Comparison of the Field-Based Intermittent Running Fitness Test 30-15 and the Treadmill Multistage Incremental Test for the Assessment of Cardiorespiratory Fitness in Elite Handball Players

The aim of the present study was to investigate whether the physiological parameters indicative of cardiorespiratory fitness obtained during the 30-15 intermittent fitness (30-15IFT) test and the multistage laboratory treadmill endurance (TR) test differ. Nineteen elite handball players were recruited for the current study and assigned in a cross-over manner to one of two tests to be performed 48 h apart at each visit to the testing facility. The results showed that VO2max (percentage difference [PC] = 6.1%; p = 0.004) and maximal running velocity (V) (PC = 19.4%; p < 0.001) were significantly higher for the 30-15IFT test than that obtained during the TR test. Furthermore, the onset of blood lactate accumulation was shown to be significantly higher for all measures considered to predict it during 30-15IFT compared to TR as follows: VO2max (PC = 12.6%; p = 0.001), running speed (PC = 33.9%; p < 0.001), and maximal heart rate (PC = 7.5%; p < 0.001). The current study highlights the importance of sport-specific testing, particularly for measuring individual cardiorespiratory fitness in elite handball players, as TR may underestimate crucial variables used for both diagnostics and training prescription.


Introduction
Handball, which has been known in its current form since 1954, has developed rapidly in recent decades as an Olympic and professional sport. Handball is a game characterized by frequent role changes of teams in attack and defense [1], with specific movement patterns consisting of different types of fast and explosive muscle actions, such as jumping, forward and backward sprinting, turning, and various changes in direction, with constant changes in tempo in combination with different types of ball throwing [2,3]. The characteristic movement patterns of the players, which are repeated in the different phases of the game, have different effects on their physiological load [4,5]. In addition to anthropometric measures, ball throwing speed, and technical and tactical skills, studies have shown that players' success depends in part on their ability to repeat short high-intensity runs [4,6]. Moreover, handball players have been shown to have a heart rate greater than 80% of their individual maximum heart rate (HR max ) during 50% of their total effective playing time [5], suggesting that cardiorespiratory fitness (CRF) is one of the most important determinants of performance in handball.
Strength and conditioning professionals in handball must be able to administer timeefficient, valid, and reliable tests to evaluate the various fitness capacities of their players. Indeed, direct measurement of CRF during a multistage laboratory treadmill endurance test (TR) is considered the "gold standard" for estimating maximal oxygen uptake (VO 2max ) [7]. Although not handball specific, TR is still routinely used to assess CRF in handball players [6,8,9]. However, due to its high cost, complex measurement procedures, and inability to measure more than one athlete at a time, this method is not the most appropriate for most handball teams. In addition, various blood lactate indices have been proposed to measure athletes' ability to exercise without accumulating lactate [10]. For example, onset of blood lactate accumulation (OBLA) was found to be strongly correlated with endurance performance (r = 0.96) [11] and has been proposed as a significant predictor of running performance [12], sensitive enough to discriminate between elite and non-elite athletes [13]. Therefore, it is important for coaches to determine the running intensity at which OBLA occurs to monitor CRF performance and prescribe endurance training intensities to their athletes. However, the point at which OBLA occurs may vary when different types of exercise [14] and testing are considered [15].
In practice, various laboratory and field tests are used to determine the physiological responses of athletes to endurance exercise. The advantage of field testing is that a larger number of athletes can be assessed at the same time, which saves time and other resources. Field tests have emerged that are better adapted to the specific requirements of various sports than laboratory tests [9,16]. They allow more suitable assessment of CRF that can be more specific to the actual sport in question [17,18]. In this way, field tests have evolved in different directions in terms of modality, i.e., the continuous test (the University of Montreal track test [19]), and/or the intermittent multistage fitness test [20] or Yo-Yo test. In these tests, it is often the case that athletes with a lower maximal running velocity (V) run at higher levels of intensity just to maintain the pace of athletes with a higher V [21]. Consequently, using the Yo-Yo test to prescribe a training intensity would not be a best option [21]. The desire to solve this problem, while also improving interval training prescription in team sports, led to the development of the 30-15 Intermittent Fitness Test (30-15 IFT ) [22]. The 30-15 IFT consists of 30 s shuttle runs interspersed with 15 s active recovery periods. At the beginning of the test, a running speed is set at 8 km/h for the first 30 s run and increased by 0.5 km/h in each 30 s phase thereafter. Subjects are required to run back and forth between two lines 40 m apart at the preset pace determined by a pre-recorded beep. The speed of the last successfully completed stage is recorded as the test result, i.e., the maximum running speed (V) during 30-15 IFT (V IFT ) [23]. By implementing this basic idea, the 30-15 IFT is useful for assessment of cardiorespiratory fitness by providing accurate estimation of maximal oxygen uptake (VO 2max ), HR max , and other components relevant to sports-related performance and training optimization, such as maximal aerobic speed and intermittent running effort recovery capacity [22]. Buchheit [16] showed that when using continuous CRF tests, running speed at maximal oxygen uptake is significantly lower than running speed at 30-15 IFT , suggesting that metabolic load at 30-15 IFT is much closer to loads observed during a handball match. Thus, compared to TR tests, the 30-15 IFT seems to be more specific to handball match demands [9]. In addition, V IFT can be considered to be an optimal tool for individualizing short intermittent run distances in handball players [23].
The first application and validation of the 30-15 IFT was performed on a sample of handball players [16]. With time and the popularization of the test, it was also used in other sports, with 30-15 IFT validations conducted in basketball, field hockey, football, and rugby [16]. The mentioned studies also confirmed the high reliability of different parameters estimated from the 30-15 IFT (ICC = 0.90-0.96). In practice, studies comparing the 30-15 IFT with other tests (YO-YO IR1, YO-YO IR2, 20 m shuttle run test) are becoming more common to determine its validity and potential for use in different sports [16,23,24].
To date, the concurrent validity of the 30-15 IFT has not been investigated in elite handball players compared with the standard continuous incremental running test. Moreover, the relationship between VO 2max measured with a portable metabolic measurement system (VO 2maxIFTK4 ) and the predicted value (VO 2maxIFT ) has yet to be determined. Therefore, the aim of this study was to experimentally test whether the 30-15 IFT can be used as valid indicator of CRF and HR max . We also wanted to investigate the differences between the 30-15 IFT and the TR test considering a point at which OBLA occurs. This information may help coaches determine CRF and prescribe endurance training intensities for their athletes when considering different testing protocols.

