The Effects of Sprint Interval Training on Aerobic Fitness ... · The Effects of Sprint Interval Training on Aerobic Fitness: A Systematic Review. Author(s): Kent W. Abstract. Background:

Article ID: WMC003284 ISSN 2046-1690

The Effects of Sprint Interval Training on AerobicFitness: A Systematic ReviewCorresponding Author:Dr. William Kent,Foundation doctor and Sports Scientist, Royal Sussex County Hospital, Brighton - United Kingdom

Submitting Author:Dr. William Kent,Foundation doctor and Sports Scientist, Royal Sussex County Hospital, Brighton - United Kingdom

Article ID: WMC003284

Article Type: Systematic Review

Submitted on:13-May-2012, 08:58:11 PM GMT Published on: 14-May-2012, 12:17:46 PM GMT

Article URL: http://www.webmedcentral.com/article_view/3284

Subject Categories:SPORTS MEDICINE

Keywords:Anaerobic, Exercise, Endurance, Health, Human

How to cite the article:Kent W. The Effects of Sprint Interval Training on Aerobic Fitness: A Systematic Review .WebmedCentral SPORTS MEDICINE 2012;3(5):WMC003284

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The Effects of Sprint Interval Training on AerobicFitness: A Systematic ReviewAuthor(s): Kent W

Abstract

Background: Greater aerobic fitness is associatedwith improved cardiovascular health. Traditionally highvolume (long duration and low intensity) endurancetraining (ET) has been used to increase aerobic fitness.Sprint interval training (SIT) is a low volume and highintensity form of interval training. Energy for SIT isproduced via both the aerobic and anaerobicmetabolic pathways. SIT is therefore a potentiallyefficient and effective means to improve aerobicfitness. Objectives: Systematically review the effects of SITon aerobic endurance performance in untrainedindividuals.Methods: The following databases were searched upto August 2008: Pubmed, the Cochrane Library,ScienceDirect, and SportsDiscuss. The reference listsof relevant reviews and identified papers were alsosearched. Included articles were limited to Englishlanguage, controlled studies, investigating SIT inhealthy untrained or recreationally active participants(VO2max < 55ml/kg/min)(1) with an outcome measureof endurance performance (as a measure of aerobicfitness). Studies fulfilling the selection criteria wereassessed for methodological quality and relevantoutcome data extracted. Results: A qualitative evaluation of the availableevidence demonstrated a consistent associationbetween sprint interval training and improved aerobicperformance. SIT produced an improvement of ~ 4%(95%CI -0.2 to 8.7), ~8 to 10% (0.4 to 16), and ~15%(0.8 to 29) after 1, 2 and 6 weeks respectively. Theseimprovements were of the same magnitude as theimprovements noted for ET but the training volumerequired was approximately 90% less. Conclusions: Short duration SIT is an effective andefficient form of improving aerobic fitness in untrainedindividuals.

Introduction

Aerobic f i tness is associated with reducedcardiovascular disease.(2) Traditional programmes toimprove aerobic fitness and thus enduranceperformance rely upon high-volume training of long

duration and low intensity (e.g. 20 to 60 min, 3-5 timesa week at 50 to 85% VO2max).(3) Energy productionwith this type of training is almost exclusively fromaerobic metabolic pathways.(3; 4; 1) Interval trainingwhere periods of hard exercise (work intervals) areinterspersed with periods of rest or lighter work (restintervals) can also enhance endurance performance ofrecreationally active individuals.(1) It is suggested thattraining at or near VO2max is the most effectivetraining intensity to enhance VO2max and enduranceperformance.(5) This is reflected by the majority of theinterval training studies which use exercise intensitiesat or below VO2max in an attempt to optimise trainingand thus endurance performance enhancement.(1; 5)This type of training also predominantly relies onaerobic energy pathways but a larger proportion ofenergy is produced anaerobically.(6)

