With the exception of track and field or other

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1 THE INTERMITTENT FITNESS TEST: ACCURACY FOR INDIVIDUALIZING INTERVAL TRAINING OF YOUNG INTERMITTENT SPORT PLAYERS MARTIN BUCHHEIT Institute of Physiology, Faculty of Medicine, Strasbourg, France and APS and Motor Patterns: Adaptations-Rehabilitation, Sport Science Department, Picardie Jules Verne University, Amiens, France ABSTRACT The objective of this study was to gather evidence supporting the accuracy of the Intermittent Fitness Test (30-15 IFT )for individualizing interval training of young intermittent sport players. In 59 young intermittent sport players (age, years), we observed the relationships between the maximal running speed (MRS) reached at the end of the IFT (MRS 30-15IFT ) and physiological variables elicited by shuttle intermittent runs, including maximal oxygen uptake, explosive power of lower limbs, and the ability to repeat intense exercise bouts through cardiorespiratory recovery kinetics during exercise. To observe the capacity of the IFT to prescribe suitable running intensities for interval training sessions, we compared heart rates (HRs) reached during 3 series of intermittent runs, where distances were set according to the MRS 30-15IFT and to MRS reached with 2 popular continuous field tests: the University of Montreal track test and the 20-m shuttle run test. The results show that the MRS 30-15IFT is significantly correlated with all physiological variables elicited by shuttle intermittent runs (P, 0.05). Although mean HR were not different among the 3 series of intermittent runs, HR recorded during the runs based on MRS 30-15IFT presented significantly less interindividual variation than when the continuously determined MRS were used as reference speeds. In conclusion, we can say that the IFT leads to an MRS that simultaneously takes into account various physiological qualities elicited when performing shuttle intermittent runs. For scheduling interval training sessions, the MRS 30-15IFT appears to be an accurate reference speed for getting players with different physiological profiles to a similar level of cardiorespiratory demand and thus for standardizing training content. KEY WORDS reference maximal running speed, maximal oxygen uptake, explosive strength, quickness, heart rate recovery Address correspondence to Martin Buchheit, martin.buchheit@ u-picardie.fr. 22(2)/ Journal of Strength and Conditioning Research Ó 2008 National Strength and Conditioning Association INTRODUCTION With the exception of track and field or other endurance activities (e.g., rowing, triathlon, cycling, swimming), the nature of most sport exercise is intermittent (2,6,21,38). Performance in intermittent sports has been related more to speed, agility, strength, explosive power, and the ability to repeat brief supramaximal exercise bouts than to the capacity to maintain continuous steady submaximal mechanical work (5,6). Thus, from a strength and conditioning approach, intermittent sport coaches priorities are to increase and maintain those physiological variables predictive of performance during intermittent sports through athletic drills and maximal strength programs (17,27). High-intensity anaerobic interval training is also regularly planned in the majority of team sports (20,27) to develop maximal oxygen uptake ( _ Vo 2 max) and enhance the ability to repeat intense exercise bouts (23,41). Although a direct relationship between _ Vo 2 max and repeated sprint ability is still not well established (4,10,24), it has been shown that high aerobic capacity might have a beneficial effect on recovery kinetics during high-intensity intermittent exercise (41) and that _Vo 2 max may influence game performance and total highintensity running distance during a soccer match (26,29). Until now, running intensities of such intense interval training sessions have been set according to individual maximal aerobic speed (MAS, determined by gas exchange analysis, is the lowest running speed that elicits _ Vo 2 max) (7,8,20). These exercises are often composed of shuttle runs so as to introduce accelerations, decelerations and changes of direction, which are, on the one hand, running patterns specific to intermittent sports, and, on the other, more adapted for short anaerobic interval training sessions aimed at preferentially enhancing peripheral components of cardiorespiratory function (7,27). In the past 2 decades, numerous field tests have been proposed to determine MAS and consequently indirectly _Vo 2 max of athletes (31 33). These famous tests are based on continuous linear runs (31) or shuttle runs (32,34), and the maximal running speeds (MRSs) reached at the end of the tests (which are not far from the MASs) are obtained through an effort different from that of intermittent sports and the way VOLUME 22 NUMBER 2 MARCH

