International Journal of Sports Physiology and Performance, 2014, 9, 845-850 http://dx.doi.org/10.1123/ijspp.2013-0384 © 2014 Human Kinetics, Inc.
www.IJSPP-Journal.com ORIGINAL INVESTIGATION
No Improvement of Repeated-Sprint Performance With Dietary Nitrate Kristy Martin, Disa Smee, Kevin G. Thompson, and Ben Rattray Purpose: Nitrate supplementation improves endurance exercise and single bouts of high-intensity activity, but its effect on repeated sprints is unclear. This study is the first to investigate the effects of acute dietary nitrate supplementation during a high-intensity intermittent-sprint test to exhaustion. Methods: Team-sport athletes (9 male, age 22.3 ± 2.1 y, VO2max 57.4 ± 8.5 mL · kg–1 · min–1; 7 female, age 20.7 ± 1.3 y, VO2max 47.2 ± 8.5 mL · kg–1 · min–1) were assigned to a double-blind, randomized, crossover design. Participants consumed 70 mL of concentrated beetroot juice containing a minimum of 0.3 g of nitrate (NT) or 70 mL of placebo (PL) 2 h before a repeated-sprint protocol involving repeated 8-s sprints with 30-s recovery on a cycle ergometer to exhaustion. Results: Fewer sprints (NT = 13 ± 5 vs PL = 15 ± 6, P = .005, d = 0.41) and less total work (NT = 49.2 ± 24.2 kJ vs PL = 57.8 ± 34.0 kJ, P = .027, d = 0.3) were completed in NT relative to PL. However there was no difference in overall mean power output or the mean power output for each individual 8-s sprint. Conclusions: These findings suggest that dietary nitrate is not beneficial for improving repeated-sprint performance, at least when such sprints are near-maximal and frequent in nature. The lack of an effect of nitrate at near-maximal oxygen uptake supports the suggestion that at greater exercise intensities nitrate does not have an ergogenic effect. Keywords: supplementation, exercise performance, team sport Dietary nitrate is a widely used exercise supplement buoyed by research regarding its beneficial effect on health and exercise.1–3 It has been shown to reduce resting blood pressure through vasodilation of capillaries and to affect fundamental physiological parameters during exercise.1 Although nitrate supplementation appears to be ergogenic for the performance of prolonged (>6 min), continuous endurance exercise (time to exhaustion increased by up to 25%4,5) and single bouts of high-intensity exercise (time-trial performance enhanced by nearly 3%2), to date no study has examined the effect of nitrate supplementation on intermittent-sprint exercise. During high-intensity intermittent exercise, insufficient energy supply and the accumulation of intramuscular metabolic by-products are believed to be factors responsible for fatigue.6 Nitrate may negate such causes of fatigue by reducing the estimated total ATP cost of exercise4 and decreasing the breakdown of phosphocreatine (PCr).1,4,5 Furthermore, during exercise in hypoxic conditions, the severity of PCr degradation, inorganic phosphate accumulation, and the fall of muscle pH reduce with nitrate supplementation, and in addition nitric oxide pathways also appear to be enhanced under hypoxic or acidic conditions.7 During submaximal exercise, nitrate lowers oxygen uptake (VO 2) without any effect on minute ventilation, heart rate, respiratory-exchange ratio, or accumulation of blood lactate.1,4,5 Cardiovascular efficiency has also been reported to improve with nitrate supplementation, through an enhanced diffusion of oxygen to tissues farther away from capillaries, resulting in a more precise local matching of oxygen delivery to metabolic rate.8
As a number of mechanisms affected by dietary nitrate might enhance intermittent-sprint exercise, it is surprising that the effect of supplementation on team-sport athletes is yet to be fully explored. A common requirement of many team sports is the ability to produce near-maximal sprints of short duration with brief recovery periods over an extended period of time.9 An integral part of many sports, therefore, is the capacity to perform well in what has now been termed repeated-sprint ability.9 It has been suggested that athletes with enhanced repeated-sprint ability are likely to perform better than those who are less able to replicate high-intensity efforts,10 and superior repeated-sprint ability in both speed maintenance and fatigue resistance is characteristic of better team-sport players.11 Any noted supplement offering a performance advantage in intermittent-sprint exercise would therefore be highly sought after. The aim of the current study was to investigate the effect of dietary nitrate on intermittent-sprint exercise performance. We hypothesized that, relative to placebo, nitrate supplementation would enable a greater amount of work to be completed, resulting in more sprints and larger mean power output during a high-intensity, intermittent-sprint test to exhaustion. We reasoned that as highintensity exercise places the body under a great amount of metabolic stress, the effects of nitric oxide would work preferentially under such conditions by improving oxygen delivery to the exercising muscle, enhancing aerobic energy contribution during sprints, and promoting enhanced PCr recovery.7
Methods Participants
The authors are with the University of Canberra National Inst of Sport Studies (UCNISS), Canberra, ACT, Australia. Address author correspondence to Kristy Martin at [email protected].
Sixteen participants, moderately trained in team sport, volunteered to participate in this study (9 male, age 22.3 ± 2.1 y, VO2max 57.4 ± 8.5 mL · kg–1 · min–1; 7 female, 20.7 ± 1.3 y, VO2max 47.2 ± 8.5 mL 845
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· kg–1 · min–1). All procedures were approved by the University of Canberra Committee for Ethics in Human Research. Participants gave written informed consent to participate.
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Design A double-blind, randomized crossover design was employed. Participants took part in initial testing and familiarization sessions to establish reliable baseline measures for the high-intensity intermittent-sprint test (HIIST) before placebo and treatment trials. They were instructed to arrive in a rested and fully hydrated state and at least 3 hours postprandial. Participants were to avoid strenuous exercise 24 hours preceding the exercise test and to refrain from consuming alcohol for 24 hours and caffeine for 6 hours before each test. For 24 hours before the first HIIST, participants recorded their food and fluid intake and physical activity. They were instructed to replicate these conditions 24 hours before each subsequent HIIST. Compliance with these requests was confirmed through a pretrial questionnaire.
