Jozo Grgic *,Bruno Lzinic ,Zeljko Pedisic

a Institute for Health and Sport(IHES),Victoria University,Melbourne,VIC 3011,Australia

b Faculty of Education,Department of Kinesiology,J.J.Strossmayer University,Osijek 31000,Croatia

Abstract Purpose:This review aimed to synthesize previous findings on the test－retest reliability of the 30－15 Intermittent Fitness Test(IFT).Methods:The literature searches were performed in 8 databases.Studies that examined the test－retest reliability of the 30－15 IFT and presented the intraclass correlation coeff icient (ICC) and/or the coeff icient of variation (CV) for maximal velocity and/or peak heart rate were included.The consensus-based standards for the selection of health measurement instruments (COSMIN) checklist was used for the assessment of the methodological quality of the included studies.Results:Seven studies,with a total of 10 study groups,explored reliability of maximal velocity assessed by the 30－15 IFT.ICCs ranged from 0.80 to 0.99,where 70%of ICCs were ≥0.90.CVs for maximal velocity ranged from 1.5%to 6.0%.Six studies,with a total of 7 study groups,explored reliability of peak heart rate as assessed by the 30－15 IFT.ICCs ranged from 0.90 to 0.97 (i.e.,all ICCs were ≥0.90).CVs ranged from 0.6%to 4.8%.All included studies were of excellent methodological quality.Conclusion:From the results of this systematic review,it can be concluded that the 30－15 IFT has excellent test－retest reliability for both maximal velocity and peak heart rate.The test may,therefore,be used as a reliable measure of fitness in research and sports practice.

Keywords:Data synthesis;Intermittent exercise;Repeatability;Reproducibility

1.Introduction

Many team sports have an intermittent structure;that is,they involve repeated high-intensity efforts that are interspersed with periods of lower-intensity activity.1,2Intermittent exercise is often characterized by short periods of high exertion that are associated with increases in oxygen consumption,heart rate,and blood lactate concentration,as well as decreases in blood and muscle pH.3Several tests have been developed for estimating an individual’s capacity to perform intermittent exercise.Some of these tests include the Yo－Yo intermittent recovery test,3,4the 20-m shuttle run test,5the University of Montreal track test,6and the 30－15 Intermittent Fitness Test(IFT).7－11These tests are popular among sport and exercise practitioners because they give insights into the current levels of fitness of their athletes,providing a basis for the development of training programs.3,11

The 30－15 IFT has gained popularity in recent years in both research and sports practice.7－11The test requires a short amount of time to be completed (around 20－30 min) and allows for testing large groups of athletes simultaneously.The test protocol includes 30-s shuttle runs (between 2 lines that are 40 m apart)that are interspersed with 15 s of passive recovery initiated and ended by audio beeps.The starting velocity for this test is 8 km/h,and it is increased by 0.5 km/h in each successive 30-s stage.The test is terminated when the individual volitionally stops due to accumulated fatigue or when the individual is not successful in reaching the next 3-m zone(near the marked line) at the beep on 3 successive occasions.This test is valid for estimating cardiorespiratory fitness.12The most commonly assessed outcomes of this test are the maximal achieved velocity and peak heart rate.Scott et al.13reported that the maximal velocity in this test correlates with 300-m shuttle time,repeated-sprint time,“Agility 505”test time,and 20-m flying sprint time.Maximal velocity in the 30－15 IFT is commonly used for the individual exercise prescription of high-intensity intermittent running,an aspect of the test that makes it very appealing for sports practitioners.7,14－16

Regardless of whether a test is used for an athlete’s fitness assessment,for exercise prescription,or for estimating the effectiveness of a given exercise program,it is important to know its reliability.17,18Reliability refers to the independence of a test from measurement error.17,18For practitioners,information on reliability can help to determine whether the test scores are trustworthy enough to allow monitoring changes in an athlete’s performance.In research,the reliability of tests is important,given that the use of tests with poor reliability may attenuate effect sizes and increase the probability of type II error.17,18The current body of evidence on the test－retest reliability of the 30－15 IFT is unclear.For example,Thomas et al.19reported a test－retest intraclass correlation coeff icient(ICC)for maximal velocity of the 30－15 IFT of 0.80,whereas Kelly et al.20reported an ICC of 0.99.According to the classif ication of ICC values suggested by Koo and Li,21the test could be classif ied as having moderate reliability(if informed by the Thomas et al.19study)or as having excellent reliability(if informed by the Kelly et al.20study).Therefore,there appears to be no scientif ic consensus on the test－retest reliability of the 30－15 IFT.Given the inconsistent findings of previous studies,the present systematic review aimed to synthesize individual studies that have explored the test－retest reliability of the 30－15 IFT.

