Acute Effects of Beetroot Juice Supplements on Lower-Body Strength in Female Athletes: Double-Blind Crossover Randomized Trial

Abstract

Background:

Beetroot juice (BRJ) is used as an ergogenic aid, but no previous study has analyzed the effect this supplement has on the production of explosive force and muscular endurance in physically active women.

Hypothesis:

BRJ improves explosive force and muscular endurance in the lower limbs of physically active women.

Study Design:

Randomized double-blind crossover study.

Level of Evidence:

Level 3.

Methods:

Fourteen physically active women performed a countermovement jump (CMJ) test, a back squat test for assessing velocity and power at 50% and 75% of one-repetition maximum (1RM), and the number of repetitions on a muscular endurance test consisting of 3 sets at 75% of 1RM in a resistance training protocol comprising 3 exercises (back squat, leg press, and leg extension). The participants performed the test in 2 sessions, 150 minutes after ingesting 70 mL of either BRJ (400 mg of nitrate) or a placebo (PLA).

Results:

A greater maximum height was achieved in the CMJ after consuming BRJ compared with a PLA (P = 0.04; effect size (ES) = 0.34). After a BRJ supplement at 50% 1RM, a higher mean velocity [+6.7%; P = 0.03; (ES) = 0.39 (–0.40 to 1.17)], peak velocity (+6%; P = 0.04; ES = 0.39 [−0.40 to 1.17]), mean power (+7.3%; P = 0.02; ES = 0.30 [−0.48 to 1.08]) and peak power (+6%; P = 0.04; ES = 0.20 [−0.59 to 0.98]) were attained in the back squat test. In the muscular endurance test, BRJ increased performance compared with the PLA (P < 0.00; η_p² = 0.651).

Conclusion:

BRJ supplements exert an ergogenic effect on the ability to produce explosive force and muscular endurance in the lower limbs in physically active women.

Clinical Relevance:

If physically active women took a BRJ supplement 120 minutes before resistance training their performance could be enhanced.

Supplementation with beetroot juice (BRJ) is considered an ergogenic aid for endurance sports. ⁴² However, the consumption prevalence of BRJ supplements is low in well-trained athletes of both sexes.^7,63 BRJ is a dietary supplement rich in inorganic nitrate (NO₃^–), this being a precursor of nitric oxide (NO) through the NO₃^––nitrite (NO₂^–)–NO pathway. ¹⁸ NO₃^– is considered biologically inert, but in the oral cavity NO₃^– is reduced to NO₂^– by the salivary glands and the facultative anaerobic bacteria nitrate reductase present on the dorsal surface of the tongue.^39,51 In the stomach, NO₂^– is partially reduced to NO, resulting in increased levels of NO₂^– and NO in the systemic circulation. ³⁸ NO₂^– in blood and other tissues could be reduced to NO under hypoxic and acidotic conditions. ³⁸ NO has numerous physiological effects, including dilation of the vascular endothelium, ⁴⁴ and there is evidence of NO₃^– supplements’ improving vascular conductance and skeletal muscle blood flow during submaximal efforts, ²³ favoring and enhancing oxygen delivery and extraction, ⁴³ and facilitating muscle work during exercise. A meta-analysis reported reduced oxygen consumption (VO₂) during moderate and heavy intensity endurance exercises, implying an enhanced economy-limiting factor for endurance performance.^5,47 Considering that energy expenditure is the product of VO₂ and the respiratory exchange ratio, it is proposed that there is a lower rate of muscle glycogen depletion, ⁵⁹ which contributes to the ergogenic effect of NO₃^– supplements in time-to-exhaustion tests. ⁴⁷

One of the main determinants of sporting performance is muscular power production. ¹⁵ The countermovement jump (CMJ) is an action that requires brief contraction times and high rates of force development. ⁵⁵ The height reached during CMJ is considered an indicator of lower-limb muscular power ⁴¹ and has been associated with speed and acceleration in sprinting, ⁵⁸ very important in team sports such as soccer, volleyball, and basketball. ^34,50 Resistance training (RT) causes structural and neural adaptations that lead to increases in hypertrophy, strength, and muscle power. ¹⁹ The main fuel during explosive effort (maximal efforts <6 seconds), such as in a CMJ, is phosphocreatine (PCr) metabolism ¹¹ ; and the depletion of this substrate could contribute to fatigue during RT. ¹ NO₃^– has been demonstrated to diminish the reduction of PCr levels for a determined task ² and increase muscle force production. ²⁸ Nevertheless, the results of the studies analyzing the effect of BRJ on the height reached in a CMJ are unclear.^32,35 In RT, BRJ has been shown to enhance power output production in bench presses with a load corresponding to 70% of the 1 repetition maximum (1RM), ⁶⁶ but not when a stop of 1 to 1.5 seconds is included between the eccentric and concentric phases, either in bench presses or back squats. ⁵² In terms of muscular endurance, it has been reported that BRJ supplements increase the number of repetitions until concentric failure, with load ranging from 60% to 80% 1RM on the upper and lower limbs.^45,66

