Effect of In-Season Generic and Soccer- Specific High-Intensity Interval Training in Young Soccer Players

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1 International Journal of Sports Science & Coaching Volume 9 Number Effect of In-Season Generic and Soccer- Specific High-Intensity Interval Training in Young Soccer Players Zbigniew Jastrzebski 1, Wojciech Barnat 2, Robert Dargiewicz 1, Ewelina Jaskulska 1, Andrzej Szwarc 1 and Łukasz Radzimiński 1 1 Gdansk University of Physical Education and Sport, K. Górskiego 1, Gdansk, Poland zb.jastrzebski@op.pl; e@kiero.net; ewelina.jask@gmail.com; szwarc@awf.gda.pl; lukaszradziminski@wp.pl; 2 Malbork Football Academy, Plac Narutowicza 14, , Malbork, Poland wojtunia_barnat1@wp.pl ABSTRACT The main purpose of this study was to determine the effect of different interval training regimes on the physical capacity and soccer-specific skills of young soccer players. Twenty-two subjects were divided into two groups: small-sided games group (SSGG; age: 15.8 ± 0.63 y, body mass: 61.6 ± 8.97 kg, height: ± 6.23 cm) or interval-running group (IRG; age: 15.8 ± 0.55 y, body mass: 62.7 ± 8.69 kg, height: ± 6.48 cm). The groups completed two different 8-week training programs. The intervention consisted of two training sessions per week (IRG 7 x 3 min running where each bout consisted of 15 s of high-intensity running and 15 s of jogging with an active recovery period of 3 min; SSGG 3 vs. 3 games for 7 x 3 min, with an active recovery period of 3 min). Significant changes over time were found in V O 2 max (p = 0.009, h 2 = 0.29) and peak power (p = 0.014, h 2 = 0.26). Post-hoc analysis revealed a significant increase in the maximal oxygen uptake only in the SSG group (p = 0.019). No significant improvement in running speed was found. The mean HR at AT increased significantly in both groups. A relevant correlation (group x time) was found for oxygen uptake at the AT intensity (p = 0.015, h 2 = 0.26). A significant time effect was also present with respect to this variable (p = 0.000, h 2 = 0.61). Post-hoc analysis of the V O 2 /AT index revealed no relevant (p = 0.19) improvements in the IRG (from 44.1 ± 5.72 to 46.0 ± 5.03 ml kg -1 min-1 ), but there was a significant (p = 0.000) increase in the SSG (from 40.9 ± 3.51 to 46.0 ± 3.88 ml kg -1 min-1 ). The improvement in soccer-specific skills was not significant (3.2 % in IRG and 5.3 % in SSGG). The results of this study show that small-sided games and running can both be efficient training strategies for developing aerobic capacity and soccer-specific skills. Key words: Aerobic Capacity, Association Football, Interval Training, Skill, Small-Sided Games, Youth Soccer Reviewers: Matt Barr (Edith Cowan University, Australia) Michael Chia (Nanyang Technological University, Singapore)

2 1170 Soccer-Specific High-Intensity Interval Training in Young Soccer Players INTRODUCTION Extensive research data are available concerning the effectiveness of small-sided games and their influence on the development of physical capacity [1, 3]. This form of training can be used as a substitute for traditional running fitness drills because of the similarity of its effect on aerobic capacity levels [1, 2]. Moreover, as a soccer-specific training method, small-sided games affect players in a multifactorial manner by simultaneously developing typical soccer skills. Development of skills and tactics is as important as raising physical capacity levels. Therefore, this form of practice should be applied to young soccer players often. To develop soccer-specific skills, which involve high-precision performance, games with a larger number of players and larger surface are applied. According to Rampinini et al. [4], the intensity of small-sided games depends on the pitch area, number of players, coach encouragement and rule modifications. Dellal et al. [5] investigated the influence of limited numbers of ball contacts on performance intensity and the distance covered during a smallsided game, and they reported that the longest distances for sprints or high-intensity runs were covered during one touch games. Furthermore, the highest-intensity effort, referred to as %HR max, occurred during these games. However, the highest blood lactate concentration was observed during a free game, which involved the largest number of 1x1 actions, contact situations, and resistance situations. Several studies [4, 6, 7] confirmed that the highest intensity can be achieved during a small-sided game with a 3 vs. 3 form, when the pitch area