In contrast to the endurance training protocols above,sprint interval training (SIT) predominantly stresses theanaerobic pathway to produce energy.(7) SIT is aneffective means of increasing anaerobic endurance,(8)but has also been found to significantly enhanceaerobic performance in untrained individuals.(1; 9) ASIT protocol consists of repeated intervals of shortduration at maximal or near maximal intensity,interspersed with recovery periods of rest or lowintensity exercise. Due to the high intensity, theintervals do not generally last longer than 60 secondsand most protocols utilise intervals of 30 seconds orless.(1) Muscle hypertrophy is not associated with SITbecause unlike resistance training which also employsbrief intermittent exercise, SIT is usually performedusing activities such as cycling or running whichprovide lower resistance.(10) The aerobic adaptationsfollowing SIT have been observed to be as great asthose elicited by endurance training.(9) Further, theadaptations occur rapidly and require a significantlysmaller training volume than previously estimated forthese adaptations to occur.(9) SIT is therefore apotentially efficient and effective means forrecreationally active people to improve aerobic fitnesswhich is associated with improved cardiovascularhealth.(2)

Two previous reviews have highlighted the potential ofSIT to improve endurance performance.(1; 9) However,SIT in an untrained population was not the main focus



of one of these reviews(1) and it included uncontrolledstudies which only reported surrogate markers ofaerobic exercise capacity. The more recent review byGibala et al(9) reported the results from studies withcomparison groups and aerobic performancemeasures but neither review used a systematicapproach to identifying articles or evaluated the qualityof studies included in their reviews. These methods donot allow for the impact of bias and confoundingfactors to be adequately evaluated which could lead toerroneous conclusions. The purpose of this reviewwas to use a systematic methodology to evaluate theeffects of SIT on the aerobic energy system. Adiscussion of study methodology, quality and thepossible impact of these factors on the results will alsobe presented along with suggestions for futureresearch.

Methods

Search strategyThe methods of this review followed the guidelines ofthe Cochrane Handbook for Systematic Reviews ofInterventions.(11) Research articles review wereidentified in two stages 1) by using freetext searchesin electronic databases and 2) by hand searching thereference lists of the included studies and reviewarticles of interval training which included SIT. Included articles were limited to English language,controlled studies, investigating SIT in healthyuntrained or recreationally active participants(VO2max < 55ml/kg/min)(1) that included an outcomemeasure of endurance performance. A free textsearch strategy was developed using these limitationsand their synonyms joined by Boolean operators(appendix 1). The text was then entered into thefollowing databases Pubmed, Sciencedirect,sportsdiscuss, and the Cochrane library. From the listof journal articles produced (appendix 2) potentiallyrelevant papers were then identified by a review oftheir title and abstract. Fourteen potentially relevant papers were identifiedfrom the electronic and hand searches combined.These articles were read in full and appraised using astandardised procedure. Critical appraisal excluded afurther 10 articles. The remaining four papers(12-15)met the defined inclusion criteria and were judged of ahigh enough quality to inform the results of the mainreview (fig 1).

Analysis of Endurance PerformanceEndurance performance data was extracted from thefour included papers. To allow the comparison of

endurance performance, the data was transformed(where needed) into percentage change in meanpower output (using the methods of Hopkins et al(16)).If exact data was not presented, data was obtained bymeasuring the data points on the presented figures.Effect size and 95% confidence intervals (95% CI)were also calculated for the outcomes to help interpretthe clinical as well as the statistical significance of theoutcomes. The 95% CI were calculated using aspreadsheet(17) which required the effect statistic andthe p value to calculate the upper and lower limits ofthe confidence interval. To allow an approximaterange where the exact p values were not given, thevalue 0.04 was substituted for a p value of p<0.05 andthe value 0.06 was given to p values of p>0.05. Thesevalues were chosen as they represent “just significant”and “almost significant” findings. These confidenceintervals combined with the magnitude of the effectsize should allow the reader to interpret the likelysignificance of the intervention for themselves. Toguard against error data were double entered andcross-checked.

Analysis of secondary outcomesData for the secondary outcomes: change in VO2maxand change in anaerobic performance (Wingate Peakand mean anaerobic power) were taken from thearticles Bergamaster et al(18) and Barnett et al.(19)These papers were judged of high enough quality tobe included in the review but did not include ameasure of endurance performance so could not beused in the main review. These results are presentedalong with changes in physiological markers to supportthe evidence from the endurance performanceoutcomes. To aid interpretation data is presented aspercentage change with 95%CI and effect sizes.