2 The Intermittent Fitness Test for Interval Training Individualization interval training sessions are programmed. Using these MRSs in the field may be the first objective way to individualize highintensity intermittent runs (7,8,20), but certain physiological determinants of performance during intermittent and shuttle efforts are not evaluated by these field tests. How do subjects adapt to the intermittency of the runs? How do they tolerate changes in direction when running at supramaximal speeds? Since shuttle speeds (32,34) are lower compared to linear ones (1), coaches have to convert them to get more appropriate reference speeds. Such conversion tables are based on theoretical data that do not provide for individual adjustment. Sometimes to get different players to reach a similar internal load, a distinct percentage of the continuous MRS is used (external load), depending on how each athlete adapts to the intermittency of a run and on how each tolerates the changes in direction. Unfortunately, these empirical manipulations diminish the precision of a training prescription. In view of these field test defects, a protocol that includes intermittent and shuttle runs simultaneously appeared necessary. A few intermittent field tests already exist, for example, the yo-yo test (29) and the Intermittent Shuttle Run Test (ISRT) (35), but they provide only an index of intermittent aerobic performance (22,30). They do not give an MRS that can be used as a reference speed for training purposes. We have thus been led to develop the Intermittent Fitness Test (30-15 IFT ) (12,13), the reliability of which has already been shown (13), as an alternative to these other tests. Being both intermittent and shuttle, its main interest is that it involves physiological variables similar to those solicited during shuttle interval training sessions (i.e., explosive power of lower limbs when changing direction, aerobic qualities, and ability to recover between exercise bouts). It allows players reach a final MRS (MRS 30-15IFT ) that we hypothesize to be more accurate for scheduling shuttle interval training sessions than a usual continuously determined MRS. The aims of this work are (a) to show the capacity of the IFT to determine a particular speed (MRS 30-15IFT ) reflecting the multiple physiological qualities elicited when performing shuttle intermittent runs and (b) to illustrate the accuracy of the MRS 30-15IFT for individualizing high-intensity anaerobic interval training. We propose that the first merit of the IFT lies in its ability to assess various physiological variables related to performance during intermittent runs instead of just a single specific quality [usual continuous tests (31 33) estimate only _ Vo 2 max, which is used to examine criterion validity]. We also suggest that the accuracy of the MRS 30-15IFT to individualize interval training sessions will be demonstrated if cardiorespiratory responses during intermittent runs based on the MRS 30-15IFT present little interindividual dispersion. This would indeed help offer a similar training load to all athletes of a team by standardizing training contents. METHODS Experimental Approach to the Problem We first examined the associations between the MRS 30-15IFT and certain physiological abilities elicited when performing shuttle intermittent runs, such as explosive power of lower limbs (estimated by 10-m sprint and counter movement jump tests), _ Vo2 max and cardiorespiratory recovery ability (evaluated via short-term heart rate (HR) recovery kinetics during exercise). In order to judge the accuracy of the IFT as training prescription for intermittent sport athletes, we observed the variability of cardiorespiratory responses (internal load, inferred from the percentage of HR reserve, %HR res ) elicited by 3 series of short intermittent runs. The running distances (similar relative external load) were set at fixed percentages of the MRS 30-15IFT and of the MRS reached at the end of 2 popular continuous tests, the University of Montreal track