Preliminary Testing Initially, participants underwent preliminary testing and familiarization with the HIIST. Preliminary testing involved a maximal ramped test to exhaustion on a cycle ergometer (High Performance Ergometer, Schoberer Rad MeBtechnik, Germany), used to identify individual workloads for the HIIST. The test to exhaustion began at 100 W for men and 50 W for women for 5 minutes and then increased by 5 W every 15 seconds. During the maximal ramped test, exhaustion was determined by volitional fatigue or inability to maintain cadence over 70 rpm and a respiratory-exchange ratio ≥1.1. Resistance for the HIIST was then set at 200% of the last fully completed resistance level during the preliminary testing. Two familiarization sessions were conducted within a week, as this has been shown to improve the reliability of repeated-sprint data12; they consisted of up to ten 8-second sprints at the previously determined workload. Two familiarization sessions have previously shown to improve reliability of power-output data of repeated sprints.12
High-Intensity Intermittent-Sprint Test Participants performed the HIIST at the same time of day on 2 occasions, separated by a washout period of at least 72 hours. Plasma nitrate and nitrite have been shown to return to near baseline levels 24 hours after ingestion of 500 mL of beetroot juice.3 The HIIST consisted of a 2-minute warm-up, cycling at 100 W at a self-selected cadence, followed by repeated 8-second bouts of high-intensity exercise at the predetermined, individualized workload, set using the hyperbolic mode of the cycle ergometer. The cadence selected during the warm-up was replicated in the subsequent trials. Sprints were interspersed with 30-second periods of active rest, consisting of cycling at 50 W at a self-selected cadence (Figure 1). Participants repeated these sprint efforts until volitional fatigue or when they were unable to increase cadence to over 100 rpm. If fatigue was not reached after 45 sprints the test was terminated, this being equal to 30 minutes of cycling. Participants received verbal encouragement to continue for as long as possible from a single, blinded researcher. Environmental conditions in the laboratory were kept consistent between participants and trials. Geometric ergometer setup selected in the first test was maintained in subsequent sessions, and the ergometer was calibrated on each occasion. The protocol was derived from the repeated-sprint efforts undertaken in team sports. This protocol elicits a 1:3.75 work:rest ratio similar to that of previous team-sport repeated-sprint studies.13 While the test took place on a stationary ergometer, it has previously been reported that there is a strong correlation between repeated-sprint cycling and running performance.14
Intervention Two hours before the HIIST, participants consumed 70 mL of nitrate-rich beetroot juice containing a minimum of 0.3 g of nitrate (NT) or 70 mL of manufacturer-supplied nitrate-depleted beetroot juice (PL; Beet It, James White Drinks Ltd, UK). Nitrate was removed from the placebo product before pasteurization by passing beetroot juice through a column containing Purolite A520E ion-exchange resin, which is specific for nitrate. Ingestion of the
Figure 1 — Schematic of high-intensity intermittent-sprint protocol. Abbreviations: O, oxygen uptake; B, plasma lactate and glucose, RPE, rating of perceived exertion; WU, warm-up; W, week; F, finish.
Nitrate and Intermittent Exercise 847
supplement occurred 2 hours before the HIIST, as plasma nitrate has been shown to significantly increase (~16-fold) 30 minutes after beetroot ingestion, peaking at 1.5 hours and then remaining stable at this level up to 5 hours postingestion.3 Participants were asked to abstain from using antibacterial mouthwash and chewing gum throughout the study to preserve commensal oral bacteria, which reduce nitrate to nitrite.15
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Measures HIIST Performance. Performance was determined from the number of 8-second sprints completed at the required cadence and from the actual power output during the sprints, recorded at 2 Hz using SRM software (SRM Training System, Schoberer Rad MeBtechnik, Germany). Mean power output was recorded as the average power output during the test, and peak power output was recorded as the greatest power output achieved. Mean power output of each individual, 8-second sprint was recorded as the average power output for each sprint, and total work done was calculated as the product of the average power output during the completed 8-second sprints throughout the test in watts and time in seconds.
Results Maximal Ramped Test Mean maximal VO2 obtained during the ramped test was 49.56 ± 11.8 mL · kg–1 · min–1. There was a weak positive relationship between maximal VO2 and sprints completed in the control and nitrate trials of the HIIST (r = .28 and r = .20, respectively). There was a weak negative relationship between maximal VO2 and response to nitrate supplementation, as determined by the change in number of sprints completed in the nitrate trial (NT) relative to placebo (PL; r = –.16).
HIIST Performance Compared with placebo, fewer sprints were completed in NT (NT = 13 ± 5 sprints vs PL = 15 ± 6, P = .005, d = 0.41); this was the case for 11 of 13 participants (Figure 2). Less total work during the sprints was also done with nitrate (NT = 49.2 ± 24.2 kJ vs PL = 57.8 ± 34.0, P = .027, d = 0.3; Figure 3). There was no difference
VO2 and Heart Rate. During the initial maximal ramped test, VO2
was recorded using an open-circuit indirect calorimetry system (TrueOne 2400 Metabolic Measurement System, Parvo Medics, Sandy, UT, USA) throughout the entire test. During the HIIST, VO2 was measured continuously and then averaged over 30 seconds, from the beginning of warm-up through to the end of the fifth sprint. Pilot testing had revealed that gas analysis interfered with performance of the HIIST, and hence as the primary measure of the current study, VO2 recording was discontinued after the fifth sprint. Gas analyzers were calibrated using primary standard reference gases before each test. Heart rate was recorded during testing by a heart-rate monitor fitted by a chest strap (T34 noncoded heart-rate transmitter, Polar, Finland). Plasma Lactate and Rating of Perceived Exertion. Capillary
blood samples were collected from the earlobe before the HIIST, in the first 10 seconds after the fourth sprint, and immediately after termination of exercise. Samples were analyzed using a lactate analyzer (Lactate Pro, Arkray, Japan). These samples were used to compare plasma lactate concentrations between conditions. Rating of perceived exertion using a 0-to-1016 scale was recorded every 3 sprints and at the point of fatigue as a measure of perceived effort encountered during work.
Figure 2 — Sprints completed in the control and nitrate trials for each participant. *Significantly fewer sprints were completed in the nitrate trial (P = .005).
Statistical Analyses Differences in the number of sprints completed, total work, and mean and peak power were compared between trials using a 1-way ANOVA with repeated measures, as was the number of sprints completed in the first or second trial. Two-factor repeated-measures ANOVA was used to compare differences between individual sprints in terms of work done and mean power output and to compare repeated measurements of plasma lactate concentration, VO2 during the warm-up and first 5 sprints of the HIIST, rating of perceived exertion, and heart rate. Statistical significance was accepted at the P < .05 level, and data are presented as mean ± SD. Effect sizes for mean differences are expressed using Cohen d. After testing was completed, 3 participants’ data sets were excluded from analysis due to their having completed the maximum 45 sprints in 1 or both of the trials without reaching terminal fatigue.
Figure 3 — Total work done during the sprint efforts of the control and nitrate trials. *Significantly less total work was completed in the nitrate trial (P = .027).
