. One set of the canisters contained a placebo (PBO), consisting of equal parts maltodextrin and glycine (3.4 g of each per serving herein). The other set of canisters contained a multi-ingredient PWS supplement (Reckless™; Maximum Human Performance, West Caldwell, NJ USA). Contents of the PWS and dosages consumed with a serving herein are shown in Table 1. PWS ingredients included the aforementioned blood flow potentiators (i.e., L-arginine, L-citrulline, creatine, Lnorvaline, elevATP®, and Spectra™) as well as ingredients , L-theanine), and recovery (folate). A single serving of both the PBO and PWS was two level "scoops" (~6.8 g) of the powder dissolved in 16 oz. of water. Participants ingested a serving of their respective supplement daily through the end of the entire study.

Venipuncture
Venipuncture was performed on three occasions at visits #2 and #3; at baseline, 45-min post-PWS or PBO ingestion, and 5-min following leg extensor exercise for assessment of plasma NOx (nitrate + nitrite), as described in detail below. Briefly,~4 mL of venous blood was collected from the antecubital space in tubes containing EDTA and immediately centrifuged at 4000 g at 4°C for 5-min. Plasma was then separated in cyrotubes and immediately placed in a − 80°C freezer for later analysis of NOx. Plasma samples were frozen for no more than 3 months. Following deproteinization (cold-ethanol preparation), batch processing for assessment of plasma NOx was performed by ozone-based chemilluminescence using a NO analyzer (Sievers NOA 280i, Zysense, Waxhaw, NC USA). To determine NOx, samples were added to 0.1 M vanadium chloride in 1 M hydrochloric acid refluxing at 95°C under nitrogen. The respective reduction to NO was detected by a cooled, photomultiplier tube housed in the NOA and concentrations of NOx were determined by plotting signal (mV) against a calibration plot.
Heart rate and blood pressure measurement HR and BP were measured on three occasions each during visits #2 and #3; at baseline, 45-min after supplement ingestion, and 5-min post-workout. For baseline and 45min post supplement ingestion measures, participants rested in a supine position in a quiet, temperature controlled room for 10-min prior to measurements. Postworkout measures were made at exactly 5-min following completion of the workout with the participant having 1-2-min of supine rest time before measurement commenced. At rest, and 45-min post supplement ingestion HR, brachial systolic (SBP) and diastolic (DBP) BP measurements were made in duplicate at the brachial artery of the left arm using an automated oscillometric device (OMRON BP785, Omron Corporation, Kyoto, Japan). If consecutive SBP or DBP measures were ≥6 mmHg apart, an additional measurement was made. The two closest values were then averaged to represent BP and HR at a given time point. A single BP measurement was made at the post-exercise time point to assure consistent measurement timing of BP and femoral artery blood flow measures. Mean arterial pressure (MAP) was calculated as [PDBP +1/3(PSBP-PDBP)].

Femoral artery blood flow measurement
Immediately following HR and BP measurements, blood flow through the right common femoral artery was assessed using high resolution ultrasound (Logiq S7 R2 Expert; General Electric, Fairfield, CT USA) with a 3 to 12 MHz multi-frequency linear phase array transducer. The femoral artery was imaged longitudinally with the transducer placed 2-3 cm proximal to the bifurcation. Simultaneous measurement of artery diameter and blood velocity was performed using duplex mode imaging (Bmode and Doppler) and video was captured through a digital interface at 30 frames/s with real time analysis (FMD Studio, Pisa, Italy). Measurements were captured for 1 continuous min with the transducer held manually in the same position. Vessel diameters were determined frame-by-frame via automatic edge detection software (FMD Studio, Pisa, Italy) measuring the distance between the near and far wall of the intima. Blood velocity was determined via selection of a region of interest around the Doppler waveform and a trace of the velocity-time integral was used to calculate mean velocity for each cardiac cycle. Blood flow from continuous diameter and mean blood velocity measurements during ultrasonography were calculated as [Π * (diameter/2) 2 * time average mean velocity * 60].