To date, all the studies that have analyzed the effects of NO₃^– supplementation on the CMJ and RT have recruited male participants. It has been proposed that in women, the reduction of NO₂^– to NO could be blunted, based on the fact that they have a greater oxidative skeletal muscle phenotype than men. ⁶⁵ However, it has been observed that there is a greater increase in plasma NO₂^– levels after supplementation compared with male participants, ³³ as well as enhanced muscle contractile function. ¹³ Therefore, based on the absence of any studies analyzing the effect of NO₃^– supplements on jumping ability and power production in the female population in terms of exercises employed in RT programs, the purpose of this study was to investigate the possible ergogenic effect of a BRJ supplement on the production of explosive force and muscular endurance in physically active women. The study hypothesis was that BRJ supplementation might increase muscle force production and muscular endurance in physically active women.

Methods

Design

The study involved an experimental double-blind crossover randomized trial. The study was designed following the CONSORT (Consolidated Standards of Reporting Trials online supplement file). The experimental procedure consisted of 3 visits, 1 week apart. In the first visit, body composition was assessed, and the participants performed a familiarization protocol and a 1RM test. During the second and third sessions, the participants carried out the experimental protocol, after ingesting either a BRJ supplement or a placebo (PLA). In line with previous studies on diurnal variation in strength and muscle power in RT, ³⁶ and a study on the different effects of BRJ based on the time of day, ²⁰ the experimental measurements were taken in the morning, at the same time of day (±0.5 h) for each individual, to standardize the influence of the circadian rhythm, at a temperature of 23°C (±1°C). This study was registered at ClinicalTrials.gov (NCT04987619).

Participants

To assess the eligibility of physically active women with experience of RT programs, the following inclusion/exclusion criteria were considered: (1) age between 18 and 30 years; (2) more than 6 months doing RT training programs; (3) familiarity with back squat, leg press, and leg extension exercises; (4) no consumption of any type of nutritional supplement or anabolic substances in the preceding 3 months or during the study period itself; (5) no musculoskeletal injuries that could interfere with the exercise protocol during the research. These criteria were verified through personal interviews. The researchers, at each of the visits, asked the participants about their menstrual periods and the consumption of birth control pills, to keep track of changes during the study. The women were cited when they were in the follicular phase. Only 2 women out of the 14 who participated in the study were taking contraceptives. The type of contraceptives was monophasic oral contraceptive with 3 mg of drospirenone and 0.02 mg ethinylestradiol.

The participants recruited were briefed on the protocol and purpose of the study and all signed a written consent prior to the start of the research. The study was conducted in accordance with the Declaration of Helsinki, ⁶⁷ and the project protocol was approved by the ethics committee of the Hospital Universitario Reina Sofía (reference 4284).

Study Interventions

Familiarization Protocol and One-Repetition Maximum Testing

In an initial session, the participants familiarized themselves with the test protocol proposed for this study. First, they performed 5 CMJs of increasing intensity; next they lifted 20 kg in a back squat using a Smith machine (Technogym); and finally, they pressed 80 kg on a leg press and 40 kg on a leg extension machine, using the maximum explosive velocity possible for a total of 3 repetitions. Their technique was evaluated and corrected whenever necessary for all the exercises.

Before attempting the protocols, the participants first completed a specific warm-up. Based on previous research,^45,66 the warm-up began with 10 minutes of pedaling on a cycle ergometer (the first 4 min at a free intensity and the following 6 min at 75% of maximum heart rate [Polar H10]). They then performed 1 warm-up set of the different exercises to be evaluated, but using very light weights.