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in overall mean power output of the combined sprints (NT = 447 ± 104 W vs PL = 444 ± 117, P = .797), mean peak power (NT = 447 ± 104 W vs PL = 444 ± 117, P = .196), the mean power output, or total work done for each individual 8-second sprint (P > .05). A comparison of sprints completed in the first versus the second trial, was carried out to determine any order effects, but analysis revealed no difference (P > .05).
VO2 and Heart Rate
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There was no difference in VO2 at baseline. Nitrate supplementation resulted in a significantly reduced VO2 during the warm-up of the HIIST (NT = 22.0 ± 4.6 mL · kg–1 · min–1 vs PL = 24.7 ± 2.8, P = .002, d = 0.71; Figure 4). VO2 attained during the first 5 sprints of the HIIST was not different between trials (P > .05). There was no difference between conditions for heart rate at any time point (P > .05).
Plasma Lactate and Rating of Perceived Exertion There was no difference in plasma lactate concentration between trials at baseline, after the first 4 sprints, or at termination of exercise (P > .05). No significant differences between trials were observed for rating of perceived exertion at baseline, after 3 sprints, or at termination of exercise (P > .05).
Discussion The primary finding of the current study was that an acute dose of dietary nitrate reduced performance during a high-intensity intermittent-sprint test to exhaustion. Capacity to complete sprints was decreased after supplementation, as was total work. This reduction in work capacity occurred without any difference between trials for mean power output or the total work completed during each individual sprint. As observed previously,1,4,5 VO2 was reduced with nitrate during the submaximal warm-up, although this trend did not extend to the HIIST. The reduced VO2 during the submaxi-
Figure 4 — Oxygen uptake profile of the high-intensity intermittent-sprint test. Shaded area indicates warm-up (2 min cycling at 50 W). *Nitrate was significantly reduced during warm-up compared with placebo (P = .002).
mal warm-up suggests that the ingestion of beetroot juice 2 hours before exercise, previously shown to result in an increase in plasma nitrite,17 resulted in the uptake of nitrate by our participants. To our knowledge this is the first study to observe a decrease in performance after nitrate supplementation. Conflicting with our hypothesis, fewer sprints were completed in the nitrate condition. Previously, a number of studies found an increased time to exhaustion after nitrate supplementation during combined arm and leg cranking, 2-legged knee extension, and incremental cycling to fatigue.4,5 In those studies, nitrate appears to be ergogenic for controlled, single, and largely submaximal efforts. Although 1 study used an incremental, intermittent recovery protocol to assess the possible benefits of nitrate supplementation for team-sport players,18 this is the first study to investigate the effect of dietary nitrate on intermittent-sprint exercise to exhaustion. The HIIST protocol was designed to investigate the potential effect of nitrate on intermittent-sprint exercise, similar to that experienced by athletes during team-sport play. However, despite previous findings, exercise was terminated earlier with nitrate supplementation than with placebo. A possible cause for the lack of agreement between this study and earlier research is the difference in intensity between the HIIST and protocols previously used. Several recent studies indicate that nitrate supplementation results in a significant reduction in pulmonary VO2 during submaximal exercise, an effect that appears to be related to enhanced skeletal-muscle efficiency. A reduced VO2 was observed during the warm-up component of the HIIST, although this did not extend to the higher-intensity sprints. The reduced VO2 during the warm-up in the nitrate trial was foreseeable given that this has been shown during short submaximal steady-state exercise previously.1,4,5 In addition, it has previously been reported that nitrate supplementation is only effective in reducing VO2 at intensities up to 80% of maximum.4 VO2 in the current study immediately after the fifth sprint was >75% VO2max,4 and thus it appears likely that in subsequent sprints, the VO2 of participants would reach levels greater than the proposed threshold, negating the positive effects of nitrate. The HIIST requires short periods of maximal effort to be repeated with insufficient recovery and hence requires participants to be working at a high intensity from the outset, a protocol more representative of on-field performance. A similar 10 × 6-second repeated-sprint protocol with 30-second recovery found that absolute PCr concentration decreased to 57% of resting value after sprint 1 and progressively declined to only 16% after the final sprint, suggesting that the majority of PCr resynthesis is not complete within 30 seconds of recovery.19 This brief recovery period, insufficient for complete PCr resynthesis, may not be great enough in duration for a nitric-oxide-mediated effect. Therefore it is possible that the intensity of the HIIST and inadequate recovery might have been too severe for the nitrate supplementation to have a significant effect on aerobic metabolism. Recently, shorter repeated-sprint protocols have been used to assess the effect of nitrate supplementation on intermittent exercise. No difference in peak or mean power output between nitrate and placebo conditions was found during 6 × 20-second sprints performed by elite cyclists20 or 5 × 10-second maximal sprints in trained male flat-water kayakers.21 These results support our finding of no difference between trials for the mean power of individual sprint efforts. In contrast, a 0.4% improvement was reported over 6 × 500-m maximal ergometer rowing efforts.17 Those sprints, however, were unlike those in the HIIST. The maximal 500-m rowing efforts
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Nitrate and Intermittent Exercise 849
were much longer in duration, lasting ~90 seconds, with longer rest periods and an approximate 1:1 work-to-rest ratio. In addition, all of these protocols had a predetermined endpoint. Only 1 study has attempted to measure the influence of nitrate supplementation on intermittent exercise, by using the Yo-Yo Intermittent Recovery Test, Level 1 (YIRT1).18 The principal finding of that study was that supplementation of concentrated beetroot juice improved intermittent exercise performance, measured by an increased distance covered.22 However, although statistically significant, the difference covered in the YIRT1 between beetroot and placebo trials was only 4.2% and 68 m on average, whereas 14% fewer sprints were completed during the HIIST. Authors of that paper also noted that the practical significance of this difference was minor and particularly so when compared with 16% and 29% improvements observed in the YIRT2 (version of the test for elite athletes) after caffeine ingestion23 and sprint and speed training,24 respectively. In addition, it is possible that these small changes, unseen in previous repeated-sprint protocols, may have been due to the much greater preloading of nitrate, as well as increased preexperimental dose. In total, ~2.17 g of nitrate was consumed over 36 hours in the YIRT1, versus ~0.31 g in the current study. In addition, the 0.4% improvement reported in repeated maximal ergometer rowing was preluded by 6 days of chronic nitrate supplementation.5 Moreover, it has been shown that performance in certain exercise tests is improved with more chronic nitrate supplementation,23 and responses may be dose-specific; however, further research is required to determine whether performance is increased to a greater extent with longerterm nitrate supplementation. Furthermore, although the YIRT1 was specifically developed to replicate the high-intensity running bouts in football match play,22 the progressive increase in pace and intensity until participants reach fatigue is much different from the HIIST, which requires maximal effort from the outset. The YIRT1 protocol is more similar to a traditional aerobic-fitness test than a snapshot of team-sport play. Inconsistent research findings are also available on the effect of nitrate on force production. In the current study, the reduction in the number of sprints completed with nitrate was not explainable by difference in the mean power output of the sprints. Studies have shown that nitric oxide depresses force output during isometric contractions in single muscle fibers25 and in muscle-fiber-bundle preparations.26 However, in human research, treatment with nitric oxide donors has been shown to result in an increase in the force generated during maximum voluntary contractions but a decrease in force at submaximal frequencies.27 Thus, it would appear that the effect of nitrate depends on the mode, intensity, and duration of muscle contraction.28
Practical Applications The current study investigated the effect of a single dose of dietary nitrate on intermittent-sprint exercise, up to a maximum of 30 minutes, using a protocol that relates to the short duration and repeated sprints that team-sport athletes typically undertake during a match. Using this protocol, the capacity to complete sprints was reduced; hence, dietary nitrate might not be beneficial for improving repeated-sprint performance, at least when such sprints are near maximal and frequent in nature. Further research involving sport- and match-specific protocols is needed to clarify the effects of supplementation during real on-field sporting performance.