per player ranges from 60 to 125 m 2. The important aspect of playing small-sided games is that this practice develops power and anaerobic endurance in soccer players in addition to aerobic endurance. These benefits are attributed to the repetition of runs that are alternated with short to long periods of recovery, which could be active or passive, in soccer [8]. Therefore, it is advisable to assess the anaerobic capacity of soccer players after a few weeks of a small-sided game regime. The correct assessment of training effects is an important element of the training process in soccer and involves both aerobic and anaerobic performance. Although numerous studies have addressed aerobic capacity, anaerobic power still requires further investigation. Popular means of assessing anaerobic performance include Wingate tests and running speed tests. During a 30-second pedaling sprint on a cycle ergometer (Wingate test), 38% of the total energy cost is covered by the aerobic system, 45% by anaerobic glycolysis, and 17% by the breakdown of phosphocreatine (PCr) + ATP. During a 10-second all-out effort, these proportions are 31, 47, and 22%, respectively [9]. Gaitanos et al. [10] claim that during 6- second work, 44% of the total energy is generated through the anaerobic glycolysis pathway, 50% through PCr, and 6% from ATP breakdown. These numbers apply to a single maximal exercise effort. However, a soccer match requires repetitive exertions. Few detailed studies have determined the engagement of each system in energy generation during repetitive efforts [11]. Bogdanis et al. [12] administered tests with two 30-second pedaling periods at maximal speed that were alternated with a 4-minute recovery. During the second performance, the proportion of the anaerobic energy system involved in the total energy production was 45% less than that during the first trial. Additionally, higher oxygen uptake was noted during the second exercise, which demonstrated increased involvement of the aerobic system in total energy generation (up to 49% during the second trial). This result indicates that during high-intensity exercise, the replenishment of ATP is driven by the oxidation of substrates and anaerobic pathways in proportions similar to those utilized during a match. Therefore, the aerobic and anaerobic capacities of the players are important factors to monitor during training, especially when small-sided games or runs of varied intensities are the dominant forms of training.

3 International Journal of Sports Science & Coaching Volume 9 Number Another criterion for assessing the effectiveness of a training program is skills progress. Special attention should be applied to planning the training of young players because at this age, significant improvement can be achieved within a relatively short time. Few studies have analyzed the influence of small-sided games or interval-running training on sportspecific skills in soccer players [6, 13, 14, 15]. Most published training programs (SSG training regimes) last from 6 to 12 weeks [3], and it is unclear whether such a short practice time is sufficient to enable significant progress in soccer-specific skills. In many team sports, small-sided games or interval running are the basic forms of fitness training. Therefore, the main aim of this study was to determine the influence of training regimes on physical capacity and soccer-specific skills. It was hypothesized that interval training in the form of either runs of various intensities or small-sided games would significantly improve the maximal oxygen uptake (V O 2max ) and anaerobic threshold (AT) that determine the aerobic capacity of young soccer players, but would have no influence on running speed. Moreover, we assumed that only training with small-sided games would develop sport-specific skills and anaerobic capacity in young soccer players during an 8- week regime. METHOD SUBJECTS A total of 24 under-16 soccer players participated in the experiment. Two players were excluded from further study because of health problems and absence during the training. Consequently, 22 subjects were analyzed (Table 1). All players belonged to the same team and had 6 to 7 years of sports experience. Before the experiment, participants were deliberately divided into two groups; i.e., the small-sided games group (SSGG) or the interval-running group (IRG), such that the maturation status, physical capacity and soccerspecific skills of the sets were homogeneous. Before the experiment, all subjects and their parents or