test (UM-TT) (31) and the 20-m shuttle run test (20mSRT) (32). A low variability (i.e., coefficient of variation,5%) of %HR res is taken as evidence of accuracy. Subjects Participants were 59 young athletes (27 women, 32 men; age, years; weight, kg; height, cm; Tanner stages 4 and 5). Their sexual development was determined by the usual recommendations (39) from values assigned to each maturity indicator on a scale of 1, representing immaturity, to 5, indicating full maturity. The youths were randomly selected from 2 competitive basketball and handball training camps that worked out at the same gymnasium and benefited from similar medical and testing conditions. Participants were involved in hours of physical training per week plus a match each weekend (usually at a regional level). The athletes underwent medical screening and did not present any contraindications for vigorous exercise. Subjects and their parents were informed about the study and gave their consent to participate. The study protocol, which was approved by the CNIL (French National Committee for Informatics and Liberties) and the local ethics committee (Consultative Committee for Human Protection in Biomedical Research), conforms to the Declaration of Helsinki. Testing Procedures Although the 59 subjects all performed a laboratory-graded continuous running test on an electronic treadmill and jumping and sprinting tests (see below), we had to divide them into 2 groups for field testing because of methodological constraints. The first group (group 1) included 34 athletes taken randomly who performed explosive tests, Vo2 _ max testing, and the IFT plus the UM-TT. They thus performed intermittent runs on the basis of the MRS 30-15IFT and the MRS UM-TT. The 25 others constituted the group 2. They performed explosive tests, Vo2 _ max testing, and the IFT together with the 20mSRT. They had their training sessions scheduled with the MRS 30-15IFT and the MRS 20mSRT. The Intermittent Fitness Test Protocol. All 59 subjects did the IFT, which consists of 30-second shuttle runs interspersed with 15-second passive recovery periods. Velocity was set at 8 kmh 21 for the first 30-second run and was increased by 0.5 kmh 21 every the 366 Journal of Strength and Conditioning Research

3 the Journal of Strength and Conditioning Research 45-second stage thereafter. Calculation of targeted distances to run during each 30-second period took into account the fact that the effort to turn is increased when running speed is increased. We subtracted an empirical value of 0.7 second from the 30-second running periods for each change of direction. For example, at 11.5 kmh 21, one would cover 96 m in a 30-second straight line run, but covering the same distance over a 40-m shuttle distance requires 2 direction changes taking seconds, which brings the corrected distance run to 91.6 m. The subjects had to run back and forth between two lines set 40 m apart at a pace governed by a prerecorded beep at appropriate intervals that helped them adjust their running speed by entering into 3-m zones at each extremity and in the middle of the field while the short beep sounds (Figure 1). During the 15-second recovery period, the subjects walked in the forward direction to join the closest line (at the middle or at one end of the running area, depending on where the previous run stopped) from where they started the next run stage. Subjects were instructed to complete as many stages as possible. The test ended when a subject could no longer maintain the imposed running speed or when he or she was unable to reach a 3-m zone around each line at the moment of the audio signal consecutively 3 times. The velocity attained during the last completed stage is taken as the MRS 30-15IFT. Physiological Basis of Exercise/Recovery Pattern. The respective durations of exercise and recovery periods were chosen (a) with regard to most intermittent sports characteristics (2,6,21,38) and (b) according to various physiological Figure 1. Area prepared for the Intermittent Fitness Test (30-15 IFT ) and example of 2 intermittent runs. For the run at 8.5 kmh 21 (about 69.2 m in 30 seconds), subjects start at line A, run to line C crossing line B, and then return. After crossing line B again, they stop after 8.5 m and walk to line A during the 15-second recovery to be ready for the next stage. For the run at 11.5 kmh 21 (about 91.2 m in 30 seconds), subjects start at line A, make one complete round trip, stop