Limitations Protocols of this nature are prone to a learning effect. However, it is unlikely that this occurred in the current study, as 2 familiarizations were included and supplementation was randomized to eliminate order effects due to learning or training. Analysis confirmed that there was no difference between the first and second trials. A further strength of the current study was the use of a nitrate-depleted beetroot juice as a placebo; this enabled a double-blinded study design. A placebo indistinguishable from the treatment condition was used, decreasing the likelihood of psychological factors or other nutrients contaminating the results. This has been a limitation in previous studies.5 It is also pertinent to mention that the concept of responders and nonresponders to nitrate supplementation has arisen previously.29 Wilkerson et al29 reported that while group mean 50-mile cycle time-trial performance was not significantly improved by acute nitrate supplementation, participants who saw a greater increase in plasma nitrite (responders) improved their performance by more, whereas those whose plasma nitrite did not change appreciably did not improve their performance. The current study did not measure changes in plasma nitrite; future research should consider both changes and degree of change in plasma nitrite when analyzing results.
Conclusions Overall, these findings indicate that under the conditions of the current study, nitrate supplementation potentially leads to a decrement in the number of repeated sprints that can be maintained. The lack of an effect of nitrate at near-maximal oxygen uptake supports the suggestion that at greater exercise intensities nitrate does not have an ergogenic effect. Differences in intensity therefore may have contributed to the dissimilarities in results between protocols. Acknowledgments No direct funding was received for the study. Remuneration for all supplements and equipment was provided by the University of Canberra. All procedures were approved by the University of Canberra Committee for Ethics in Human Research.
References 1. Vanhatalo A, Fulford J, Bailey SJ, Blackwell JR, Winyard PG, Jones AM. Dietary nitrate reduces muscle metabolic perturbation and improves exercise tolerance in hypoxia. J Physiol. 2011;589(Pt 22):5517–5528. PubMed 2. Lansley KE, Winyard PG, Bailey SJ, et al. Acute dietary nitrate supplementation improves cycling time trial performance. Med Sci Sports Exerc. 2011;43(6):1125–1131. PubMed doi:10.1249/ MSS.0b013e31821597b4 3. Webb AJ, Patel N, Loukogeorgakis S, et al. Acute blood pressure lowering, vasoprotective, and antiplatelet properties of dietary nitrate via bioconversion to nitrite. Hypertension. 2008;51:784–790. PubMed 4. Larsen FJ, Weitzberg E, Lundberg JO, Ekblom B. Effects of dietary nitrate on oxygen cost during exercise. Acta Physiol (Oxf). 2007;191(1):59–66. PubMed doi:10.1111/j.1748-1716.2007.01713.x 5. Bailey SJ, Winyard P, Vanhatalo A, et al. Dietary nitrate supplementation reduces the O 2 cost of low-intensity exercise and enhances tolerance to high-intensity exercise in humans. J Appl
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850 Martin et al Physiol. 2009;107(4):1144–1155. PubMed doi:10.1152/japplphysiol.00722.2009 6. Bishop DJ. Fatigue during intermittent-sprint exercise. Clin Exp Pharmacol Physiol. 2012;39(9):836–841. PubMed doi:10.1111/j.14401681.2012.05735.x 7. Modin A, Herulf A, Weitzberg L. Nitrite-derived nitric oxide: a possible mediator of ‘acidic–metabolic’ vasodilation. Acta Physiol Scand. 2001;171(1):9–16. PubMed 8. Thomas DD, Liu X, Kantrow SP, Lancaster JR. The biological lifetime of nitric oxide: implications for the perivascular dynamics of NO and O2. Proc Natl Acad Sci USA. 2001;98(1):355–360. PubMed doi:10.1073/pnas.98.1.355 9. Spencer M, Bishop D, Dawson B, Goodman C. Physiological and metabolic responses of repeated-sprint activities. Sports Med. 2005;35(12):1025–1044. PubMed doi:10.2165/00007256-20053512000003 10. Bishop D, Spencer M, Duffield R, Lawrence S. The validity of a repeated sprint ability test. J Sci Med Sport. 2001;4(1):19–29. PubMed doi:10.1016/S1440-2440(01)80004-9 11. Fitzsimons M, Dawson BT, Ward D, Wilkinson A. Cycling and running tests of repeated sprint ability. Aust J Sci Med Sport. 1993;25:82–87. 12. McGawley K, Bishop D. Reliability of a 5×6-s maximal cycling repeated-sprint test in trained female team-sport athletes. Eur J Appl Physiol. 2006;98(4):383–393. PubMed doi:10.1007/s00421-0060284-8 13. Pyne DB, Saunders P, Montgomery PG, Hewitt A, Sheehan K. Relationships between repeated sprint testing, speed, and endurance. J Strength Cond Res. 2008;22(5):1633–1637. PubMed doi:10.1519/ JSC.0b013e318181fe7a 14. Balsom PD, Seger JY, Sjodin B, Ekblom B. Physiological responses to maximal intensity intermittent exercise. Eur J Appl Physiol Occup Physiol. 1992;65:144–149. PubMed doi:10.1007/BF00705072 15. Govoni M, Jansson EÅ, Weitzberg E, Lundberg JO. The increase in plasma nitrite after a dietary nitrate load is markedly attenuated by an antibacterial mouthwash. Nitric Oxide. 2008;19(4):333–337. PubMed doi:10.1016/j.niox.2008.08.003 16. Borg GA. Pyschophysical bases of perceived exertion. Med Sci Sports Exerc. 1982;14(5):377–381. PubMed doi:10.1249/00005768198205000-00012 17. Bond H, Morton L, Braakhuis AJ. Dietary nitrate supplementation improves rowing performance in well-trained rowers. Int J Sport Nutr Exerc Metab. 2012;22:251–256. PubMed
18. Wylie LJ, Mohr M, Krustrup P, et al. Dietary nitrate supplementation improves team sport–specific intense intermittent exercise performance. Eur J Appl Physiol. 2013;113(7):1673–1684. PubMed doi:10.1007/s00421-013-2589-8 19. Gaitanos GC, Williams C, Boobis LH, Brooks S. Human muscle metabolism during intermittent maximal exercise. J Appl Physiol. 1993;75(2):712–719. PubMed 20. Christensen PM, Nyberg M, Bangsbo J. Influence of nitrate supplementation on VO2 kinetics and endurance of elite cyclists. Scand J Med Sci Sports. 2013;23(1):e21–e31. PubMed doi:10.1111/sms.12005 21. Muggeridge DJ, Howe CCF, Spendiff O, Pedlar C, James PE, Easton C. The effect of dietary nitrate supplementation on performance in trained male flatwater kayakers. Nitric Oxide. 2012;27(Suppl):S12. 22. Bangsbo J, Iaia FM, Krustrup P. The Yo-Yo Intermittent Recovery Test: a useful tool for evaluation of physical performance in intermittent sports. Sports Med. 2008;38:37–51. PubMed doi:10.2165/00007256200838010-00004 23. Mohr M, Nielsen JJ, Bangsbo J. Caffeine intake improves intense intermittent exercise performance and reduces muscle interstitial potassium accumulation. J Appl Physiol. 