guardians were fully informed and signed a consent form. The protocol was fully approved by the ethical committee of the local medical association. Table 1. Characteristics of the subjects (mean ± SD) IRG (n=11) SSGG (n=11) Age [y] 15.8 ± ± 0.63 Mass [kg] 62.7 ± ± 8.97 Height [cm] ± ± 6.23 STUDY DESIGN The experiment was conducted during the competitive season (September - October). The experimental training program was integrated into the standard regime of the team. Both groups underwent high-intensity interval training twice a week (Tuesday and Thursday). On the other days, all participants were subjected to the same practice, which they performed together. After the warm-up for both groups, which lasted for 15 min followed by 5 min of passive stretching, the SSGG performed 3 vs. 3 small-sided games without goalkeepers on an 18 x 30 m field. The pitch area per player was 90 m 2. The games consisted of seven 3 min bouts with 90 s active recovery between each bout. The character of the games was free with no limits on the touches per possession. Moreover, the coaches motivated the players to engage maximally during the game. Simultaneously, the IRG performed seven 3-min runs with 90 s active recovery. Each course consisted of 15 s of high-intensity running and 15 s

4 1172 Soccer-Specific High-Intensity Interval Training in Young Soccer Players of jogging. The average intensity of each run ranged between 85 and 90% of the HR max. The subjects controlled their own heart rates using heart rate monitors (Polar Electro, Kempele, Finland). The data were recorded in 5-s intervals. Table 2. Overview of the weekly training load during the 8-week training intervention Monday Tuesday Wednesday Thursday Friday Saturday Sunday Regeneration/compensation training session Small-sided games/interval running Technical, tactical and coordination drills Small-sided games/interval running Technical, tactical and speed drills League match Free day PROCEDURE Because of the number and intensity of the tests, measurements were obtained on 5 consecutive days in the morning. The running speed test was conducted on Monday, the Wingate test on Tuesday, the aerobic endurance test (a continuous graded exertion until exhaustion) on Wednesday and Thursday and the soccer-specific skills test on Friday. During each of these days in the afternoon, players participated in 60-min low-intensity skills training. Running speed tests and DFB (Deutsche Fussball Bund) soccer-specific skills tests [16] were performed on a football pitch with a synthetic surface. Weather conditions were similar during all the tests (wind speed < 2 m s -1, air temperature of C, air humidity of 50-55% and air pressure of 1,012-1,015 hpa). Both the Wingate test and the continuous graded exertion until refusal were conducted in the laboratory (temperature of C, air pressure of 1,021 hpa, and air humidity of 30-32%). Sprint test. Before the test, the players performed a 20-min warm-up involving three 5 m sprints and two 30 m sprints. The sprint times were recorded by double photocells (TAG Heuer, la Chaux-de-Fonds, Switzerland) positioned at the starting (0 m) and finishing lines (5 m and 30 m) at a height of 0.9 m and 1.1 m.. The subjects performed two maximal attempts for the 5 and 30 m distances. Only the best (the shortest) times were used in the subsequent analysis. Each participant started from a standing position, with his front leg on the starting line (0 m). The resting periods were 90 s after the 5-m sprint and 240 s after the 30-m sprint. Peak power. A 30-s Wingate test on a cycle ergometer (Monark Ergomedic 894 E, Monark, Sweden) was used to assess the peak power. A relative load corresponding to 7.5% of the subject s body mass was applied. Before performing the test, the participants completed a 10-min warm-up, including pedaling at a frequency of 60 rotations per minute (RPM), with a relative load of 1.2 W kg -1 and three rapid accelerations between the 7th and 10th minute. After the warm-up, the subjects performed five minutes of stretching and relaxing exercises and then started the test. Aerobic capacity (V O 2 max). All of the subjects performed a continuous graded exercise test on an electronically braked cycle ergometer (Oxycon Pro, Erich JAEGER GmbH, Hoechberg, Germany) to determine their maximal oxygen uptake. The test began with 5 min of continuous pedaling, with a frequency of 60 RPM and a relative load of 1.5 W kg -1. After this phase, the workload was systematically increased by 15 W each minute until exhaustion.