Near infrared spectroscopy (NIRS) measures
Participants wore the NIRS apparatus during the leg extensor exercise sessions for recordings during sets of exercise and the respective rest intervals in-between. Briefly, the skin over the left vastus lateralis was prepped (i.e., hair removal, skin abrasion) and cleaned with alcohol, and the NIRS device (optodes) was secured with an elastic bandage and flexible tape mid-way between the anterior superior iliac spine and the lateral border of the patella. Per the manufacturer instructions, a black, light-absorbing cloth was also placed over top of the NIRS device to limit the amount of ambient light detected by the device. Relative changes from baseline were measured by a continuous wavelength NIRS device that used spatially resolved spectroscopy with a sampling rate of 10 Hz. Oxyhemoglobin and deoxyhemoglobin concentration changes were calculated from light absorbance at 758 and 847 nm using the modified Lambert-Beer Law. Following data collection, the traces were smoothed using custom-written software (Lab-VIEW 16.0, National Instruments, Austin TX USA) to remove residual noise and movement artifact due to rhythmic muscle contractions. The primary outcomes of interest included Min and Max oxygenated hemoglobin (O 2 Hb), deoxygenated hemoglobin (HHb), difference between O 2 Hb and HHb (HbDiff ), and total hemoglobin (tHb). Both minimum and maximum NIRS derived variable values were of interest to determine the extent to which each variable was reduced during a set of leg extensions and the extent to which they recovered by the end of the respective rest period(s).

Leg extensor exercise
After 45-min of passive rest, subjects completed four sets of as many technically-proficient repetitions as possible of bilateral leg extensions at a load of either 30% or 80% of their tested 1-RM. Each repetition was performed in cadence with a metronome set at 40 bpm. A first metronome beep initiated concentric movement, the second sound indicated that the legs should be in full extension and held there until the third sound, at the third beep the eccentric phase began, the forth sound indicated the legs should be back to the starting position and the fifth/first sound initiated the next repetition. Thus, the estimated time under tension for each repetition was approximately 4.5-s with~1.5-s of rest between consecutive repetitions. An unsuccessful repetition, characterized by incomplete extension of the legs at the requisite tempo of the metronome, resulted in termination of the set. Subjects were given 3-min of rest between sets. Volume loads for each set were calculated as the product of repetitions and load.

Statistical analysis
For all statistical analyses, an alpha level of p ≤ 0.05 was required for statistical significance. Between groups (i.e. PWS vs. PBO) comparisons for participant characteristics were evaluated using independent-tests. Three-way repeated measures ANOVAs were used to evaluate the continuous primary dependent variables associated with this study with Huynh-Feldt correction applied to hypothesis testing. Time and load were included as within-subjects factors and supplement was included as a between-subjects factor. Posthoc testing of main effects and interactions were performed using t-tests (paired t-tests for exclusively within subjects factors and independent t-test for all others) with Bonferroni adjustments for multiple comparisons (α/[number of comparisons]). Grouped data are presented as mean ± standard deviation and data respective to post-hoc comparisons are presented as the mean difference and [95% confidence interval (lower limit, upper limit)]. All statistical analyses were performed using IBM SPSS Statistics 22 for Windows (Chicago, IL USA).

Subject characteristics
Subject characteristics are provided in Table 2. There were no significant between group (i.e., PWS vs. PBO) differences in age, height, weight, BMI or 1-RM (p > 0.05).

Leg extensor exercise performance
Data regarding the number of repetitions performed and volume load for each set and load condition is presented in Table 3. As expected, there were significant main effects of load and set and a set*load interaction for number of repetitions performed (p < 0.001 for all). The number of reps performed during set 1, 2, 3 and 4 were all significantly greater during the 30% load compared to the 80% load (p < 0.001 for all). For volume load, a significant main effect of set (p < 0.001) and a set*load interaction (p = 0.002) was observed though post-hoc analysis revealed no significant differences in volume load between load conditions for any set (p > 0.0125 for all). No significant effect of treatment or treatment interactions were observed for repetitions or volume load (p > 0.05). Finally, for total volume load (sum of all four sets), there were no main effects or intensity*treatment interaction (p > 0.05). The total volume load in the 30% load condition was 5912 ± 1097 kg and 4877 ± 1904 kg for PBO and PWS, respectively. For the 80% load condition, total volume load was 5137 ± 1410 kg and 5794 ± 1894 kg for PBO and PWS, respectively.