The Experimental Approach to the Problem
All handball players performed two maximal exercise tests, one on the field (IFT  ) and one in the laboratory (i.e., TR), to evaluate their cardiorespiratory fitness in the middle of the regular training season. During this period, athletes performed mainly tactical and technical training with handball game-specific movement patterns and physiological efforts (4 to 7 sessions), while strength and power training was performed once or twice a week. During the first visit to the testing facility, the TR was performed indoors on a treadmill, whereas the field test was performed on a standard indoor handball court with a hardwood floor. The tests were conducted with a rest period of 48 h between them. Both tests were conducted between 10 am and 11 am at an ambient temperature of 20 to 22 • C.

Participants
During the conceptualization of the study design, an a priory power analysis was conducted based on the correlation coefficient, as recommended [25]. Based on a previous study with a similar aim and design [22], we expected to find a moderate to high relationship between continuous and intermittent run tests (r = 0.76) for VO 2max . Therefore, with a probability of failing to reject the null hypothesis of β = 0.2 and two-tailed α = 0.05, a minimum sample size of 11 subjects was shown to be sufficient to detect a value of ≥0.76 for r. As a result, twenty-four elite handball players (mean age: 24.2 ± 5.5 years; height 188.6 ± 6.5 cm; body mass 89.4 ± 9.4 kg; training status 12.6 ± 3.1 years) were recruited for the current study. The inclusion criteria were as follows: handball players who are members of a national team and/or play at the international level and regularly participate in European Cups, who have not had any serious injury or illness that could limit maximal performance for the six months prior to the start of the study, who have not had acute pain, and who have participated in a regular training process. To avoid unnecessary fatigue, players and coaches were instructed to avoid intense sporting activities one week before actual testing and during the study period. All subjects were informed of the benefits and potential risks of the study and provided written informed consent to participate in the current study. All procedures were conducted in accordance with the ethical standards of the 1964 Declaration of Helsinki and approved by the Ethics Committee of the Faculty of Sport (University of Ljubljana, Ljubljana, Slovenia).

Procedures
Immediately prior to testing, participants completed a standard 25 min warm-up program consisting of 10 min of self-paced jogging, 10 min of dynamic stretching, and 5 repetitions of 30 m of fast running. After the warm-up routine participants each participant performed the TR or IFT test.