2.Methods

The guidelines proposed by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses were employed for the purpose of this review.The search for the studies was conducted through Academic Search Elite,CINAHL,ERIC,PsycINFO,PubMed/MEDLINE,Scopus,SPORTDiscus,and Web of Science (including all Web of Science Core Collection:Citation Indexes) databases.Two authors (JG and BL)performed the searches for the studies independently.After conducting the searches,the included studies were cross-checked between the 2 authors,and a mutual consensus was reached for the final included/excluded studies.In all of the databases that were searched,the following combination of keywords and Boolean operators was used to identify the studies:(“30－15”OR “30 15” OR “intermittent fitness” OR “intermittent-f itness”OR “IFT”) AND (“reliability” OR “repeatability” OR“reproducibility”).The search was conducted on January 6,2020.

Studies that satisf ied the following criteria were included in the present review:(1) published in the English language,(2)published in a peer-reviewed journal,(3) examined the test－retest reliability of the 30－15 IFT,and(4)presented ICC and/or coeff icient of variation(CV)for maximal velocity and/or peak heart rate.We considered all studies,regardless of the participants’training status or their respective sport.

The following data were extracted from the included studies:(1) details of the sample,including the sample size,sex,and participants’ sports experience;(2) the number of days between the first assessment (test) and the second assessment(retest);and (3) ICC and/or CV values.Two authors (JG and BL) independently extracted the data from the included studies.After data extraction,the authors compared the files.Any observed differences in the extracted data were rechecked in the original studies.We based our interpretation of the ICC values on the following thresholds:poor reliability (ICC ＜0.50),moderate reliability(ICC=0.50－0.75),good reliability(ICC=0.76－0.90),or excellent reliability(ICC ＞0.90).21

Form B of the consensus-based standards for the selection of health measurement instruments (COSMIN) checklist(designed for reliability studies) was used for the assessment of the methodological quality of the included studies.22This form has 11 items that refer to the number of measurements and their administration,the time between test and retest,the adequacy of the sample size,the reporting of ICCs,and any other methodological limitations of the study.The checklist is described in more detail elsewhere.22A maximum of 1 point per item is assigned to a study.The overall score on the checklist,therefore,ranges from 0 to 11 points.The following thresholds were used to classify the studies according to their overall methodological quality:excellent quality (9－11 points),moderate quality(6－8 points),and poor quality(fewer than 6 points).Two authors (JG and BL) of the review independently conducted the quality assessment;any differences between the independent assessments were resolved through discussion between the authors until they reached an agreement.

3.Results

Of the 547 search results identif ied through the databases,516 were excluded based on their titles and abstracts,whereas 31 full-text papers were read.Seven studies were found to satisfy the inclusion criteria.12,19,20,23－26More details about the search and study selection process are presented in the flow diagram(Fig.1).

The average number of participants per study was 23,while the pooled number of participants across all of the included studies was 159.All of the studies included athletes as participants.They were competitors in basketball,ice hockey,soccer,rugby,wheelchair rugby,and futsal.Four of the studies used a 7-day period between the tests,whereas the remaining 3 studies used 2-,5-,and 9-day periods(Table 1).All included studies established reliability for maximal velocity,but one of them did not assess test－retest reliability for peak heart rate(Table 1).All included studies presented both ICCs and CVs as measures of test－retest reliability.The included studies are summarized in Table 1.

All studies scored 9 points on the COSMIN checklist and were,therefore,classif ied as being of excellent methodological quality(Table 2).None of the included studies received a point on Item 3(“Was the sample size included in the analysis adequate?”)or on Item 5(“Were the administrations independent?”).

Table 1Summary of the studies included in the review.

Table 2Results from the methodological quality assessment using Form B of the COSMIN checklist.

Seven studies,with a total of 10 study groups,explored reliability of maximal velocity.ICCs ranged from 0.80 to 0.99,where 70% of ICCs were ≥0.90.CVs in the included studies for maximal velocity ranged from 1.5%to 6.0%.

Fig.1.Flow diagram of the search and study selection process.

Six studies,with a total of 7 study groups,explored reliability of peak heart rate.ICCs ranged from 0.90 to 0.97 (i.e.,all ICCs were ≥0.90).CVs in the included studies for peak heart rate ranged from 0.6%to 4.8%.

4.Discussion

This review found that the 30－15 IFT has excellent test－retest reliability for maximal velocity,as shown by high ICCs(≥0.80)and low CV values(≤6.0%).We also found that the 30－15 IFT has excellent test－retest reliability for peak heart rate,given that the ICCs for this outcome were high(≥0.90) and the CVs were low (≤4.8%).All included studies were classif ied as being of excellent methodological quality,which adds to the credibility of these findings.

As noted previously,the 30－15 IFT is widely used by practitioners.7－11Therefore,it is important to compare the reliability of this test with other similar tests that are popular in practice,such as the Yo－Yo test.A recent review summarized findings from 19 studies that explored the test－retest reliability of the Yo－Yo test.27Across the included studies,ICCs for the Yo－Yo test ranged from 0.78 to 0.98(97%of all ICCs were above 0.80),whereas CVs ranged from 3.7% to 19.0% (75% of all CVs were below 10%).27Based on this comparison of results,the reliability of the 30－15 IFT seems to be similar to the reliability of the Yo－Yo test.