FindingsQuality of published workAll of the studies included in this review have beensubjectively rated as having a moderate risk ofbias.(20) The use of controlled studies, no drop-outsand objective markers limits the influence of bias in thestudies but the lack of adequate randomisation andblinding leave the studies open to the influence of biasand over estimation of outcome measures.(21-24)Small sample sizes leave studies underpowered suchthat significant effects with large effect sizes are likelyto be true results (but may be due to the play ofchance). However, small but important effects arelikely to be missed as the study is underpowered todetect these changes.(25) The small sample sizes,reliance on the p value to ascertain significancecombined with the selective reporting of data can also



be problematic for reviewers. Only reporting significantresults in detail does not allow the reader to interpretthe changes in results which “just missed” statisticalsignificance. Due to the underpowered studies theseresults could represent a type II error and not reportingthe data for findings which were not statisticallysignificant can lead to reporting bias. The inclusion ofdata with precision estimates for all measuredoutcomes will allow for subsequent re-analysis orinclusion in a metaanalysis. If present the direction of bias in the studies is likely tobe away from the null towards a significant finding forthe experimental condition. This possibility must beconsidered when interpreting the results of thesestudies. The large effect sizes of the main resultssuggest that these are true results, but their truevalues may not be as large. The impact of this bias willlikely effect secondary measures which have smallereffect sizes thus bias could account for all or most ofthis apparent effect. To improve future studies greaterconsideration should be given to reducing the possibleinfluence of bias. This could be achieved by improvedimplementation and reporting of randomisation andblinding. A justification of sample size should also beincluded and data summaries presented with effectsizes and precision estimates for each primary andsecondary outcome.(26)

Interpreting effectsThe use of different intervention durations andperformance measures in the included studiesprevents the reliable use of meta-analysis to quantifyan overall pooled estimate of effect. The consistentresults across the studies allow a confident qualitativeinterpretation of SIT effects on enduranceperformance. Cautious interpretation of the secondaryoutcome measures is also presented considering thelimitations of the studies outlined above.

Primary Outcome Measures Endurance Performance fol lowing SITinterventionsThe homogeneity of the results for enhancement ofendurance performance is clear from table 1. Despitesmall samples and the inherent variability of data froma non-athletic population, significant increases inperformance are seen in all of the studies afterSIT.(12-15) Due to the imprecision of the data theeffect size statistic is only small to moderate and the95%CI ranges are wide. A dose-response relationshipis also evident from the results in table 1 with anapproximate 4% (95%CI -0.2 to 8.7) improvementafter 1 week increasing to ~8 to 10% (95%CI 0.4 to 16)after 2 weeks training and ~15% (95%CI 0.8 to 29)

after six weeks training. In the one study whichcompared SIT to endurance training(14) the absoluteincrease in performance following SIT was greaterthan ET (10.4 v 6.5%), but there was no difference ineffect size (both moderate) for these results andconsiderable overlap between the confidence intervals.This indicates that both forms of training modalitiesproduce rapid adaptations in the aerobic metabolicpathways with no identifiable difference between them.Despite the two interventions producing the samemagnitude of effect this result has clinical significancebecause the volume of exercise used in the SITprotocol was approximately 90% less than the volumeof exercise used in the endurance training protocol.This has potential benefits for population health wherelack of time is quoted as one of the biggest reasons foradults not committing to regular exercise.(27) Howeverit needs to be established whether the other healthbenefits associated with ET are also produced throughthe shorter bouts of exercise associated with SIT.Recent research has begun to investigate thesequestions with promising results. High intensity intervaltraining has been found to produce greater reductionsin fat mass and insulin resistance in overweight youngwomen compared with steady state endurancetraining.(28) SIT using the protocol of Bergamaster etal(13) has also been found to improve peripheralvascular structure and function comparable with thechanges induced by higher volume ET.(29) Combinedthese findings suggest that SIT could have asignificant impact upon the exercise recommendationsfor physical fitness and health promotion and deservefurther research. In comparison to the intervention groups the controlgroups demonstrated no significant changes frombaseline. The one exception to this was the controlgroup from Barnett et al(19) which demonstrated astatistically significant increase in VO2max frombaseline (table 2). Although Barnett et al(19) did notmonitor habitual activity this change was ascribed tonatural variations in habitual activity over the eightweek duration of the study. This change could also beexplained by a Hawthorne effect or an increase inactivity because the control subjects were aware thattheir fitness would be tested again. Whatever thereason for this change, it demonstrates the importanceof a control group to help control confounders and theinterpretation of the magnitude of the effect that canbe ascribed to the intervention.