Field Test
Aerobic capacity was measured using the field based 30-15 IFT test, as previously recommended [23,26]. This intermittent, incremental test consists of 30 s shuttle runs interspersed with 15 s active recovery periods. Running speed was set at 8 km/h for the first 30 s run and increased by 0.5 km/h in each 30-s phase thereafter. Players were required to run back and forth between two lines 40 m apart at the preset pace determined by a pre-recorded beep. The prerecorded beep allowed players to adjust their running speed when they entered a 3 m zone in the middle and at each end of the test field. During the 15 s recovery period, players walked forward to the nearest line (either in the middle or at the end of the running area, depending on where their previous run had ended); from this line, they began the next running phase. Players were instructed to complete as many stages as possible. The test ended when the player could no longer maintain the required running speed or failed to reach the 3 m zone three consecutive times in the period before the sound signal. The speed of the last successfully completed stage was recorded as the test result, i.e., the maximum running speed (V) during 30-15 IFT (V IFT ) [23]. The VO 2max was calculated by following equation [23]: where (G) stands for gender, (A) for age and (BM) for subjects' body mass.

Incremental Treadmill Test
An incremental step test was performed on a treadmill (hp Cosmos Saturn, hp Cosmos, Traunstein, Germany) with a constant gradient of 1% inclination [27]. The initial velocity was set at 8 km/h and increased by 2 km/h every 4th minute until the subject was no longer able to maintain the velocity. The achievement of VO 2max was identified as the plateauing of VO 2 (<2.1 mL/kg/min decrease) despite an increase in workload [28]. If the above-stated criterion was not fulfilled, the participants were asked to perform a further constant-speed test equal or higher than the highest speed achieved at the end of the incremental test, as recommended [29]. Throughout the test, respiratory gases were continuously measured breath-by-breath and reduced to 10 s averages [30]. The rest period between different stages was 1 min, which was used to take lactate samples. The last running velocity reached during the test was defined as V TR.

Maximum Aerobic Performance, Heart Rate and Blood Sampling Testing Equipment
A portable gas analyzer K4b 2 (COSMED, Rome, Italy) was used to obtain physiological parameters. The device provides reliable values for oxygen uptake (O 2 ), carbon dioxide production (CO2), and pulmonary ventilation (VE) breath-by-breath [31,32]. In addition, blood samples (20 µL) were collected from the earlobe for both tests and the samples were analyzed for blood lactate concentration (LA-) using a Kodak EKTACHROME analyzer. However, due to the different natures of the tests performed, blood samples were collected at the following intervals: TR test: before the test (T1); at running speeds of 10 km/h (T2), 12 km/h (T3), 14 km/h (T3), 16 km/h (T4), and 18 km/h (T5); and at 3rd (T6) and 5th (T7) minutes after the end of the test.
Simultaneously, a heart rate was measured by Polar S-610 heart rate pulse-meters (Polar Electro, Kempele, Finland). The data were recorded in 5 s intervals and automatically analyzed using the original Polar software. The OBLA that is the running speed corresponding to the [LA] of 4 mmol/L was detected by interpolation from the [LA]-running speed relationship curve [33].

Statistical Analysis
All data are presented as mean ± SD and 95% confidence intervals. Statistical analyses were undertaken with SPSS statistical software (version 27, IBM corp., Chicago, IL, USA). Normality was confirmed by visual inspection and using the Shapiro-Wilk test. The paired Student's t test was used to compare physiological data obtained by laboratory and fieldbased tests. One-way ANOVA was used to compare the VO 2max values obtained on TR (VO 2maxTR ), VO 2maxIFTK4 , and VO 2maxIFT . Hedges' g effect sizes (ES) with 95% confidence intervals were calculated to show practical differences between legs and were interpreted as: trivial: <0.20, small: 0.20-0.50, moderate: 0.50-0.80, or large: >0.80 [34]. Bland-Altman analysis was used to determine absolute limits of agreement between (a) VO 2maxTR and VO 2maxIFTK4 ; (b) VO 2maxIFTK4 and VO 2maxIFT ; and (c) V TR and V IFT . In addition, a Pearson correlation coefficient was used to evaluate the association between VO 2max variables obtained from TR and IFT tests, whereas Spearman's rank-order correlation was used if the assumption of normality of data distribution was violated. The following thresholds of the correlation coefficient were used to assess the magnitude of the relationships analyzed: weak ≤ 0.35; 0.36 ≤ moderate < 0.67; 0.68 ≤ high < 1 [35]. A level of significance for all analyses was accepted at p ≤ 0.05.