The testing protocol was performed to find the 1RM testing load for the back squat, leg press, and leg extension, according to the suggestion of Brown and Weir. ⁶ All 3 exercises were performed on the same day, and the protocol was replicated for the back squat, leg press, and leg extension. During the first set, the participants performed 5 repetitions at 50% of the estimated 1RM. In the second set, they performed 3 repetitions at 75% of the estimated 1RM, with 3-minute intervals between them. After the second set, the participants rested for 3 minutes. Then, the participants had up to 5 attempts to achieve the 1RM load, with a 3-minute interval between attempts. This protocol was used to determine the intensity percentages of the exercise in later stages.

Supplement Protocol

The BRJ and PLA were presented in opaque jars. Moreover, adequate transport and storage conditions for the products were ensured. 70 mL of the BRJ (400 mg NO₃^–, BEET It Sport; James White Drinks Ltd), or 70 mL of the PLA, a blackcurrant beverage that depletes NO₃⁻, unlike BRJ (Capri-Sun), was taken 120 minutes before each visit. ¹⁷

As an individual’s diet could affect energy metabolism during exercise, the participants were given nutritional guidelines to ensure that 48 hours before each of the test sessions, they followed a similar diet consisting of 60% carbohydrates, 30% lipids, and 10% proteins using a food group exchange diet.^10,17,30,54 The nutritional guidelines included avoiding foods with a high or moderate NO₃⁻ content (beetroot, celery, arugula, lettuce, spinach, turnip, endives, leak, parsley, cabbage). In the 24 hours prior to each experimental session, the participants were instructed to avoid any products that contained caffeine, due to its ergogenic effect. In addition, in the 24 hours preceding the test sessions, the participants were asked to refrain from brushing their teeth or using a mouthwash, chewing gum, or eating any sweets that could contain a bactericidal substance such as chlorhexidine or xylitol.

Study Outcomes

Anthropometry and Body Composition

The anthropometric measurements and body composition of all the participants were recorded during their first visit. The participants’ height (portable stadiometer; Seca 214) and bioelectrical impedance (MC-780MA; Tanita) were measured to report body composition, with special emphasis on the body fat percentage. Skinfold thickness (Holtain DIM-98.610ND, sensitivity 0.2 mm, Gales) was measured in defined areas, always avoiding muscle (sensitivity 0.1 mm; Cescorf Scientific). The anthropometric measurements consisted of 6 skinfold thickness analyses (triceps, subscapularis, supraspinal, abdominal, thigh, and leg). The measurement was taken according to the International Society for the Advancement of Kinanthropometry (ISAK) protocol. ⁴⁰ All the anthropometric measurement data were collected by a technician certified by ISAK, with an actual technical error of measurement of 0.57%. Therefore, the technical error of measurement was within 5% agreement for skinfolds and within 1% for circumferences.

Jumping Ability (CMJ Test)

The CMJ test comprised 3 CMJs with a recovery period of 45 seconds between jumps. To ensure that it was performed correctly, the participants were instructed to execute a downward movement followed by complete extension of the legs, and they were free to determine the countermovement amplitude necessary to avoid changes in jumping coordination. The CMJ was performed with the hands on the hips. ³⁷ During the jump, an evaluator was 1.5 m distant in the frontal plane, to record the jump with a cell phone (iPhone 7; Apple) at a sampling rate of 240 Hz, using the My Jump 2 App, which has been shown to have good validity (r = 0.997) and reliability (r = 0.995; CV = 3.4%) when used with a force platform. ³ The maximum height (cm) reached for the 3 CMJs was recorded.

Movement Velocity and Power in Back Squat at 50% and 75% 1RM (Mean and Peak Measurements)

The movement velocity was controlled by the researchers using a recently validated linear position transducer (Version 4.1; Speed4Lift), with respect to the “gold standard” (Version 120:Trio; OptiTrack; NaturalPoint, Inc). ⁴⁸ Two back squat tests using a Smith machine were performed, with the movement velocity and power parameters being measured. The first consisted of 2 repetitions at 50% 1RM. The second involved 2 repetitions at 75% of the estimated 1RM, both performed at maximum velocity. The back squat technique involved positioning the back straight, the feet aligned at the width of the shoulders and the hands holding the bar located on the traps. The participants flexed their knees and hips to an angle of greater than 90° and then returned to the initial position. ³⁷ The participants were motivated to perform at their maximum velocity in the concentric phase of each repetition to ensure the use of maximum muscle strength. ²⁵ The mean velocity (MV) in meters per second of the 2 repetitions was recorded, as well as the peak velocity (PV) of these repetitions. Similarly, the mean power (MP) and the peak power (PP) were recorded (in watts).