Secondary Outcome MeasuresVO2max DataThe finding of improved endurance performance issupported by the increases in VO2max seen after



training in all but one of the studies (table 2). SIT wasfound to increase VO2max by between 5.5% (-0.9 to6.3) to 8.2 (3.9 to 12.0) over a period of two to eightweeks respectively.(15; 18; 19) In the one study thatcompared improvement in VO2max between SIT andET the increases in VO2max following ET weresmaller in absolute terms after 2 weeks training butgreater compared with SIT after 6 weeks (moderate vsmall effect size), but these findings were notsignificantly different (table 2). This finding promptedthe authors of the study to suggest that differences inthe time course for adaptations may be found. Toinvestigate this hypothesis studies with longer followup durations and larger samples would be needed.Only one study(13) did not find an improvement inVO2peak for either the SIT or ET interventionshowever the lack of reported data prevented anyfurther interpretation of this result. Support for thechanges in endurance performance was also providedby the increased physiological markers of aerobicperformance following training which coincided withincreased performance. for a comprehensive review ofthese changes in physiological markers see Gibala etal.(9)

Anaerobic Performance fol lowing SITinterventionsThe results for anaerobic performance (for which SITis more traditionally associated) were less consistentthan the results for endurance performance (table 4).Mean power in the Wingate anaerobic Test (WAnT)was enhanced in all of the studies. The results ofBurgomaster et al(18) suggest that the increase ispossibly greater than the improvements seen with ET(table 4) but more research is needed to confirm this.Peak Power results were less consistent. Burgomasteret al(18) found SIT increased PP by 17% (0.9 to 33.0)which was greater than the 7% (0.4 to 14) increaseseen with ET and Burgomaster et al(15) reported a5.4% increase. In contrast Barnett et al(19) found nodifference in PP between the SIT group and a controlgroup after 8 weeks of training. Burgomaster et al(15)also reported the fatigue index which was significantlyreduced by 20% (4.9% to 82%) after SIT with amoderate effect size. These results are notunexpected because of the design of the studies anduse of the specific testing measure. SIT is known toimprove repeated sprint endurance(8) but the WAnT isa single 30s test designed to test anaerobic power. Asexpected mean power over 30s and the fatigue indexis improved reflecting an increased ability to generateenergy using anaerobic metabolism. Improved aerobicmetabolism will also improve these two measuresbecause the aerobic system contributes significantly to

the WAnT especially at the end of the test. Theinconsistent results seen for the PP are notunexpected because PP measurement is taken over aperiod of 1s (depending upon the protocol). Theseshort measures of power output are associated withless reliable data because each cycle revolutioncontributes a greater proportion to the overall scorethan in longer tests.(30) Individual errors thereforehave a greater impact on the measure. This increasein noise makes it less likely to identify changes insmall samples where the precision is low. Further,peak power is more closely correlated with maximumstrength than endurance capacity.(3) SIT has beenshown to cause preferential changes in the aerobicmuscle fibres (type IIa and type I fibres)(9) which arelikely to have a smaller effect on peak power thaninterventions such as resistance training or plyometrictraining which stimulates development in the moreanaerobic explosive type IIb fibres and have beenfound to increase WAnT PP.(31)

Critical Analysis and Suggestions for ImprovingFuture ResearchStudy PopulationThe populations used in the included studies weresamples of convenience utilising the Universitystudents at the researcher’s institutions. Their age,demographics, anthropometric, habitual activity, andthus physical fitness do not represent the healthyrecreationally active population as a whole. Theselimitations restrict the generalisabilty of the results torecreationally active university students. SIT needs tobe tested in the general untrained population and notunder strict laboratory conditions so that itseffectiveness (as opposed to efficacy) as a method ofendurance enhancement can be evaluated. Underthese conditions motivation to perform taxing exercisemay not be as high, which could lead to a reduction inthe effectiveness of the interventions. Further researchis also needed on individuals with greater trainingexperience (both sprint trained an endurance trained)to quantify the benefits of this form of exercise forthem.