after 9.5 m when going toward line B, and then walk to line B during the 15- second of recovery for the next start. Note that calculation of targeted distances takes into account the time needed for the direction changes. considerations. Concerning exercise periods, 30 seconds is close to the time of cardiorespiratory on-response kinetics at the beginning of exercise (16,19) and is also the time in which HbO 2 resources have been shown to be consumed at intense exercise (36). Since the half-time for recovery of phosphocreatine stores has been reported to be seconds (25), the 15 seconds of recovery would allow sufficient but incomplete recovery of energy substrates as during intermittent games (6,21,38). Finally, compared to the ISRT (35), the choice of longer shuttle distances (40 m vs. 20 m) is intended to diminish muscular lactacidemia (1) and perceived exercise painfulness in the lower limbs, which helps reach a supramaximal MRS (higher than MAS). Reliability of the Intermittent Fitness Test. The reliability of the test has been described elsewhere (12,13). We previously reported that test-retest yielded similar results in 19 subjects (9 women, 10 men; age, years) with intraclass correlation coefficient (ICC) r = 0.96 for the MRS 30-15IFT.The mean MRS 30-15IFT was similar ( vs kmh 21 ). Physiological parameters were also not different: maximal HR: vs bmin 21 ; blood lactate 3 minutes after the test: vs mmoll 21. The University of Montreal Track Test The UM-TT was conducted on a 400-m outdoor track according to the recommendations of Leger and Boucher (31) on 34 subjects. Pylons were placed at every 50 m of the track. The 34 subjects ran continuously and were paced with a sound signal emitted at specific intervals from a prerecorded tape. The velocity was set initially at 8 kmh 21. Thereafter, it was increased by 1 kmh 21 every 2 minutes. The subjects were instructed to complete as many stages as possible, and the test was stopped when the subjects were at least 5 m behind the appropriate pylon at the sound signal 2 successive times or when they felt they could not complete the stage. Although we believe that the test leads to MAS, with no gas analysis data, we prefer to note the final velocity as MRS UM-TT. The reliability of this test has already been investigated (ICC r = 0.94; n = 60) (31). The 20-Meter Shuttle Run Test The 20mSRT was performed as formulated by Leger and Lambert (32) by 25 subjects. It involves continuous running between 2 lines set 20 m apart on a nonsliding surface at running speeds governed by a prerecorded beep at appropriate intervals. Velocity was set at 8 kmh 21 for the first minute, increasing by 0.5 kmh 21 every minute thereafter. Subjects were instructed to complete as many stages as possible, and the test was stopped when a subject was unable 3 consecutive times to reach a 3-m zone situated ahead of each 20-m line at the moment of the audio signal. We believe that the test leads to MAS, but without gas analysis data, we prefer to note the final velocity as MRS 20mSRT. The reliability of this test has already been investigated (ICC r = 0.98; n = 50) (32). VOLUME 22 NUMBER 2 MARCH

4 The Intermittent Fitness Test for Interval Training Individualization TABLE 1. Physiological capacities elicited when performing intermittent runs and maximal running speeds reached at the end of each of 3 field tests. MRS Subjects _ Vo2 max MAS Treadmill IFT UM-TT 20mSRT 10 m CMJ HR RE Total (N = 59) 45.8 (5.8) 14.1 (0.9)* 14.7 (1.9)* 18.2 (1.6) 1.98 (0.14) 42.5 (7.7) 6.6 (1.3) Group 1 (n = 34) 49.2 (4.2) 14.8 (1.1)* 15.1 (1.7)* 18.6 (2.1) 14.9 (1.9)* 1.90 (0.09) 47.2 (5.3) 6.5 (1.5) Group 2 (n = 25) 41.9 (3.6) 13.2 (1.4)* 13.9 (1.2)* 16.9 (1.2) 11.5 (0.9)* 2.09 (0.10) 36.7 (4.9) 6.8 (1.9) Mean values (SD) of maximal oxygen uptake ( _ Vo 2 max, mlmin 21 kg 21 ), maximal aerobic speed (MAS), maximal running speed (MRS, kmh 21 ) reached at the end of each test (treadmill for the laboratory exercise test, Intermittent Fitness Test [30-15 IFT ], University of Montreal track test [UM-TT], and 20-m shuttle run test [20mSRT]), explosive power of lower limbs (10-m sprint time [10 m], in seconds; countermovement jump [CMJ], in centimeters), and heart rate recovery index (HR RE ) measured during the IFT. Group 1 = subjects who performed the IFT and the UM-TT; group 2 = subjects who performed the IFT and the 20mSRT. *Significant difference (P, 0.001) vs. MRS 30-15IFT. Series of Intermittent Runs