Muscular Endurance Test (Total Number of Repetitions)

To reach the maximum number of repetitions in the muscular endurance test, 3 exercises were employed: thrust, leg press, and leg extension. For each exercise, three sets were performed until concentric failure at 75% 1RM, with 3 min rest between each. ¹⁶ During the leg press exercise, the participants sat with similar 90° knee angles and, in the back squat, fully extended their lower limbs. In addition to the global orientation of the body, the hip angle for the seated leg press compared with the standing squat differs by about 90°. For the leg extension, the participants were seated in the machine with their hands clasping the handles. They then raised their legs to approximately 30°, 60°, and 90° of knee flexion successively. Finally, they returned to the starting position. ²¹

Rating of Perceived Exertion

As soon as the participants had completed the muscular endurance test, they evaluated their rating of perceived exertion (RPE) on a scale of 1 to 10. ⁵³

Sample Size

As the CMJ is one of the main outcomes for this study, the sample size was determined by calculating the statistical power based on previous studies, ¹² together with a similar female population, ⁶⁰ with a power of 0.80 and a 2-tailed α level set to 0.05; the minimum number of participants required to detect an 8% difference in CMJ performance was estimated as 14.

Randomization

To ensure the double blinding, an external researcher randomly allocated all the participants’ supplements in a counterbalanced fashion; in each session, 50% of the participants ingested BRJ and 50% took the PLA (https://www.randomlists.com/team-generator).

Statistical Analyses

The data are presented as mean ± SD. In addition, we calculated the coefficient of variation (CV) expressed as percentage. To assess the normality of the variables, Shapiro-Wilk tests were performed, and the equality of variance was contrasted with Levene test. To analyze the effect of BRJ versus PLA on the maximum value attained during the CMJ tests, the MV, PV, MP, and PP at 50% and 75% 1RM for the back squat, and RPE of the session, a paired-samples t test was employed. For a practical significance of the results, effect size (ES) were calculated using Hedges g for repeated measures, with their respective 95% CIs. ESs were considered to have large (ES > 0.8), moderate (ES = 0.8-0.5), small (ES = 0.5-0.2), or trivial (ES < 0.2) effects. ²⁷ To analyze the number of repetitions in each set of the muscular endurance test, a 2-way analysis of variance (ANOVA) with repeated measures was applied for the effect of time (sets), supplement type (BRJ versus PLA), and the time-supplement interaction. The Greenhouse-Geisser adjustment was calculated for sphericity. After running an F test for overall significance, differences between means were identified using pairwise comparisons with Bonferroni adjustment. The ES of the ANOVA-RM were calculated using partial eta squared (η_p²), with small considered to be under 0.25, medium as 0.26 to 0.63, and large above 0.63. ²² Significance was set at P < 0.05. SPSS software (Version 22.0, IBM SPSS Statistics for Windows, 2013; IBM Corp) was used for the statistical analysis.

Results

A total of 14 physically active women were recruited. All the participants completed the trial (Figure 1).

Figure 1.

Flow diagram utilizing Consolidated Standards of Reporting Trials (CONSORT) guidelines.

The descriptive characteristics and anthropometric data of participants is listed in Table 1.

Table 1.

Descriptive characteristics and anthropometric data of the physically active female participants

Variable	Value
Age (years)	25.36 ± 3.97
Height (m)	1.64 ± 0.04
Body mass (kg)	56.98 ± 5.42
BMI (index)	20.97 ± 1.48
Body fat (%)	21.99 ± 3.75
Sum of 6 skinfolds (mm)	90.45 ± 23.64
RT experience (years)	4.14 ± 2.14
1RM back squat (kg; kg/body mass)	60.53 ± 11.31; 1.06 ± 0.18
1RM leg press (kg; kg/body mass)	159.28 ± 44.80; 2.8 ± 0.73
1RM leg extension (kg; kg/body mass)	84.28 ± 14.39; 1.48 ± 0.22

1RM, one-repetition maximum; BMI, body mass index; RT, resistance training.

In terms of jumping ability, it was observed a significant effect of BRJ on the maximum height reached in the CMJ (+6.02%; 1.82 ± 3.05 cm; P = 0.04; ES = 0.36 [−0.42 to 1.15]) (Figure 2).

Figure 2.

Differences in countermovement jump (CMJ) performance between the placebo (PLA) and beetroot juice (BJ) supplement.