Use of control or comparison groups and samplesizeIn their earlier review Laursen et al(1) rely almostexclusively upon studies which did not include acontrol, this leaves outcomes open to the influence ofconfounding factors. The value of only includingcontrolled studies is demonstrated by the example ofBarnett et al(19) where a significant improvement inperformance was seen in the control group who didnot complete a training intervention. This improvement



was explained by Barnett et al(19) as seasonalimprovements associated with changing levels ofhabitual fitness in active individuals. This highlights apotential problem with working with recreationallyactive individuals as small fluctuations in trainingvolume represent a much greater proportionalincrease in training volume for them than moderatelytrained or elite athletes. Combined with their lowerbaseline of physical fitness these small changes couldlead to dramatic improvements in physical ability.Indirect evidence for this is seen in the findings of ametanalysis investigating the reliability of power outputdata from endurance performance tests.(30)Non-athletes were 1.3 times less reliable than athletesand in addition non-athletic females produced lessreliable data than non-athletic men (1.4x). This is apotential source of confounding factors especially asone of the studies (Bergomaster et al(13)) included 2females in the SIT group but the control group was allmen. To adequately control for these fluctuationsfuture studies should include sample size calculationsand equal comparison groups.

Reliability and validityReliable and valid measures of endurance capacityare also needed and the data should be presentedwith enough information to allow transformation intoother forms to allow future comparisons and/ormetanalysis. This suggestion is supported by theobservation that the performance enhancement in onepaper(13) was substantially greater in absolute termsthan the other papers.(12; 14; 15) In their review ofSIT Gibala et al(9) noted this difference inperformance change but because they only reportedon the outcome data as it was presented in theindividual papers they did not adequately explain thisoutlying result. The inclusion of untrained female datain the SIT group only and the unblinded protocol inBurgomaster et al(13) is likely to have contributed tothis finding. The greatest contributor to this disparitywas the use of incomparable outcome measures.Burgomaster et al(13) measured the change inendurance capacity using a constant power test untilvolitional fatigue, the other papers(12; 14; 15) usedconstant work tests (time trials). These two types oftests both produce reliable results demonstrated bylow average coefficients of variations(30) but a smallchange in endurance capacity wil l cause asubstantially larger improvement in performance in theconstant power test.(30) This difference accounts forthe substantial difference in absolute performancechange between this paper and other papers usingsimilar protocols (96.6%(13) vs. ~10%(12; 14; 15)).However, by transforming the outcome data into the

same form (change in mean power) directcomparisons can be made and the results are inagreement with the other papers (table 1). The use of both performance tests and physiologicalmeasures in combination improved the confidence ofthe findings. Studies which only rely upon surrogatemeasures do so based on assumptions which relyupon current understanding linking them to outcome ofinterest. Understanding is constantly evolving andsuch assumptions can be misleading if ourunderstanding is incomplete.(32) Surrogate markersshould not be relied upon when the outcome ofinterest can be readily measured. Performance data istherefore the main outcome measure of interest butsurrogate markers of aerobic performance canincrease the confidence in positive findings inunblinded studies such as these. If an increase inperformance is found in conjunction with increases inthese markers then the increase in physicalperformance is unlikely to be completely due to aplacebo effect.(16) Further, using criteria such as theHill(33) criteria the strength of association for causeand effect can be evaluated and evidence formechanisms of improvements in performance can beelicited. The important questions for researchers tofocus on in the future are if the physiologicaladaptations from SIT that lead to improved endurancecapacity are due to the same physiologicaladaptations that occur with endurance training in boththe short-term and the long-term. The time course ofthe adaptations should also be investigated along withany additive effects of a combination of both low andhigh intensity training verses either type of trainingalone.