2011;111(5):1372–1379. PubMed doi:10.1152/japplphysiol.01028.2010 24. Mohr M, Krustrup P, Nielsen JJ, et al. Effect of two different intense training regimens on skeletal muscle ion transport proteins and fatigue development. Am J Physiol Regul Integr Comp Physiol. 2007;292:R1594–R1602. PubMed doi:10.1152/ajpregu.00251.2006 25. Reid MB. Nitric oxide, reactive oxygen species, and skeletal muscle contraction. Med Sci Sports Exerc. 2001;33:371–376. PubMed doi:10.1097/00005768-200103000-00006 26. Perkins WJ, Han Y-S, Sieck GC. Skeletal muscle force and actomyosin ATPase activity reduced by nitric oxide donor. J Appl Physiol. 1997;83:1326–1332. PubMed 27. Folland JP, Maas H, Jones DA. The influence of nitric oxide on in vivo human skeletal muscle properties. Acta Physiol Scand. 2000;169:141– 148. PubMed doi:10.1046/j.1365-201x.2000.00725.x 28. Murrant CL, Frisbee JC, Barclay JK. The effect of nitric oxide and endothelin on skeletal muscle contractility changes when stimulation is altered. Can J Physiol Pharmacol. 1997;75:414–422. PubMed doi:10.1139/y97-096 29. Wilkerson DP, Hayward GM, Bailey SJ, Vanhatalo A, Blackwell JR, Jones AM. Influence of acute dietary nitrate supplementation on 50 mile time trial performance in well-trained cyclists. Eur J Appl Physiol. 2012;112:4127–4134. PubMed doi:10.1007/s00421-012-2397-6
www.IJSPP-Journal.com ORIGINAL INVESTIGATION
No Improvement of Repeated-Sprint Performance With Dietary Nitrate Kristy Martin, Disa Smee, Kevin G. Thompson, and Ben Rattray Purpose: Nitrate supplementation improves endurance exercise and single bouts of high-intensity activity, but its effect on repeated sprints is unclear. This study is the first to investigate the effects of acute dietary nitrate supplementation during a high-intensity intermittent-sprint test to exhaustion. Methods: Team-sport athletes (9 male, age 22.3 ± 2.1 y, VO2max 57.4 ± 8.5 mL · kg–1 · min–1; 7 female, age 20.7 ± 1.3 y, VO2max 47.2 ± 8.5 mL · kg–1 · min–1) were assigned to a double-blind, randomized, crossover design. Participants consumed 70 mL of concentrated beetroot juice containing a minimum of 0.3 g of nitrate (NT) or 70 mL of placebo (PL) 2 h before a repeated-sprint protocol involving repeated 8-s sprints with 30-s recovery on a cycle ergometer to exhaustion. Results: Fewer sprints (NT = 13 ± 5 vs PL = 15 ± 6, P = .005, d = 0.41) and less total work (NT = 49.2 ± 24.2 kJ vs PL = 57.8 ± 34.0 kJ, P = .027, d = 0.3) were completed in NT relative to PL. However there was no difference in overall mean power output or the mean power output for each individual 8-s sprint. Conclusions: These findings suggest that dietary nitrate is not beneficial for improving repeated-sprint performance, at least when such sprints are near-maximal and frequent in nature. The lack of an effect of nitrate at near-maximal oxygen uptake supports the suggestion that at greater exercise intensities nitrate does not have an ergogenic effect. Keywords: supplementation, exercise performance, team sport Dietary nitrate is a widely used exercise supplement buoyed by research regarding its beneficial effect on health and exercise.1–3 It has been shown to reduce resting blood pressure through vasodilation of capillaries and to affect fundamental physiological parameters during exercise.1 Although nitrate supplementation appears to be ergogenic for the performance of prolonged (>6 min), continuous endurance exercise (time to exhaustion increased by up to 25%4,5) and single bouts of high-intensity exercise (time-trial performance enhanced by nearly 3%2), to date no study has examined the effect of nitrate supplementation on intermittent-sprint exercise. During high-intensity intermittent exercise, insufficient energy supply and the accumulation of intramuscular metabolic by-products are believed to be factors responsible for fatigue.6 Nitrate may negate such causes of fatigue by reducing the estimated total ATP cost of exercise4 and decreasing the breakdown of phosphocreatine (PCr).1,4,5 Furthermore, during exercise in hypoxic conditions, the severity of PCr degradation, inorganic phosphate accumulation, and the fall of muscle pH reduce with nitrate supplementation, and in addition nitric oxide pathways also appear to be enhanced under hypoxic or acidic conditions.7 During submaximal exercise, nitrate lowers oxygen uptake (VO 2) without any effect on minute ventilation, heart rate, respiratory-exchange ratio, or accumulation of blood lactate.1,4,5 Cardiovascular efficiency has also been reported to improve with nitrate supplementation, through an enhanced diffusion of oxygen to tissues farther away from capillaries, resulting in a more precise local matching of oxygen delivery to metabolic rate.8
As a number of mechanisms affected by dietary nitrate might enhance intermittent-sprint exercise, it is surprising that the effect of supplementation on team-sport athletes is yet to be fully explored. A common requirement of many team sports is the ability to produce near-maximal sprints of short duration with brief recovery periods over an extended period of time.9 An integral part of many sports, therefore, is the capacity to perform well in what has now been termed repeated-sprint ability.9 It has been suggested that athletes with enhanced repeated-sprint ability are likely to perform better than those who are less able to replicate high-intensity efforts,10 and superior repeated-sprint ability in both speed maintenance and fatigue resistance is characteristic of better team-sport players.11 Any noted supplement offering a performance advantage in intermittent-sprint exercise would therefore be highly sought after. The aim of the current study was to investigate the effect of dietary nitrate on intermittent-sprint exercise performance. We hypothesized that, relative to placebo, nitrate supplementation would enable a greater amount of work to be completed, resulting in more sprints and larger mean power output during a high-intensity, intermittent-sprint test to exhaustion. We reasoned that as highintensity exercise places the body under a great amount of metabolic stress, the effects of nitric oxide would work preferentially under such conditions by improving oxygen delivery to the exercising muscle, enhancing aerobic energy contribution during sprints, and promoting enhanced PCr recovery.7
Methods Participants
The authors are with the University of Canberra National Inst of Sport Studies (UCNISS), Canberra, ACT, Australia. Address author correspondence to Kristy Martin at [email protected].