For movement velocity and power measurement, differences were observed at 50% 1RM in favor of BRJ, in MV (+6.7%; 0.04 ± 0.1 m/s; P = 0.03; ES = 0.39), PV (+6%; 0.4 ± 0.07 m/s; P = 0.04; ES = 0.39), MP (+7.3%; 14.4 ± 21.1 W; P = 0.02; ES = 0.30), and PP (+6.8%; 13.8 ± 21.8 W; P = 0.03; ES = 0.20); but no statistically significant differences were observed for any parameter at 75% 1RM (Table 2).

Table 2.

Movement velocity and power measurement in back squat exercise

Variable	50% 1RM						75% 1RM
	BRJ		PLA		P		BRJ		PLA		P
	M ± SD	CV (%)	M ± SD	CV (%)		ES	M ± SD	CV (%)	M ± SD	CV (%)		ES
MV (m/s)	0.71 ± 0.1	14.0	0.67 ± 0.1	14.9	0.030*	0.39 [−0.40 to 1.17]	0.50 ± 0.06	12.0	0.48 ± 0.07	14.6	0.275	0.30 [−0.48 to 1.08]
PV (m/s)	0.73 ± 0.1	14.0	0.69 ± 0.1	14.5	0.040*	0.39 [−0.40 to 1.17	0.51 ± 0.06	11.8	0.50 ± 0.06	12.0	0.363	0.16 [−0.62 to 0.94]
MP (W)	211.3 ± 51.3	24.2	196.8 ± 43.2	22.0	0.024*	0.30 [−0.48 to 1.08]	222.2 ± 52.2	23.5	214.6 ± 50.7	23.6	0.186	0.14 [−0.63 to 0.92]
PP (W)	216.5 ± 51	23.5	202.6 ± 45.2	22.3	0.034*	0.20 [−0.58 to 0.98]	229.7 ± 54.6	23.7	220.2 ± 51.2	23.3	0.135	0.17 [−0.60 to 0.95]

1RM, one-repetition maximum; BRJ, beetroot juice; CV, coefficient of variation; ES, effect size; MP, mean power; MV, mean velocity; PLA, placebo; PP, peak power; PV, peak velocity; W, watts.

^*Significant differences (P < 0.05).

For the muscular endurance test, a statistically significant effect was found for time in the 3 exercises analyzed: back squat (F = 59.009; P < 0.00; η_p² = 0.905), leg press (F = 36.145; P < 0.00; η_p² = 0.858), and leg extension (F = 46.875; P < 0.00; η_p² = 0.887), as well as in the sum of the 3 exercises (F = 73.605; P < 0.00; η_p² = 0.925). Supplement type was also significantly different in terms of the sum of the 3 exercises (F = 23.902; P < 0.00; η_p² = 0.648), in addition to each of the 3 individual exercises: back squat (F = 18.294; P < 0.00; η_p² = 0.585), leg press (F = 21.277; P < 0.00; η_p² = 0.621), and leg extension (F = 9.487; P = 0.01; η_p² = 0.422). A statistically significant effect was also seen in terms of supplement interaction time in the sum of the repetitions of the 3 exercises analyzed (F = 24.236; P < 0.00; η_p² = 0.651), back squat (F = 18.070; P < 0.00; η_p² = 0.582), leg press (F = 27.157; P < 0.00; η_p² = 0.819), and leg extension (F = 8.724; P < 0.01; η_p² = 0.402), with a higher number of repetitions being seen after BRJ supplementation in every set in each of the exercises (P < 0.05) (Table 3).

Table 3.

Number of repetitions performed in back squat, leg press, and leg extension exercises