5 International Journal of Sports Science & Coaching Volume 9 Number The effort was ended when the pedaling frequency declined by 10%; that is, when the pedaling frequency fell below 54 RPM. The highest value of the oxygen uptake maintained for 15 s was considered as V O 2 max. The anaerobic threshold values were obtained using the V-slope method [17]. The highest HR value recorded during the test was considered as HR max. Soccer-specific technical skills. The level of soccer-specific technical skills was evaluated using a battery of tests proposed by the German Soccer Federation (e.g., the DFB). The tests included seven trials evaluating the following technical skills: juggling, dribbling, heading, passing the ball into specified areas, shooting into specified sectors of the goal from the penalty kick point, rotation pass from the corner of penalty area and passing the ball against the bench. For each attempt, the subjects could score from 0 to 60 points (420 points in total). The total number of points scored determined the level of the assessed technical skills. STATISTICAL ANALYSES The results are presented as the mean and standard deviation (M ± SD). All of the statistical analyses were conducted using StatSoft, Inc. STATISTICA software version 9.0. All of the data sets were assessed using the Shapiro-Wilk test for normal distributions. A two-way repeated measures ANOVA was applied to analyze the between-group (SSGG and IRG) and within-group (pre- and post-training) effects. Moreover, the effect size (eta squared, η 2 ) for analysis of variance was calculated. When interaction effects were significant, post-hoc analyses using the HSD Tukey test were performed. The Cronbach s alpha reliability coefficient (α) was used to determine the reliability of the DFB test. The level of significance was set at p < RESULTS Before the experiment, the measured indices did not differ between the two groups. Two-way repeated measures evaluation of the influence of regime time and applied training load on the measured indices showed significant changes in V O 2 max and peak power (Table 3). Significant changes over time were found in V O 2 max (p = 0.009, η 2 = 0.29) and peak power (p = 0.014, η 2 = 0.26). Post-hoc analysis revealed a significant increase in the maximal oxygen uptake only in the SSG group (p = 0.019). The peak power improved in both groups by 0.3 kg -1. However, the changes did not reach significance after the 8-week interval training. There was also no significant improvement in the measured running speed. Additionally, the main significant effect over time was for the heart rate at the anaerobic threshold (p = 0.000, η 2 = 0.56). An analysis of the indices determining the AT level showed that the mean HR at AT increased after eight weeks of the experiment from ± to ± 9.69 b min -1 (p = 0.038) in the IRG and from ± to ± 8.96 b min -1 (p = 0.002) in the SSGG. A relevant correlation (group x time) was found for oxygen uptake at the AT intensity (p = 0.015, η 2 = 0.26). A significant effect over time also occurred with respect to this variable (p = 0.000, η 2 = 0.61). Post-hoc analysis of the V O 2 /AT index revealed not statistically significant (p = 0.19) improvements in the IRG (from 44.1 ± 5.72 to 46.0 ± 5.03 ml kg -1 min -1 ) but a significant (p = 0.000) increase in the SSG (from 40.9 ± 3.51 to 46.0 ± 3.88 ml kg -1 min -1 ). Moreover, a relevant correlation over time was found for V O 2 /AT, expressed as V O 2 max (p = 0.015, η 2 = 0.26), and HR/AT, expressed as % HRmax (p = 0.000, η 2 = 0.59). Although the HSD Tukey statistics demonstrated a relevant increment in the % HR/AT in both the IRG (p = 0.017) and the SSGG (p = 0.002), the soccer-specific skills did not improve significantly (3.2 % and 5.3 %, respectively). The Cronbach s alpha reliability coefficient (α) for DFB test was 0.76.