Sixteen participants, moderately trained in team sport, volunteered to participate in this study (9 male, age 22.3 ± 2.1 y, VO2max 57.4 ± 8.5 mL · kg–1 · min–1; 7 female, 20.7 ± 1.3 y, VO2max 47.2 ± 8.5 mL 845
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· kg–1 · min–1). All procedures were approved by the University of Canberra Committee for Ethics in Human Research. Participants gave written informed consent to participate.
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Design A double-blind, randomized crossover design was employed. Participants took part in initial testing and familiarization sessions to establish reliable baseline measures for the high-intensity intermittent-sprint test (HIIST) before placebo and treatment trials. They were instructed to arrive in a rested and fully hydrated state and at least 3 hours postprandial. Participants were to avoid strenuous exercise 24 hours preceding the exercise test and to refrain from consuming alcohol for 24 hours and caffeine for 6 hours before each test. For 24 hours before the first HIIST, participants recorded their food and fluid intake and physical activity. They were instructed to replicate these conditions 24 hours before each subsequent HIIST. Compliance with these requests was confirmed through a pretrial questionnaire.
Preliminary Testing Initially, participants underwent preliminary testing and familiarization with the HIIST. Preliminary testing involved a maximal ramped test to exhaustion on a cycle ergometer (High Performance Ergometer, Schoberer Rad MeBtechnik, Germany), used to identify individual workloads for the HIIST. The test to exhaustion began at 100 W for men and 50 W for women for 5 minutes and then increased by 5 W every 15 seconds. During the maximal ramped test, exhaustion was determined by volitional fatigue or inability to maintain cadence over 70 rpm and a respiratory-exchange ratio ≥1.1. Resistance for the HIIST was then set at 200% of the last fully completed resistance level during the preliminary testing. Two familiarization sessions were conducted within a week, as this has been shown to improve the reliability of repeated-sprint data12; they consisted of up to ten 8-second sprints at the previously determined workload. Two familiarization sessions have previously shown to improve reliability of power-output data of repeated sprints.12
High-Intensity Intermittent-Sprint Test Participants performed the HIIST at the same time of day on 2 occasions, separated by a washout period of at least 72 hours. Plasma nitrate and nitrite have been shown to return to near baseline levels 24 hours after ingestion of 500 mL of beetroot juice.3 The HIIST consisted of a 2-minute warm-up, cycling at 100 W at a self-selected cadence, followed by repeated 8-second bouts of high-intensity exercise at the predetermined, individualized workload, set using the hyperbolic mode of the cycle ergometer. The cadence selected during the warm-up was replicated in the subsequent trials. Sprints were interspersed with 30-second periods of active rest, consisting of cycling at 50 W at a self-selected cadence (Figure 1). Participants repeated these sprint efforts until volitional fatigue or when they were unable to increase cadence to over 100 rpm. If fatigue was not reached after 45 sprints the test was terminated, this being equal to 30 minutes of cycling. Participants received verbal encouragement to continue for as long as possible from a single, blinded researcher. Environmental conditions in the laboratory were kept consistent between participants and trials. Geometric ergometer setup selected in the first test was maintained in subsequent sessions, and the ergometer was calibrated on each occasion. The protocol was derived from the repeated-sprint efforts undertaken in team sports. This protocol elicits a 1:3.75 work:rest ratio similar to that of previous team-sport repeated-sprint studies.13 While the test took place on a stationary ergometer, it has previously been reported that there is a strong correlation between repeated-sprint cycling and running performance.14
Intervention Two hours before the HIIST, participants consumed 70 mL of nitrate-rich beetroot juice containing a minimum of 0.3 g of nitrate (NT) or 70 mL of manufacturer-supplied nitrate-depleted beetroot juice (PL; Beet It, James White Drinks Ltd, UK). Nitrate was removed from the placebo product before pasteurization by passing beetroot juice through a column containing Purolite A520E ion-exchange resin, which is specific for nitrate. Ingestion of the
Figure 1 — Schematic of high-intensity intermittent-sprint protocol. Abbreviations: O, oxygen uptake; B, plasma lactate and glucose, RPE, rating of perceived exertion; WU, warm-up; W, week; F, finish.