Near-infrared spectroscopy (NIRS) during leg extensor exercise
For NIRS measurement during leg extensor exercise no effects (main or interactions) relative to treatment were observed. Thus, NIRS results by set and load during leg extensor exercise are illustrated in Fig. 1. During the leg extensor exercise sessions, a main effect of set and load and a set*load interaction was observed for Min O 2 Hb. Min O 2 Hb was significantly lower during the 80% load condition compared to the 30% load condition during set 1 (−5.3 [−2.8, −7.7]; p < 0.001) but not sets 2-4 ( Fig.  1a; p > 0.0125 for all). For Max HHb a main effect of intensity and a set*load interaction was observed with Max HHb values being significantly higher during set 3  (Fig. 1b). There was a main effect of set and load for Min HbDiff responses during exercise, but no set*load interaction (Fig. 1c) Fig. 1d).

Near-infrared spectroscopy (NIRS) between sets of leg extensor exercise
NIRS results by intensity and rest periods between leg extensor exercise sets are illustrated in Fig. 2. During the  . 2b). A main effect of rest period and load, and their interaction, was observed for MinO 2 Hb during rest periods between leg extensor exercise sets. MinO 2 Hb was significantly lower in the 80% load condition compared to the 30% load condition for rest period 1 (−6.  Fig. 2e). For Min HHb, a main effect of rest period and treatment was observed. Post-hoc testing for Min HHb did not reveal significant between rest period differences (p > 0.0167 for all; Fig. 2f). However, Min HHb values were significantly higher with PWS treatment compared to PBO treatment

Heart rate and blood pressure
Data for HR and BP measurements at BL, PRE and POST are illustrated in Fig. 4. A significant main effect of time was observed for HR, SBP, DBP and MAP. A time*treatment interaction was also observed for HR though there were no statistically significant differences observed at each time point between treatments (p > 0.0167 for all; Fig. 4a). Independent of treatment and load (i.e. overall mean), HR at POST was higher than at BL (+23.