6 1174 Soccer-Specific High-Intensity Interval Training in Young Soccer Players Figure 1. Changes in anaerobic threshold values measured as V O 2 (a), HR (c), and percentage of V O 2 max (b) and HRmax (d). * p < 0.05, ** p < 0.01, *** p < Figure 2. Mean values of %HRmax for the SSGG players during each smallsided game and for the IRG players during each running interval

7 International Journal of Sports Science & Coaching Volume 9 Number Table 3. Physical capacity and soccer-specific skills indices in young soccer players before and after high-intensity interval training IR Group SSG Group Pre Post Pre Post Differences ES V O 2 max [ml kg -1 min-1 ] 55.7 ± ± ± ± 5.44* time 0.29 Peak Power [W kg -1 ] 11.0 ± ± ± ± 0.72 time 0.26 Sprint test [s] 5 m 1.16 ± ± ± ± m 4.66 ± ± ± ± DFB test [points] ± ± ± ± *Significant pre to post changes at p < 0.05 DISCUSSION The aim of this study was to compare the effects of two training regimes on physical capacity and soccer-specific skills. It was assumed that high-intensity interval training, realized in two forms (running and small-sided games), would significantly increase aerobic physical capacity but would not improve running speed. Moreover, we hypothesized that only the small-sided games form would improve soccer skills in the players. The results of our study support these hypotheses. However, the effect of high-intensity interval training on the examined physical capacity and skills is not clear. Two of the most important factors influencing the effectiveness of the applied regime during a given time are the volume and type of load. However, these factors are difficult to measure in soccer because of the limited individualization during training. Thus, individual heart rate monitors are especially useful in this case, although some physiological indices had to be determined beforehand for optimization of performance. Figure 2 shows the mean values of the percentage HR max in young soccer players obtained during the experiment. Despite the differences between the two groups (up to 1.5%), the registered values corresponded with the assumption of 85-90% HR max intensity during interval training. In our opinion, the higher values may have resulted from natural game rivalry. However, the values are consistent with the results presented by other studies [15, 25]. Dellal et al. [22] reported similar mean values of %HR max during 15 sets of 15-s runs with passive rest and 4 vs. 4 small-sided games with goalkeepers (77 % HR reserve). Balsom [23] revealed that continuous exercise without a ball, intermittent exercise (30 30-s intermittent exercise at 100% V O 2 max), and a 3 vs. 3 small-sided game had similar effects on the circulatory system. Moreover, Dupont and Berthoin [24] considered s intermittent exercise at 120% V O 2 max to be the most effective form of interval for developing V O 2 max. The training load applied to the players during the s intermittent running in our studies turned out to be insufficient for significantly improving the maximal oxygen uptake. Dellal et.al [22] confirmed that appropriate and precise load application play a crucial role in aerobic capacity development. Several studies have examined this phenomenon. However, there was no consensus regarding the effectiveness of different forms of small-sided games or interval-running training on physical capacity. Some coaches prefer small-sided games, but others apply both forms in balanced proportions. In our opinion, this split results from coaches experience and intuition rather than scientific reasons. Dellal et al. [5] examined changes of intensity during a 4-sided game by rule modification

8 1176 Soccer-Specific High-Intensity Interval Training in Young Soccer Players (4 x 4 min with 3 min of active recovery). One-contact, 2-contact and free games were analyzed. The results showed that regardless of the rules, the intensity (%HRmax, ratings of perceived exertion, and blood lactate levels) increased in each consecutive game. This increase was probably caused by increasing fatigue, even though the distance covered by the players during subsequent games diminished. However, during matches, high-intensity performance is extended relative to that in other training forms because of the psychological factors (e.g., motivation and emotions) embedded in rivalry. Previous research confirmed the effectiveness of interval training on aerobic fitness in soccer players. Impellizzeri et al. [2] administered small-sided games (4 x 4 min, 3 min of active recovery) to players twice a week and noted a V O 2 max improvement of up to 7.1% after a 12-week training cycle. In our studies, the maximal oxygen uptake increase was similar (8.6% after 8 weeks of training). Impellizzeri et al. [2] included both the preparatory season (4 weeks) and the competitive season (8 weeks), but the V O 2 max changes during the competitive period (8 weeks) did not reach significance (<1%). These results suggest that small-sided games positively affect the players starting in the first weeks of their application. Similar research on specific training (twice a week for 4 x 4 min, 3 min of active recovery) was conducted by Chamari et al. [18]. However, there was a difference in the applied regime. The players dribbled the ball along the route designed by Hoff et al. [19] on Tuesdays and played small-sided games on Wednesdays. After 8 weeks of such practice during the competitive period, the players obtained very high values of maximal oxygen uptake (higher than 70 ml kg -1 min -1, which represents a 7.5% increase). In addition, an improvement of 9.6% in soccer skills was registered using the Hoff