Nitrate and Intermittent Exercise 847
supplement occurred 2 hours before the HIIST, as plasma nitrate has been shown to significantly increase (~16-fold) 30 minutes after beetroot ingestion, peaking at 1.5 hours and then remaining stable at this level up to 5 hours postingestion.3 Participants were asked to abstain from using antibacterial mouthwash and chewing gum throughout the study to preserve commensal oral bacteria, which reduce nitrate to nitrite.15
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Measures HIIST Performance. Performance was determined from the number of 8-second sprints completed at the required cadence and from the actual power output during the sprints, recorded at 2 Hz using SRM software (SRM Training System, Schoberer Rad MeBtechnik, Germany). Mean power output was recorded as the average power output during the test, and peak power output was recorded as the greatest power output achieved. Mean power output of each individual, 8-second sprint was recorded as the average power output for each sprint, and total work done was calculated as the product of the average power output during the completed 8-second sprints throughout the test in watts and time in seconds.
Results Maximal Ramped Test Mean maximal VO2 obtained during the ramped test was 49.56 ± 11.8 mL · kg–1 · min–1. There was a weak positive relationship between maximal VO2 and sprints completed in the control and nitrate trials of the HIIST (r = .28 and r = .20, respectively). There was a weak negative relationship between maximal VO2 and response to nitrate supplementation, as determined by the change in number of sprints completed in the nitrate trial (NT) relative to placebo (PL; r = –.16).
HIIST Performance Compared with placebo, fewer sprints were completed in NT (NT = 13 ± 5 sprints vs PL = 15 ± 6, P = .005, d = 0.41); this was the case for 11 of 13 participants (Figure 2). Less total work during the sprints was also done with nitrate (NT = 49.2 ± 24.2 kJ vs PL = 57.8 ± 34.0, P = .027, d = 0.3; Figure 3). There was no difference
VO2 and Heart Rate. During the initial maximal ramped test, VO2
was recorded using an open-circuit indirect calorimetry system (TrueOne 2400 Metabolic Measurement System, Parvo Medics, Sandy, UT, USA) throughout the entire test. During the HIIST, VO2 was measured continuously and then averaged over 30 seconds, from the beginning of warm-up through to the end of the fifth sprint. Pilot testing had revealed that gas analysis interfered with performance of the HIIST, and hence as the primary measure of the current study, VO2 recording was discontinued after the fifth sprint. Gas analyzers were calibrated using primary standard reference gases before each test. Heart rate was recorded during testing by a heart-rate monitor fitted by a chest strap (T34 noncoded heart-rate transmitter, Polar, Finland). Plasma Lactate and Rating of Perceived Exertion. Capillary
blood samples were collected from the earlobe before the HIIST, in the first 10 seconds after the fourth sprint, and immediately after termination of exercise. Samples were analyzed using a lactate analyzer (Lactate Pro, Arkray, Japan). These samples were used to compare plasma lactate concentrations between conditions. Rating of perceived exertion using a 0-to-1016 scale was recorded every 3 sprints and at the point of fatigue as a measure of perceived effort encountered during work.
Figure 2 — Sprints completed in the control and nitrate trials for each participant. *Significantly fewer sprints were completed in the nitrate trial (P = .005).
Statistical Analyses Differences in the number of sprints completed, total work, and mean and peak power were compared between trials using a 1-way ANOVA with repeated measures, as was the number of sprints completed in the first or second trial. Two-factor repeated-measures ANOVA was used to compare differences between individual sprints in terms of work done and mean power output and to compare repeated measurements of plasma lactate concentration, VO2 during the warm-up and first 5 sprints of the HIIST, rating of perceived exertion, and heart rate. Statistical significance was accepted at the P < .05 level, and data are presented as mean ± SD. Effect sizes for mean differences are expressed using Cohen d. After testing was completed, 3 participants’ data sets were excluded from analysis due to their having completed the maximum 45 sprints in 1 or both of the trials without reaching terminal fatigue.
Figure 3 — Total work done during the sprint efforts of the control and nitrate trials. *Significantly less total work was completed in the nitrate trial (P = .027).
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in overall mean power output of the combined sprints (NT = 447 ± 104 W vs PL = 444 ± 117, P = .797), mean peak power (NT = 447 ± 104 W vs PL = 444 ± 117, P = .196), the mean power output, or total work done for each individual 8-second sprint (P > .05). A comparison of sprints completed in the first versus the second trial, was carried out to determine any order effects, but analysis revealed no difference (P > .05).
VO2 and Heart Rate
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There was no difference in VO2 at baseline. Nitrate supplementation resulted in a significantly reduced VO2 during the warm-up of the HIIST (NT = 22.0 ± 4.6 mL · kg–1 · min–1 vs PL = 24.7 ± 2.8, P = .002, d = 0.71; Figure 4). VO2 attained during the first 5 sprints of the HIIST was not different between trials (P > .05). There was no difference between conditions for heart rate at any time point (P > .05).
Plasma Lactate and Rating of Perceived Exertion There was no difference in plasma lactate concentration between trials at baseline, after the first 4 sprints, or at termination of exercise (P > .05). No significant differences between trials were observed for rating of perceived exertion at baseline, after 3 sprints, or at termination of exercise (P > .05).
Discussion The primary finding of the current study was that an acute dose of dietary nitrate reduced performance during a high-intensity intermittent-sprint test to exhaustion. Capacity to complete sprints was decreased after supplementation, as was total work. This reduction in work capacity occurred without any difference between trials for mean power output or the total work completed during each individual sprint. As observed previously,1,4,5 VO2 was reduced with nitrate during the submaximal warm-up, although this trend did not extend to the HIIST. The reduced VO2 during the submaxi-
Figure 4 — Oxygen uptake profile of the high-intensity intermittent-sprint test. Shaded area indicates warm-up (2 min cycling at 50 W). *Nitrate was significantly reduced during warm-up compared with placebo (P = .002).