Here the aim was to bring all players to a similar level of metabolic demand during each of 3 series of intermittent runs, using in each series 1 of the 3 MRSs as the reference running intensity. Each series consisted of repeated runs for 15 seconds at high intensity alternated with 15 seconds of passive recovery lasting 10 minutes. The targeted running distance of each intermittent run was calculated as a given percentage of each MRS. Awareness of MRS differences among the 3 tests [Table 1; MRS 30-15IFT is higher than MRS UM-TT (by 20%) Figure 2. Zero-order linear regressions between the maximal running speed (MRS) reached at the end of the Intermittent Fitness Test (MRS 30-15IFT )and physiological qualities elicited when performing shuttle intermittent runs in the 59 subjects. Maximal oxygen uptake ( _ Vo 2 max), explosive power of lower limbs (10-m sprint time [10 m] and countermovement jump [CMJ]) and heart rate recovery index (HR RE ). the 368 Journal of Strength and Conditioning Research

5 the Journal of Strength and Conditioning Research TABLE 2. Targeted distance and heart rate reserve observed during 10-minute intermittent runs. Subjects Reference speed Targeted distance (m) for 15-s runs HR %HR res CV (%) Total (N = 59) MRS 30-15IFT 69.5 (7.2) 91.2 ( ; 2.8) 3.1 Group 1 (n = 34) MRS 30-15IFT 72.4 (6.1) 92.4 ( ; 2.1) 2.3 MRS UM-TT 69.9 (12.2) 89.1 ( ; 8.3)* 9.2 Group 2 (n = 25) MRS 30-15IFT 66.6 (4.7) 91.7 ( ; 2.7) 2.9 MRS 20mSRT 66.5 (10.5) 89.5 ( ; 9.5) 10.6 Mean values (SD) of targeted distance runs and mean percentage (range; SD) of heart rate reserve (%HR res ) reached during each series of intermittent runs (15-second exercise/15-second rest) based on the maximal running speed (MRS) of each test [MRS IFT = MRS reached with the 30-15IFT = Intermittent Fitness Test, MRS UM-TT for the University of Montreal track test, and MRS 20mSRT for the 20-m shuttle run test (kmh 21 )]. CV = coefficient of variation; Group 1 = subjects who performed both the IFT and the UM-TT; Group 2 = subjects who performed both the IFT and the 20mSRT. *Six subjects (18%) were not able to finish the 10-minute intermittent runs based on the UM-TT. Four subjects (16%) were not able to finish the 10-minute intermittent runs based on the 20mSRT. and higher than MRS 20mSRT (by 35%)] and based on previous experimentation (14), we used the following percentages that are intended to represent equivalent metabolic demands: 110% of MRS UM-TT (group 1, n = 34), 140% of MRS 20mSRT (group 2, n = 25) and 95% of MRS 30-15IFT (pooled, N = 59). For example, for one subject displaying typical MRS values (MRS UM-TT = 16.5 kmh 21 and MRS 30-15IFT = 19.0 kmh 21 ), running at 95% of an MRS 30-15IFT consisting of covering [(19/3.6) ] 3 ( ) = 71.7 m in 15 seconds and running at 110% of an MRS UM-TT consisting of covering [(16.5/3.6) ) 3 ( ) = 72.0 m in 15 seconds, which are quite similar distances. Instrumentation Maximal Oxygen Uptake. A maximal graded continuous running test was performed on an electronic treadmill (Cardiovit 100; Schiller, Baar, Switzerland) where _ Vo 2 max was determined. All athletes performed a standardized 5-minute warm-up, and the test began at a running speed of 8 kmh 21,whichwas increased by 1 kmh 21 every 2 minutes until exhaustion. The treadmill grade was set to 1%. After a standard calibration procedure of all apparatus, HR and gas exchange parameters (minute ventilation, _ Vo2, CO 2 output) were continuously recorded with a commercially available system (Breath-by-- Breath Metabolic Measurement; Sensor Medic MSE, Rungis, France). _ Vo2 max was determined by the criteria of Taylor et al. (40): a plateau in _ Vo 2 despite an increase in running speed and HR.90% of the predicted maximal value. The velocity associated with _ Vo 2 max (MAS) was the lowest running speed that elicited a _ Vo 2 value equal to _ Vo 2 max (9) and represented 92.2% of the MRS reached during the treadmill test. Explosive Power of Lower Limbs After a supervised warm-up, muscular explosive power of lower limbs was assessed by jumping and sprinting abilities (18). Following Bosco et al. (11), jump testing consisted of a vertical countermovement jump (CMJ; in centimeters) on a Bosco jumping mat (Ergojump; Globus Italia, Codogne, Italy) that calculates jumping height from flying times. Since this method of assessment can have a methodological bias (notably landing with leg flexion), an experienced investigator validated each trial visually. Sprint