RandomisationThe main strength of randomisation is in theexplanation of cause and effect.(34) Only one of theincluded studies(1) reported using matching andrandomisation to allocate participants to theintervention groups, but the reporting was inadequateto ascertain if the methods were rigorous enough toprevent subversion and bias. Each stage ofrandomisation (allocation concealment, randomisation,and implementation) should be adequately describedin the methods of a study so that the likelihood of biasin group allocation can be evaluated.(26) Withoutadequate allocation concealment even random,unpredictable allocations can be corrupted.(21; 26)Well meaning or not, such tampering undermines thevalidity of a trial. Strong empirical evidence has beenreported that the lack of adequate randomisation isassociated with overestimates of treatment effects.(21;22; 24) These f indings support the use of



randomisation in studies and as a criterion in criticalappraisal. It is therefore important that appropriatemethods of randomisation are used and how it wasimplemented is adequately described so that theoverall quality of the study and its conclusions can beevaluated with confidence.(26)

BlindingKnowledge, experiences, and beliefs can all influencephysical and psychological responses to anintervention.(35) This is an important consideration forinterventions like the ones in this review becausephysical performance relies upon a multitude ofvariables, including motivation of the subjects andknowledge of intervention allocation is associated withlarger estimates of treatment effects.(23) Blinding(concealing the intervention from the participant,investigator, or both) can be utilised to avoidparticipants’ or investigators’ expectations impactingupon the results. Unfortunately none of the studiesincluded in this review reported using blinding in theirmethods. It is accepted that blinding is more difficult innon-pharmacological interventions(36) but blinding ofsome form can still be implemented (e.g. blinding thestatistical analysts). In an attempt to increase andimprove the use of blinding, descriptions of blindingm e t h o d s h a v e b e e n p u b l i s h e d f o rnon-pharmacological interventions(36) and guidelineshave been written for the correct reporting ofblinding.(26)All studies have limitations but good study designreduces the potential for bias and combined with thegood reporting increases our confidence in theconclusions of a trial. The lack of randomisation andblinding does not automatical ly indicate amethodologically unsound trial as the design mayprevent bias in other ways.(35; 23) The studiesincluded in this review all used objective measures ofphysiological markers to support their findings. Thisincreases our confidence in the validity of the findings.Attrition bias was also not a problem as there were noreported drop outs. This indicates that the motivationof the subjects was high (as the training wasstrenuous) which increases the confidence in thereliability of the performance test which can beinfluenced by fluctuating motivation. The inclusion offamiliarisation trials and the use of reliableperformance tests in all of the included papers alsoincreases the reliability of the performance data.(30)Furthermore unlike randomisation which has stringempirical evidence for its inclusion the influence ofblinding on reducing bias has yet to be fullyascertained. Studies are required to assess itsinfluence on preventing bias because its actual impact

may be considerably different to its theoreticalinfluence.(37)

Review

Strengths & Weaknesses of the Review The systematic methodology is the main strength ofthis review compared with other reviews.(1; 9) Theexclusion of uncontrolled trials and critical appraisal ofstudies reduces the influence of bias and confoundingand produces more accurate and reliable conclusions.Limitations of the methods are its restriction to theEnglish language and published articles which mayresult in publication bias. The measurement of datafrom graphs where primary data was not presented inthe original articles could have also introduced error tothe results as could the assumptions used to transformdata. These limitations together with the limitations ofthe included studies outlined above need to beconsidered when interpreting the results and shouldbe addressed in future research and reviews of SIT forendurance performance.

Suggested Further ResearchStudies with improved research designs are requiredto allow more precision to investigate the speed ofadaptations seen with SIT compared with endurancetraining. Longer studies investigating the long termadaptations to SIT compared with endurance trainingwill also provide evidence for the efficacy of thistraining modality to enhance aerobic fitness andprovide health benefits as the fitness of theparticipant’s changes.

Conclusion

SIT protocols using short bouts of high intensityexercise can produce significant improvements inaerobic fitness and thus endurance performance in aslittle as two weeks in active adults. The improvementsin endurance performance increase for up to sixweeks but longitudinal studies are needed to quantifythe improvements beyond this time. The magnitude ofthese changes is equivalent to the changes seen withendurance training but the volume of training is ~90%less with SIT. Mean anaerobic power is also improvedconcurrently with this type of training, possibly to agreater extent than with ET but further research isneeded to confirm this finding. These results couldhave a significant impact upon physical activityrecommendations for health and fitness if furtherstudies can confirm the health benefits associated withlonger duration ET also occur with brief SIT



interventions.

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Study Selection process

Illustrations

Illustration 1

Figure 1



Summary of Endurance Performance as change in Mean Power

Illustration 2

Table 1



Summary of change in VO2peak

Illustration 3

Table 2



Wingate Anaerobic Performance Data. Change in Peak Power and Mean Power

Illustration 4

Table 3



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