Discussion
The primary aim of the study was to compare physiological parameters obtained during TR and 30-15 IFT tests in elite handball players. In addition, a concurrent validity and correlation analysis was performed between determined VO 2max and V values. Considering the characteristics of handball [1] and the principles of training specificity and adaptation, we hypothesized that the 30-15 IFT test provides better insight into the physiological parameters of handball players than the traditional TR test.
The results showed that the values for V and VO 2max were significantly higher in 30-15 IFT than in the TR test. V values obtained are consistent with previously reported results, showing between 2 and 5 km/h higher running speed during 30-15 IFT than those achieved during the TR test [16]. Later results suggest that the 30-15 IFT is a sensitive tool for detecting changes in V during performance monitoring. In addition, similar trends in VO 2max difference estimates have been reported for female soccer players [36], female basketball players [37], and male team sports athletes [22]. There are two main reasons that may explain the discrepancies observed. First and foremost, the 30-15 IFT is a field test that assesses change in direction ability, inter-effort recovery ability, and anaerobic capacity in addition to CRF [16], and is more similar to the handball game than a TR test, which uses a continuous running protocol. Furthermore, group testing, as performed in team sports during the 30-15 IFT test, has been shown to increase task motivation [38]. In addition, our results demonstrated that HR max values tend to be higher during 30-15 IFT , confirming previous findings [22,36,37], although with lack of statistical significance. This is not surprising, because sport-specific tests often result in higher HR max values, as shown in other sports such as soccer, tennis, and squash [39][40][41].
When comparing OBLA, the estimated values differ significantly between TR and 30-15 IFT . OBLA was found to be significantly higher for all measures used to predict it during 30-15 IFT compared with TR. The observed differences can be explained by the protocols used, which comprise intermittent versus continuous endurance testing. The unique feature of 30-15 IFT is that the running bouts are interrupted by a 15 s active rest, which enables athletes to partially resynthesize the energy substrates used for both the anaerobic (i.e., ATP and CP) and aerobic components of intermittent running performance (i.e., myoglobin functioning) [42]. Essen and colleagues showed that ATP and CP levels fluctuated between work and rest phases during the intermittent running protocol, but did not resynthesize to initial levels. In addition, myoglobin functioning, which acts as an oxygen store, was found to be an important factor in delaying OBLA, the time when anaerobic metabolism becomes dominant during maximal endurance testing. It appears that a 15 s rest period can delay the onset of fatigue, allowing athletes to achieve a 19.4% higher V during 30-15 IFT compared to TR.
To the best of the authors' knowledge, this is the first study to report objectively measured and estimated 30-15 IFT VO 2max values. Although several studies have shown that 30-15 IFT is reliable, valid, and useful [36,43], no author has compared the objectively measured VO 2max values and those calculated according to the formula proposed by Buchheit [23].
We found moderate and high correlations between 30-15 IFT and TR and 30-15 IFT and 30-15 IFT4K VO 2max values, respectively, whereas  and TR values had a weak, non-significant correlation. These results confirm our hypothesis that the 30-15 IFT test can provide better insight into the physiological parameters of handball players than the traditional TR test, due to its similarity to handball play. This was further supported by the significant difference between TR and the 30-15 IFT VO 2max values, which underestimated VO 2max by up to 6.1% during the continuous endurance test.
This study highlights the importance of sport-specific testing, particularly for measuring individual CRF. As noted by Basset and Boulay [44], VO 2max is highly dependent on the type of test. For example, runners are usually tested on a treadmill and cyclists on a cycle ergometer because of this specific adaptation. The 30-15 IFT test takes into account various qualities required in shuttle intermittent runs, such as lower limb power, aerobic qualities, and the ability to recover between subsequent sets of running bouts [16], and plays a very important role in diagnostics, training prescription, and optimization in handball.
We are aware that our study may have some limitations. Although we used a reliable and valid multistage laboratory testing protocol to assess CRF, it may not be the most appropriate protocol to compare physiological strain at different levels of running intensity with 30-15 IFT.

Conclusions
We found that the variations in the differences between two graded treadmills and IFT were significant and did not fall within the limits of agreement. In addition, the results showed that the values for maximum running velocity and VO 2max were significantly higher in the 30-15 IFT test than in the TR test. Therefore, the current study highlights the importance of sport-specific testing, particularly for measuring individual cardiorespiratory fitness in elite handball players, as TR may underestimate crucial variables used for both diagnosis and training prescription.
Author Contributions: U.M. and M.S. conceived the study, participated in its design, carried out the experiments. A.H.P. performed the data processing and statistical analysis, and drafted the manuscript with U.M., E.A. and S.J. All authors have read and agreed to the published version of the manuscript.
Funding: This project was supported by the Slovenian Research Agency (ARRS) (project ID: P5-0147 entitled "The kinesiology of mono-structured, poly-structured and conventional sports", project manager: Prof. Dr. Matej Supej).

Institutional Review Board Statement:
The study protocol was reviewed and received full ethical clearance.
Informed Consent Statement: Each participant provided written, informed consent to participate in the study.
Data Availability Statement: All data generated are available within the present manuscript.