abilities were evaluated by a 10-m standing-start run (10 m; in seconds) (18) recorded with photoelectric cells (Wireless Timing-Radio Controlled, Brower Timing System; Matsport, St. Ismier, France). Both tests were performed 3 times, separated by 45 seconds of passive recovery. Only the best performance was retained. Heart Rate After applying conductive gel, an electrode transmitter belt (T61; Polar Electro, Kempele, Finland) was fitted to the chest of each subject as prescribed by the manufacturer. Heart rate was measured during quiet wakefulness in the morning (15) in the supine position for 10 minutes (HR rest ). During all the tests, HR was recorded at 5-second intervals using a HR monitor (S610; Polar Electro, Kempele, Finland). When recorded data displayed aberrant values, HR was corrected by interpolation from adjacent values. Maximal HR (HR max ) was recorded in all field tests so as to verify that the subject performed at maximal effort. During field tests, data from the subject presenting at exhaustion an HR,90% of HR max recorded during the maximal graded continuous running was rejected (n = 1). Heart Rate Analysis The HR recovery index (HR RE ) (12,13) was used to evaluate cardiorespiratory recovery capacities. The HR recovery index was calculated, during the IFT only, as the sum of all differences between maximal and minimal HRs registered during each 15-second recovery period divided by the total number of beats during the entire test. Mean individual HR VOLUME 22 NUMBER 2 MARCH

6 The Intermittent Fitness Test for Interval Training Individualization recorded during the 3 series of intermittent runs are reported as a percentage of HR res as proposed by Karvonen et al. (28). In order to evaluate the %HR res disparity among athletes, we calculated the difference between the mean %HR res reached by all the subjects and the individual %HR res :DHR res = abs (mean %HR res 2 %HR res individual). The coefficient of variation (CV; in percentage) (3) was also calculated for %HR res. Statistical Analyses Statistical analyses were carried out using Minitab 13.2 Software (Minitab Inc., Paris, France). Descriptive statistics were computed as means and SD. As data were normally distributed, parametric tests were used. To avoid any gender, age, or body mass effect (all known to influence endurance, power, and speed), we adjusted data on these possible confounding factors by introducing them as variables in the analyses. A multiple linear regression model was first used to establish the link between MRS 30-15IFT and all the intermittent sport-specific physiological capacities tested in the 59 subjects. A minimal sample size of 59 subjects was determined based on the work of Pedhazur (37), who suggested a subject-to-variable ratio of 15:1 for multiple regression analyses ( = 60). Concerning HR responses during intermittent runs, DHR res calculated from each series were compared with a one-way analysis of variance with a post hoc test (Tukey). Significance was set at P # 0.05; a tendency was assumed for P, RESULTS Maximum Heart Rate at the End of Each Field Test The HR max reached at the end of the IFT was not different from those measured at the end of the other tests [respectively, vs bmin 21 for IFT vs. treadmill (N = 59), vs bmin 21 for IFT vs UM-TT (n = 34) and vs bmin 21 for IFT vs 20mSRT (n = 25)]. Physiological Qualities and Maximal Running Speeds Reached Within Each Field Test Mean (6SD) values of Vo _ 2 max, MAS, MRS, explosive power of lower limbs, and HR RE are presented in Table 1. Whatever the subgroup considered, the MRS 30-15IFT was significantly higher than the MAS, MRS UM-TT, and MRS 20mSRT (P, 0.001). Figure 3. Individual HR reserve (%HR res ) profiles during intermittent runs in 3 subjects. The exercise consisted of running 15 seconds at 110% of the maximal running speed (MRS) reached with the University of Montreal track test (MRS UM-TT, upper plots) and at 95% of the MRS reached with the Intermittent Fitness Test (MRS 30-15IFT, lower plots) intersected with 15 seconds of passive recovery during 10 minutes. Relationships Between MRS 30-15IFT and Physiological Qualities Elicited When Performing Shuttle Intermittent Runs For the 59 subjects, the MRS 30-15IFT was significantly the 370 Journal of Strength and Conditioning Research