Variable	Supplement		Set 1	Set 2	Set 3	Total Sets	P Value Time	P Value Supplement	P Value Time·Supplement
Back squat	BRJ	M ± SD	16.1 ± 2.7 a,#	14 ± 2.6 b,#	11.2 ± 1.8 #	41.4 ± 6.5 #	0.00*	0.00*	0.00*
		CV (%)	16.8	18.6	16.1	15.7
	PLA	M ± SD	11.8 ± 2.5	9.9 ± 2.4	8.2 ± 2.7	29.9 ± 7.1
		CV (%)	21.2	24.2	32.9	23.7
Leg press	BRJ	M ± SD	20 ± 5.2 a,#	15.9 ± 4.5 b,#	13.7 ± 4.1 #	49.6 ± 13.3 #	0.00*	0.00*	0.00*
		CV (%)	26.0	28.3	29.9	26.8
	PLA	M ± SD	13.2 ± 4.6	10.9 ± 3.4	8.3 ± 2.7	32.5 ± 10.5
		CV (%)	34.8	31.2	32.5	32.3
Leg extension	BRJ	M ± SD	15.6 ± 2.9 a,#	13.3 ± 3.3 b,#	11.6 ± 2.4 #	40.5 ± 8.3 #	0.00*	0.01*	0.01*
		CV (%)	18.6	24.8	20.7	20.5
	PLA	M ± SD	12.6 ± 3.2	11.1 ± 2.8	9.9 ± 2.8	33.6 ± 8.4
		CV (%)	25.4	25.2	28.3	25.0
Total	BRJ	M ± SD	51.7 ± 8.4 a,#	43.2 ± 7 b,#	36.6 ± 5.7 #	131.5 ± 20.5 #	0.00*	0.00*	0.00*
		CV (%)	16.2	16.2	15.6	15.6
	PLA	M ± SD	37.6 ± 7.8	31.9 ± 6.3	26.5 ± 5.7	96.1 ± 19.1
		CV (%)	20.1	19.7	21.5	19.9

BRJ, beetroot juice; CV, coefficient of variation; PLA, placebo.

^aThere are differences between set 1 and sets 2 and 3.

^bThere are differences between set 2 and set 3.

^*Significant differences (P < 0.05).

^#Significant differences between the same set for BRJ and PLA.

Nevertheless, no differences were reported in terms of RPE at the end of the muscular endurance test (BRJ, 7.36 ± 0.84; PLA, 7.50 ± 0.85; P = 0.655; ES = 0.17 [−0.9 to 0.58]).

Discussion

The purpose of this study was to investigate the possible ergogenic effect of BRJ on the production of muscle force and muscular endurance in physically active women. The results confirm our hypothesis that an acute BRJ supplement taken 120 minutes before exercise improved muscle force production in physically active women, as assessed by the height reached in a CMJ test and the power produced in a back squat with a load corresponding to 50% 1RM (although not at 75% 1RM). In addition, BRJ facilitates enhanced muscular endurance, as assessed by an RT test consisting of 3 sets until concentric failure at 75% 1RM in 3 lower-limb exercises.

The enhanced CMJ performance after BRJ supplementation (+6%; P = 0.04) is in line with the improvement of the PP (+6.8%; P = 0.03) and MV (+6.7%; P = 0.03) for the back squat at 50% 1RM. The height reached in a CMJ is determined by the velocity reached at takeoff. ⁴ The absence of any effect of the BRJ supplement at a load of 75% 1RM, with an increase in MV at 50% 1RM, and performance in the CMJ therefore suggests that a BRJ supplement potentiates power only during high-velocity contractions. In this sense, previous studies performed by Coggan et al^13,14 reported enhanced power output during an isokinetic leg extension exercise at high angular velocity (360 deg/s), but not a low or moderate angular velocity (90-270 deg/s). With the increase of the movement velocity occur a predominant of type II muscle fibers. ²⁹ In fact, some effects, such as increased skeletal muscle blood flow, have been reported preferentially in type II muscle fibers after NO₃^– supplementation, ²³ with the ergogenic effects acting selectively on type II muscle fibers. ³¹ It has therefore been suggested that NO₃^– supplementation has a potential ergogenic effect on RT exercises and movement performed at a high contraction velocity. ⁵⁶ Our results suggest that, in line with Williams et al, ⁶⁶ who reported an MV in a bench press exercise with a load corresponding to 70% 1RM similar to our enhancement with 50% 1RM (+6.5% vs +6.7%), this could be explained by an enhanced skeletal muscle contractile function. Conversely, the stop included in the study performed by Ranchal-Sánchez et al, ⁵² between the eccentric and concentric phase, could blur the ergogenic effect of BRJ on MV and MP by reducing the contraction velocity; this may explain the differences between the results of the present study and the results reported by Williams et al. ⁶⁶ In fact, on a flywheel device, which is a machine that maximizes tension on the plyometric phase though an eccentric overload, ¹⁰ an enhanced MP has been reported for the eccentric and concentric phases. ⁵⁴