Discussion
The primary findings of the present study are that, 1) the magnitude of reactive hyperemia (i.e., femoral artery blood flow) following leg extensor exercise was not affected by differing estimated time under tension and/or relative loads; 2) circulating concentrations of NO metabolites (i.e., NOx) at 45-min following PWS consumption were significantly elevated and remained elevated through the end of the leg extensor exercise protocol; and 3) PWS augmented the reactive hyperemic response to leg extensor exercise at a higher relative load (80%), but not a lower relative load (30%).
PWS did not have an obvious performance effect on leg extensor exercise. The different exercise loads utilized herein resulted in markedly different numbers of repetitions performed across all sets with no significant differences in total volume loads (repetitions x load). Given that all repetitions in both exercise load conditions were performed in cadence to a metronome, estimated time under tension was also markedly different, though rest intervals between sets were the same and not standardized to exercise/work time. Thus, the primary differences between the two exercise load conditions were number of contractions, estimated time under tension, and the magnitude of force development.
During the initial set of leg extensor exercise, compared to the 30% load condition, NIRS data revealed significantly lower Max tHb and Min O 2 Hb values during the 80% load condition suggesting less perfusion relative to oxygen consumption/metabolic demand during that set. However, in the subsequent sets, Max tHb was, in essence, the same between load conditions, while Min O 2 Hb remained lower and Max HHb higher in the 80% load condition. Ultimately, the lower Max tHb values in the higher load condition during the first set could be explained by greater vascular resistance [30] and/or a reduced exercise time (~297 and 90 s for 30% and 80% load conditions, respectively) in which feedback gain from local vasodilatory mediators did not reach the same levels as in the lower load condition [31,32]. In addition, the reduced Min O 2 Hb (and elevated Max HHb) values across all sets can likely be explained by a significantly higher work rate in the higher load condition [33].
Regarding the NIRS data in the rest periods between sets of leg extensor exercise, there appears to be a 'carry-over' effect from the exercise data. That is, Min O 2 Hb, HHb, tHb and HbDiff values mirror the responses observed during the exercise period. However, no differences were observed in Max O 2 Hb, HHb, tHb and HbDiff suggesting a similar 'recovery' over the duration of the recovery periods for both load conditions. Interestingly, we also observed a main effect of treatment on Min HHb with values being significantly higher across all rest periods with PWS. Given that PWS did not have an effect on tHb (surrogate of perfusion), we posit that the PWS effect is likely at the metabolic level. Dietary supplements increasing NOx concentrations have been shown to reduce the oxygen demand per unit of work [34][35][36]. Herein, the PWS acutely increased plasma NOx concen-trations~3.5-fold, a relatively large change compared to other PWS studies [37][38][39]. Moreover, several ingredients contained within the Reckless™ PWS (e.g., β-alanine, creatine, L-citrulline), albeit at relatively lower doses than  [34,[40][41][42].
As noted previously, differences in NIRS parameters as a result of exercise loads in the rest periods between sets appeared to be relatively transient suggesting resolution prior to our femoral artery blood flow measurements at 5-min following the last set of leg extensor exercise. Indeed, we did not observe a time*load interaction for mean arterial blood flow data with similar values for both load conditions at the POST time point, independent of treatment. However, mean femoral artery blood flow was significantly increased at the POST time point regardless of treatment and load indicating significant exercise-induced reactive hyperemia at the time of measurement. We had hypothesized that PWS would augment the reactive hyperemic response at both exercise intensities, so it was interesting to find that the PWS had a significant effect compared to PBO in the 80% load condition, but not the 30% load condition (Fig.  6b). No differential effects of treatment or load were observed for BP or femoral artery diameters. Moreover, although there was a time*treatment interaction for HR suggesting an elevated HR with PWS at the POST time point, post-hoc analyses did not reveal a significant difference (p = 0.027) and the absolute difference in HR between PWS and PBO was actually numerically larger in the 30% load condition than the 80% load condition (Fig. 4a). Thus, hemodynamics and/or femoral artery diameter do not appear to explain the effect of PWS on mean femoral artery blood flow in the 80% load condition but not 30% load condition.
Exercise-induced reactive hyperemia involves a complex interaction of many factors (i.e., mechanical, metabolic, endothelium-derived, and erythrocyte-derived). Further obfuscating interpretation of the responses observed herein are the interaction of these factors with unique exercise loads, estimated time-under tension, force development, and PWS ingredients. Indeed, there are a plethora of ingredients included in the PWS utilized herein that could affect the reactive hyperemic response, but the specific effect of PWS on mean femoral artery blood flow post exercise in the 80% load condition but not 30% load condition is intriguing. If PWS ingredient(s)-mediated increases in local concentrations of metabolites (e.g., adenosine) were major players independent of local factors, a similar effect would be expected across both exercise loads. One notable difference observed herein was the significantly lower Min O 2 Hb and Min HbDiff values observed during the rest periods between sets of leg extensor exercise. As noted previously, it is likely that these differences 'carry-over' from the exercise itself and is resolved over the time course of the 3-min rest period as evidenced by similar Max O 2 Hb and HbDiff values during rest. Though transient, a period of reduced oxygenation in the local tissues could interact with circulating metabolites from the PWS and have a cumulative effect with multiple sets of exercise. Indeed, a major stimulus for nitrate/nitrite conversion to NO is hypoxia [43] which could contribute to the reactive hyperemia response. However, this notion is not supported by the Max tHb data observed during and between sets of exercise (no treatment*load interaction).
Though beyond the scope of this study, we propose two possibilities for consideration as explanatory mechanisms for the intensity specific effect of PWS. First, in the 80% load condition, the greater resistance loads and subsequent force development with leg extensor contraction may be associated with increased muscle fiber recruitment [44]. With a relatively higher proportion of active muscle fibers, there may also be increased capillary network recruitment [1] potentiating delivery of PWS ingredients and metabolites to a greater area of vascular tissue. Moreover, there is likely a differential proportion of fiber types recruited between the two loads which could interact with PWS ingredients. Indeed, there is marked heterogeneity in vascular control mechanisms as a function of fiber type [45,46] and fiber type/region specific effects of dietary supplements (e.g., nitrate) is not novel [47,48]. Secondly, it is possible that with the significantly greater number of repetitions and exercise time, there are higher local levels of vasodilatory metabolites and the ability of PWS to further augment the response is limited (i.e., diminishing returns).