7 the Journal of Strength and Conditioning Research Figure 4. Individual heart rate reserve (%HR res ) profiles as a function of running speed for 2 subjects presenting significantly different maximal running speeds (MRSs) determined with the University of Montreal track test (MRS UM- TT of 15 vs. 18 kmh 21, upper) but quite similar MRSs obtained with the Intermittent Fitness Test (30-15 IFT ) (MRS IFT of 19.5 vs. 20 kmh 21, lower). correlated with all physiological variables elicited when performing shuttle intermittent runs and can be summarized by the following regression: MRS 30-15IFT = _Vo 2 max m CMJ HR RE G W A (r = 0.87 and P, for the relationship), where _ Vo 2 max is expressed in mlminkg 21, 10 m in seconds, CMJ in centimeters, HR RE with no units, G stands for gender (1 = male, 2 = female), W for weight (kilograms), and A for age (years). Zero-order linear regression analyses made between MRS 30-15IFT and _ Vo 2 max, explosive power of lower limbs, and HR RE kinetics index were all significant (P, 0.05). These 4 relationships are illustrated in Figure 2. Cardiorespiratory Demands During Intermittent Runs Based on Each Maximal Running Speed Table 2 presents mean values of targeted distances and the corresponding %HR res observed for subjects performing intermittent runs at a similar relative intensity (at a similar percentage of a given MRS). The results show that the mean %HR res reached by the subjects did not depend on the reference MRS used. Nonetheless, when scheduling intermittent runs based on a given percentage of a continuously obtained MRS, 6 and 4 subjects of the groups using, respectively, the MRS UM-TT and MRS 20mSRT as reference speeds could not finish the 10-minute series, and 7 of these subjects presented an HR res,80%. The %HR res range of values showed more disparity with the 2 continuous tests than with the IFT, and the CV of the %HR res was lower for the series based on the IFT than those based on the 2 continuous tests (Table 2). Individual differences from the mean %HR res were significantly lower in the series based on the IFT than in the series based on the UM-TT (DHR res = vs bmin 21, P, 0.05) or on the 20mSRT (DHR res = vs bmin 21, P, 0.05). Figure 3 illustrates data for 3 representative athletes who did not display comparable %HR res during 2 series when distances were planned based on the MRS UM-TT or the MRS 30-15IFT. DISCUSSION The present study shows that the IFT (12,13) leads to an MRS that is significantly associated with physiological qualities elicited when performing shuttle intermittent runs, i.e., explosive muscular power of lower limbs, aerobic power, and cardiovascular recovery capacity. In addition, the results show that using the MRS 30-15FT as a reference speed for determining intermittent run distances enables reaching VOLUME 22 NUMBER 2 MARCH

8 The Intermittent Fitness Test for Interval Training Individualization a requested level of metabolic demand with lower interindividual differences than when using continuously determined running speeds. This possibility to easily provide all members on a team a similar exercise load and especially to individuals with distinct aerobic or anaerobic capacities illustrates the good accuracy of the IFT for individualizing interval training sessions. The significant relationships between MRS 30-15IFT and CMJ, 10-m sprint time, Vo2 _ max, and HR RE exemplifies the merit of the IFT. The relationship between an MRS reached after a graded exercise test and anaerobic capacities has not been investigated before. In the present study, the physiological qualities elicited when performing shuttle intermittent runs accounted for 75% of the variance of the MRS 30-15IFT (r = 0.87). These results for adolescents suggest that MRS 30-15IFT is a unique speed that takes into account several physiological variables simultaneously. The present results also illustrate the accuracy of the IFT when testing intermittent sports players with different physiological profiles. Figure 4 illustrates the fact previously reported (5,10) that physiological capacities other than _Vo 2 max (i.e., explosive power and the ability to recover between efforts) condition shuttle intermittent performance. Here, 2 subjects with different continuous linear MRSs reached with the UM-TT (18 and 15 kmh 21 ) presented quite similar performances on the IFT (20 and 19.5 kmh 21 ). Their individual physiological profiles can explain these running speed differences. Compared to MRS UM-TT, athlete A with a clearly aerobic profile together with poor explosive strength ( Vo _ 2 max: 63.4 mlminkg 21, CMJ: 44 cm, 10 m: 