In terms of muscular endurance, an enhanced number of repetitions until concentric failure was observed with a load corresponding to 75% 1RM for the total session and each of the 3 sets performed for the 3 exercises (back squat, leg press, and leg extension). These results are in line with previous studies.^45,66 Indeed, our research group ⁵² observed an increase (+17.7%) in the total number of repetitions in a session comprising a set with a load of 60%, 70%, and 80% 1RM for bench presses and back squats after BRJ supplementation. In addition, Williams et al ⁶⁶ reported an increase (+10.7%) in the maximum number of repetitions in 3 sets with a load of 70% 1RM, with a recovery of 2 minutes between sets on a bench press, whereas Mosher et al ⁴⁵ reported an enhancement (+19.4%) with a load corresponding to 70% 1RM. Based on previous studies that observed enhanced excitation-contraction coupling of human skeletal muscle in addition to explosive force production, ²⁶ and other studies that have reported a less-pronounced decrease in PCr in exercise at different intensities, ⁵⁴ it could be speculated that BRJ supplementation could enhance muscular endurance based on improved muscle contraction function, ²⁶ delayed PCr depletion during the muscular endurance test, and a possible recovery of PCr between sets, depending on an oxidative metabolism, ⁶² as a consequence of enhanced muscle blood flow.^23,56

Although various studies have reported improved running^46,57 and cycling^24,30 performance with a lower RPE, in this study we found enhanced muscular endurance but with the same RPE. Other studies have reported improved physical performance in various types of exercise with no changes in RPE.^3,45,57,61 The higher number of repetitions performed after BRJ supplementation, but with the same RPE, therefore reflects an enhancement of the ratio between workload and RPE. ⁹ The modification that stabilizes RPE, even though there is an increased workload, could be related to the ergogenic effect of NO₃^– supplementation, as this improves the blood flow to the frontal lobe of the brain, either improving motor control and causing a subjective perception of effort ⁴⁹ or reducing the muscle metabolite accumulation ² that attenuates the development of central fatigue and decreasing the activation of type III/IV muscle afferent feedback to the central nervous system. ⁶⁴

Although women have a phenotype that is less favorable in terms of reduction to NO₂^– after NO₃^– supplementation compared with men, ⁶⁵ they do present an oral microbiome that is more conducive to oral NO₃^– reduction, resulting in an increased NO₂^– plasma level after NO₃^– supplementation. ³³ Therefore, in line with a previous study that reported an equally ergogenic effect of BRJ supplementation on running performance in male and female recreational runners, ⁸ in the present study it was found that the BRJ supplement had an ergogenic effect on lower-limb muscular power, as assessed by CMJ, and muscular endurance performance in back squats, in a similar way to previous studies carried out on men.

Although this study is the first to assess the effect of BRJ on explosive force production and muscular endurance in physically active women, there are a number of limitations that should be acknowledged. First, the number of women was small (N = 14), indicating this is a preliminary study. In addition, we used a dose of BRJ and timing relative to an expected increase in plasma NO₂^– based on previous studies, ⁶⁸ but we did not assess NO₂^– plasma values, meaning that the relationship between the ergogenic effect of BRJ and the explanation based on the increased on NO₂^– plasma levels is merely speculative. Second, we did not assess PCr concentrations after the muscular endurance test, which could reflect greater efficiency of the metabolic pathway, as reported in previous studies. ⁵⁴ Third, muscle blood flow during effort and recovery was not assessed, which could be used to quantify any possible ergogenic effect caused by an increased oxidative contribution during the muscular endurance test or subsequent recovery. We also did not assess the intrinsic contractile properties of the muscle during this study, which could confirm the contribution of this mechanism to the ergogenic effects of BRJ supplementation. Finally, although this study provides novel data on the ergogenic effect of BRJ on the production of explosive force and muscular endurance in physically active women, this study does not compare the results with an equivalent male population. Therefore, and considering a potentially different response in terms of increased NO₂^– levels and muscle contractile function according to sex,^13,46 future studies should include a group of both men and women to analyze possible sex-related differences.

Conclusion and Clinical Relevance

The acute ingestion of a BRJ supplement 120 minutes before exercise seems to produce an ergogenic effect on explosive force production as assessed through a CMJ test, as well as movement velocity and power at 50% 1RM in physically active women. Moreover, the BRJ supplement improved lower-limb muscular endurance performance in RT, as assessed by the maximum number of repetitions until concentric failure in tests comprising 3 sets of back squat, leg press, and leg extension exercises, although the RPE did not change. These findings suggest that providing a BRJ supplement could be a suitable strategy for improving lower-limb power and muscular endurance in physically active women.