mal warm-up suggests that the ingestion of beetroot juice 2 hours before exercise, previously shown to result in an increase in plasma nitrite,17 resulted in the uptake of nitrate by our participants. To our knowledge this is the first study to observe a decrease in performance after nitrate supplementation. Conflicting with our hypothesis, fewer sprints were completed in the nitrate condition. Previously, a number of studies found an increased time to exhaustion after nitrate supplementation during combined arm and leg cranking, 2-legged knee extension, and incremental cycling to fatigue.4,5 In those studies, nitrate appears to be ergogenic for controlled, single, and largely submaximal efforts. Although 1 study used an incremental, intermittent recovery protocol to assess the possible benefits of nitrate supplementation for team-sport players,18 this is the first study to investigate the effect of dietary nitrate on intermittent-sprint exercise to exhaustion. The HIIST protocol was designed to investigate the potential effect of nitrate on intermittent-sprint exercise, similar to that experienced by athletes during team-sport play. However, despite previous findings, exercise was terminated earlier with nitrate supplementation than with placebo. A possible cause for the lack of agreement between this study and earlier research is the difference in intensity between the HIIST and protocols previously used. Several recent studies indicate that nitrate supplementation results in a significant reduction in pulmonary VO2 during submaximal exercise, an effect that appears to be related to enhanced skeletal-muscle efficiency. A reduced VO2 was observed during the warm-up component of the HIIST, although this did not extend to the higher-intensity sprints. The reduced VO2 during the warm-up in the nitrate trial was foreseeable given that this has been shown during short submaximal steady-state exercise previously.1,4,5 In addition, it has previously been reported that nitrate supplementation is only effective in reducing VO2 at intensities up to 80% of maximum.4 VO2 in the current study immediately after the fifth sprint was >75% VO2max,4 and thus it appears likely that in subsequent sprints, the VO2 of participants would reach levels greater than the proposed threshold, negating the positive effects of nitrate. The HIIST requires short periods of maximal effort to be repeated with insufficient recovery and hence requires participants to be working at a high intensity from the outset, a protocol more representative of on-field performance. A similar 10 × 6-second repeated-sprint protocol with 30-second recovery found that absolute PCr concentration decreased to 57% of resting value after sprint 1 and progressively declined to only 16% after the final sprint, suggesting that the majority of PCr resynthesis is not complete within 30 seconds of recovery.19 This brief recovery period, insufficient for complete PCr resynthesis, may not be great enough in duration for a nitric-oxide-mediated effect. Therefore it is possible that the intensity of the HIIST and inadequate recovery might have been too severe for the nitrate supplementation to have a significant effect on aerobic metabolism. Recently, shorter repeated-sprint protocols have been used to assess the effect of nitrate supplementation on intermittent exercise. No difference in peak or mean power output between nitrate and placebo conditions was found during 6 × 20-second sprints performed by elite cyclists20 or 5 × 10-second maximal sprints in trained male flat-water kayakers.21 These results support our finding of no difference between trials for the mean power of individual sprint efforts. In contrast, a 0.4% improvement was reported over 6 × 500-m maximal ergometer rowing efforts.17 Those sprints, however, were unlike those in the HIIST. The maximal 500-m rowing efforts
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were much longer in duration, lasting ~90 seconds, with longer rest periods and an approximate 1:1 work-to-rest ratio. In addition, all of these protocols had a predetermined endpoint. Only 1 study has attempted to measure the influence of nitrate supplementation on intermittent exercise, by using the Yo-Yo Intermittent Recovery Test, Level 1 (YIRT1).18 The principal finding of that study was that supplementation of concentrated beetroot juice improved intermittent exercise performance, measured by an increased distance covered.22 However, although statistically significant, the difference covered in the YIRT1 between beetroot and placebo trials was only 4.2% and 68 m on average, whereas 14% fewer sprints were completed during the HIIST. Authors of that paper also noted that the practical significance of this difference was minor and particularly so when compared with 16% and 29% improvements observed in the YIRT2 (version of the test for elite athletes) after caffeine ingestion23 and sprint and speed training,24 respectively. In addition, it is possible that these small changes, unseen in previous repeated-sprint protocols, may have been due to the much greater preloading of nitrate, as well as increased preexperimental dose. In total, ~2.17 g of nitrate was consumed over 36 hours in the YIRT1, versus ~0.31 g in the current study. In addition, the 0.4% improvement reported in repeated maximal ergometer rowing was preluded by 6 days of chronic nitrate supplementation.5 Moreover, it has been shown that performance in certain exercise tests is improved with more chronic nitrate supplementation,23 and responses may be dose-specific; however, further research is required to determine whether performance is increased to a greater extent with longerterm nitrate supplementation. Furthermore, although the YIRT1 was specifically developed to replicate the high-intensity running bouts in football match play,22 the progressive increase in pace and intensity until participants reach fatigue is much different from the HIIST, which requires maximal effort from the outset. The YIRT1 protocol is more similar to a traditional aerobic-fitness test than a snapshot of team-sport play. Inconsistent research findings are also available on the effect of nitrate on force production. In the current study, the reduction in the number of sprints completed with nitrate was not explainable by difference in the mean power output of the sprints. Studies have shown that nitric oxide depresses force output during isometric contractions in single muscle fibers25 and in muscle-fiber-bundle preparations.26 However, in human research, treatment with nitric oxide donors has been shown to result in an increase in the force generated during maximum voluntary contractions but a decrease in force at submaximal frequencies.27 Thus, it would appear that the effect of nitrate depends on the mode, intensity, and duration of muscle contraction.28
Practical Applications The current study investigated the effect of a single dose of dietary nitrate on intermittent-sprint exercise, up to a maximum of 30 minutes, using a protocol that relates to the short duration and repeated sprints that team-sport athletes typically undertake during a match. Using this protocol, the capacity to complete sprints was reduced; hence, dietary nitrate might not be beneficial for improving repeated-sprint performance, at least when such sprints are near maximal and frequent in nature. Further research involving sport- and match-specific protocols is needed to clarify the effects of supplementation during real on-field sporting performance.
Limitations Protocols of this nature are prone to a learning effect. However, it is unlikely that this occurred in the current study, as 2 familiarizations were included and supplementation was randomized to eliminate order effects due to learning or training. Analysis confirmed that there was no difference between the first and second trials. A further strength of the current study was the use of a nitrate-depleted beetroot juice as a placebo; this enabled a double-blinded study design. A placebo indistinguishable from the treatment condition was used, decreasing the likelihood of psychological factors or other nutrients contaminating the results. This has been a limitation in previous studies.5 It is also pertinent to mention that the concept of responders and nonresponders to nitrate supplementation has arisen previously.29 Wilkerson et al29 reported that while group mean 50-mile cycle time-trial performance was not significantly improved by acute nitrate supplementation, participants who saw a greater increase in plasma nitrite (responders) improved their performance by more, whereas those whose plasma nitrite did not change appreciably did not improve their performance. The current study did not measure changes in plasma nitrite; future research should consider both changes and degree of change in plasma nitrite when analyzing results.
Conclusions Overall, these findings indicate that under the conditions of the current study, nitrate supplementation potentially leads to a decrement in the number of repeated sprints that can be maintained. The lack of an effect of nitrate at near-maximal oxygen uptake supports the suggestion that at greater exercise intensities nitrate does not have an ergogenic effect. Differences in intensity therefore may have contributed to the dissimilarities in results between protocols. Acknowledgments No direct funding was received for the study. Remuneration for all supplements and equipment was provided by the University of Canberra. All procedures were approved by the University of Canberra Committee for Ethics in Human Research.
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