Limitations
Notably, the PWS administered in the current study included a relatively high dose of caffeine (390 mg) and we did not include a matched, caffeine only group. However, previous work suggests that caffeine may actually attenuate limb blood flow during dynamic leg exercise [49]. Regardless, the Reckless™ PWS includes several ingredients, including caffeine, which may have impacted the findings. The intention of this study was to evaluate a PWS as a whole, though this makes it difficult to discern if the results were attributable to any one ingredient in particular or any synergy between ingredients. Notably, the relative dose delivered with a serving for some of the blood flow potentiating ingredients is less than found in more focused supplements/PWS (e.g. only 500 mg of L-citrulline) which may suggest that there is some additive effect of multiple ingredients and/or synergy between ingredients. However, in the context of the current study, the mechanisms underlying the reported effects cannot be delineated. More research is warranted on the synergistic and isolated effects of the ingredients found in PWS. In addition, our study included a cohort with a relatively high degree of experience in training (at least 6 consecutive months of lower body resistance training prior to the study), and further work should determine the effects of this PWS with varying resistance training ages. Finally, we did not monitor dietary habits throughout the study beyond the requirement of reporting for visits ≥4-h fasted. Thus, the influence of dietary behaviors in the days leading up to visits cannot be excluded.

Conclusions
In summary, we report that four consecutive sets of leg extensor exercise to failure at 30% and 80% of 1RM elicits a similar post-exercise reactive hyperemic response with greater hemoglobin deoxygenation during exercise that resolves quickly during rest (<1 min). The PWS utilized herein did not affect exercise performance or NIRS variables during exercise, though Min HHb was found to be higher with PWS. Finally, reactive hyperemia was significantly increased compared to PBO postexercise but only following the 80% exercise load condition. Local metabolic perturbations and fiber-type specific effects of the PWS should continue to be explored in an effort to further define which situations and athletes may benefit from its use. Laboratory. Neuromuscular analyses were performed in the Applied Neuromuscular Laboratory at Oklahoma State University. All authors read and approved the final manuscript. JSM procured funding for the study, was, in part, responsible for the conception and design of the experiment, assisted with experimental procedures and data acquisition, and was primarily responsible for writing of the manuscript. PWM assisted with experimental procedures, data acquisition, and manuscript editing. CTH was, in part, responsible for conception and design of the experiment, assisted with experimental procedures, data acquisition and manuscript editing. PAR assisted with experimental procedures, data acquisition and manuscript editing. RJC assisted with experimental procedures, data acquisition and manuscript editing. MPF assisted with experimental procedures, data acquisition, and manuscript editing. MJL assisted with experimental procedures, data acquisition, and manuscript editing. CSM assisted with experimental procedures, data acquisition, and manuscript editing. TWDM assisted with experimental procedures, data acquisition, and manuscript editing. KCY assisted with experimental procedures, data acquisition, and manuscript editing. DDP assisted with experimental procedures, data acquisition, and manuscript editing. JMD assisted with experimental procedures, data acquisition, and manuscript editing. NDMJ was, in part, responsible for the conception and design of the experiment and assisted with experimental procedures, data acquisition, and manuscript editing. MDR procured funding for the study, was, in part, responsible for the conception and design of the experiment, and assisted with experimental procedures, data acquisition, and manuscript editing.
Ethics approval and consent to participate All procedures described herein were approved by the Auburn University Institutional Review Board (Protocol #16-333 MR 1609) and conformed to the standards set by the latest revision of the Declaration of Helsinki. All subjects provided written and verbal consent prior to study participation.

Consent for publication
Not applicable.