1.94 seconds, HR RE : 9.7) has increased his MRS by 2 kmh 21 with the IFT. Athlete B with an anaerobic profile, despite a lower Vo _ 2 max (54.5 mlminkg 21 ) but with greater explosive power (CMJ: 69 cm, 10 m: 1.81 seconds) and higher cardiovascular recovery capacity (HR RE : 11.6) was able to run 4.5 kmh 21 faster during the IFT. In other words, compared to athlete A, athlete B tolerated the changes of direction better and derived more benefit from the intermittency of the runs. This point should be considered when programming interval training sessions, especially for athletes with different physiological profiles, as in team sports. Finally, the results demonstrate the capacity of the IFT to prescribe proper running intensities for interval training sessions. We have made the assumption that the MRS 30-15IFT helps get players with different physiological profiles to reach similar cardiorespiratory levels and that interindividual differences become less important when using the MRS 30-15IFT compared to continuously determined MRSs for prescribing the intensity of interval training sessions. When scheduling intermittent runs based on a given percentage of the MRS obtained with the two continuous tests, 17.5 and 16% of the subjects could not finish the 10-minute series, whereas many of them presented low HR res values (,80% HR res )(Table2). Our results show that there is significantly more disparity among cardiorespiratory responses (inferred by higher CVand DHR res ) during intermittent runs targeted based on the MRS UM-TT or the MRS 20mSRT than when programmed with the MRS 30-15IFT as the reference speed (Table 2, P, 0.05). The findings illustrate that the metabolic demand can be different from one subject to another, even at a similar percentage of a continuously determined MRSs (similar relative external load). These data are illustrated in Figure 3. When using MRS UM-TT as the reference running speed to calculate individual intermittent run distances, cardiorespiratory demands were clearly different for each athlete (Figure 3, top; %HR res = 95.8, 82.8, and 91.1% for athletes C, D, and E, respectively). However, when using MRS 30-15IFT as the reference speed, %HR res for the 3 athletes were similar (Figure 3, bottom; %HR res = 92.9, 94.2, and 93.1% for athletes C, D, and E, respectively). Note that since MRS 30-15IFT is 20% higher than MRS UM-TT, we used a distinct percentage of each MRS (95% MRS 30-15IFT and 110% MRS UM-TT ). Previous investigations have indicated that physiological demands are quite similar to these percentages (14). When using a continuously determined MRS, players with different cardiovascular responses may be led to produce efforts with disproportionate intensities that may have different physiological impacts and unexpected consequences on their health, fitness, or fatigue. Finally, we postulate that it is because the MRS 30-15IFT is a speed taking into account various qualities solicited during shuttle intermittent runs that it brings athletes with different physiological profiles to a similar level of cardiorespiratory demand. We can thus propose that the IFT is an accurate tool for individualizing aerobic training since it permits programming a similar workout load for each athlete. PRACTICAL APPLICATIONS Coaches and trainers can use the IFT examined in this study because it leads to a particular MRS that takes into account various qualities solicited during shuttle intermittent runs, i.e., explosive power of lower limbs, aerobic qualities, and the ability to recover between exercise bouts. The IFT has been shown to be accurate for individualizing short intermittent run distances in subjects presenting different aerobic or anaerobic profiles. Using the MRS 30-15FT as a reference speed for determining intermittent run distances enables a given level of cardiorespiratory demand with lower interindividual differences to be reached than when using continuously determined MRSs. This result has its importance in team sports since a coach can be certain that a similar training load will be offered to all the athletes. Although further study is still needed to determine at which percentage of the MRS 30-15IFT players have to train to improve one or another physical capacity, our present results already show that the accuracy of the test for a training prescription is high. the 372 Journal of Strength and Conditioning Research

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