Interestingly,a study modif ied the 30－15 IFT and explored its test－retest reliability among athletes competing in wheelchair rugby.20The test was performed on an indoor court of 28 m (the length of a court for wheelchair rugby),where the participants were required to push back and forth between the allocated lines.20In this modif ied 30－15 IFT,the authors observed high ICCs and low CVs for maximal velocity(ICC=0.99;CV=1.9%) and peak heart rate (ICC=0.95;CV=4.5%).The results of this study suggest that the modif ied 30－15 IFT is a reliable test among wheelchair rugby players.

A learning effect is a well-recognized factor that may inf luence the reliability of a given exercise test.18An individual may become more prof icient in a given test with increased experience.This may subsequently decrease the amount of random error in the test results and,hence,improve the reliability of the test.Therefore,in studies focusing on the reliability of exercise tests,Currell and Jeukendrup18have suggested that the participants should be familiarized with the testing protocol by performing at least 1 practice session before the main sessions commence.However,of the 7 studies included in our review,only Scott et al.25reported that a familiarization session was provided to the participants as a part of the study design.Despite the overall lack of familiarization with the test,both maximal velocity and peak heart rate generally had excellent test－retest reliability.These results suggest that a familiarization session is not needed for an excellent test－retest reliability of the 30－15 IFT.It might be that a familiarization session would even further increase the reliability of 30－15 IFT.It is worth noting that the sample in all the included studies in our review comprised competitive athletes who,in some cases,12,19may have already been familiar with similar testing (or training) procedures.Therefore,it remains unclear whether similar results would be observed in untrained individuals.This merits future research,given that the test may also have applications for populations of untrained individuals.

In addition to familiarization with the exercise test,practitioners should also consider that several other factors may impact test reliability.For example,encouragement during the test could introduce another source of random variation because the encouragement may differ among trials,and the participants’responses to the encouragement might not be uniform in each trial.18Research has also established that various components of exercise performance vary according to the time of day,with better performance generally observed in the evening hours.28－30Additionally,the use of certain supplements,such as caffeine and sodium bicarbonate,has been found to enhance performance in tests similar to 30－15 IFT.31,32Practitioners should attempt to standardize these factors as much as possible when conducting trials.For more details on this topic,see the work by Currell and Jeukendrup.18

Research has shown that women are more fatigue resistant than are men.33Given this physiological difference between sexes,it is possible that men and women would have different responses in the context of test－retest reliability.Of the 7 studies included in our review,only Valladares-Rodrˊıguez et al.26included both men and women in their trials.In their study,13 male and 14 female futsal players performed the 30－15 IFT on 2 occasions.The results of Valladares-Rodrˊıguez et al.26indicated that the test was highly reliable both for men (ICC=0.92;CV=1.5%) and for women(ICC=0.96;CV=1.5%).However,given that this was the only study conducted involving both sexes,future studies with similar research designs are warranted.Additionally,given that the studies included in our review were conducted among basketball,hockey,rugby,soccer,and futsal players(with preliminary data reported for handball),7,8,11more research is also needed on the reliability of 30－15 IFT among athletes competing in other sports,particularly those with an intermitted structure,where this test has high applicability.

For the assessment of study quality,we adhered strictly to the guidelines set forth by the research group that developed the COSMIN checklist.22This research group has recommended that a sample of 100 participants is needed to classify a reliability study as a study with adequate sample size.It should be noted,however,that this is an arbitrary threshold.The required sample size will depend greatly on the expected ICC and the width of its conf idence interval that is deemed acceptable.For example,to achieve the width of 95%CI of ±0.02,the required sample size estimated using the Bonett’s34calculation would be around 90 participants for the expected ICC of 0.95,or about 350 participants for the expected ICC of 0.90.Nevertheless,given that the largest sample size in the included studies had 55 participants,25future studies exploring the test－retest reliability of the 30－15 IFT would def initely benef it from using larger sample sizes.

5.Conclusion

From the results of this systematic review,it can be concluded that the 30－15 IFT has excellent test－retest reliability for both maximal velocity and peak heart rate.Therefore,this test may be used as a reliable measure of fitness in research and sports practice.In particular,the 30－15 IFT can be used as a reliable test for monitoring athletes’ performance and for determining the eff icacy of a given training program.

Authors’contributions

JG conceived the idea for the review,conducted the literature searches,data extraction,and methodological quality assessment,and drafted the manuscript;BL conducted the literature searches,data extraction,and methodological quality assessment and critically revised the manuscript content;ZP conceived the idea for the review and critically revised the manuscript content.All authors have read and approved the final version of the manuscript,and agree with the order of presentation of the authors.

Competing interests

The authors declare that they have no competing interests.