test, which was greater than that observed in both our groups. Such development could be influenced by the fact that the skill level was assessed using the test that the players performed every week for 16 min. The same test was used by McMillan et al. [20]. After a 10-week regime (6 weeks of preparatory season and 4 weeks of competitive season), the V O 2 max increased by 9.4%, but the time measured during a 10 m run did not change. These results are consistent with the SSGG results in our studies. Hill-Hass et al. [1] conducted studies regarding the effectiveness of generic training and soccer-specific training. During a 7-week experiment, one group underwent different forms of small-sided games, whereas the other group performed different forms of running (including high-intensity running, repeated sprint, sprint training, change-of-direction speed drills, speed ladder drills, and aerobic power practice). Similarly to the results in our studies, the maximal oxygen uptake and time measured during the 5 m and 20 m runs did not change significantly. However, a relevant improvement in the YYIRTL1 test occurred in the group subjected to generic training. Our results are opposite to those of Ferrari Bravo et al. [21], who examined the influence of high-intensity interval training on V O 2 max changes in soccer players. These studies revealed a considerable increase in the V O 2 max index after an 8-week regime (4 x 4 min, 3 min of active recovery) during the competitive season. According to Helgerud [26], the anaerobic threshold is more sensitive than the maximal oxygen uptake to changes in training load. In our study, indices of the anaerobic threshold (HR/AT and V O 2 /AT) increased significantly in both groups. Therefore, the relevant increment in oxygen uptake at AT and heart rate at AT with a simultaneously steady level of maximal oxygen uptake in the IR group could be explained by the higher sensitivity of the AT index to the applied load. Moreover, this finding could suggest a positive effect of the administered training program [26]. Although a V O 2 /AT improvement of 12% was registered in the SSG group, which is consistent with the results of Impellizzeri et al. [2], in the IR group, only a 4% increase occurred, which is less than that observed by Helgerud et al. [27] and Impellizzeri et al. [2]. Moreover, the AT, expressed as % V O 2 max, increased by nearly

9 International Journal of Sports Science & Coaching Volume 9 Number % (from 78.1 to 80.9 % V O 2 max) in the SSG group. However, Chamari et al. [18] reported a V O 2 max improvement of only 0.4% as a result of applying small-sided games. Such poor effect of a training regime in 14-year-old players could have been caused by their high level in this index (approximately 88% V O 2 max) prior to the experiment. Conversely, in the IR group, a minor increase in the % V O 2 max (from 79% to 80.9%) was registered. Moreover, improvements in the heart rate at AT (HR/AT) and performance intensity at AT (%HRmax) were observed in both groups (Figure 1). However, when all indices identifying AT were considered, higher improvement was found in the SSG group. From the results of the present study and those of other training programs, small-sided games are more effective than interval running for developing aerobic capacity in soccer players. No statistically significant differences in running speed were found in the tested players. Although the 30 m running time improved in the IRG (p = 0.374) and deteriorated in the SSGG (p = 0.128), both changes were irrelevant. This finding suggests a lack of a significant influence of high-intensity interval training, either generic or soccer-specific, on running speed in young players. Moreover, the IR group which did longer runs, made no improvements in acceleration (5 m sprint), while the 5 m sprint results in SSG group were not significantly better (p = 0.123). It might be the effect of applied training form. The smallsided games training involves frequent accelerations, while the longer sprints occur rarely. This result is an important tip for coaches, who plan the training load during a competitive season and should be aware of a decrease in some performance area if training methods are focused on these training forms. Given these effects, the reason for the peak power increase (2.7% in the IRG and 2.8% in the SSGG) measured by the Wingate test is unclear. In our opinion, soccer training, especially small-sided games, involves a significant amount of eccentric muscle work and resistance exercise when directly contacting an opponent or during movement on soft ground. The routine training to which the players were subjected in addition to the experimental interval regime may have resulted in a significant improvement in the W/kg index in the subjects. No statistically significant differences in soccer skills were found in either of the examined groups, possibly because the experiment duration was too short or because the experiment was conducted during a competitive season, when the players should be at their highest skill level. Moreover, the experiment followed a 5-week preparatory period during which the participants developed their technical skills. However, in accordance with our assumption, higher skills development was observed in the SSG group than in the IR group (5.3% and 3.2%, respectively). Therefore, interval training in the form of small-sided games is advisable during the competitive season to develop soccer skills in players. CONCLUSION The performance intensity of young soccer players during interval training was higher in small-sided games than in the running form, probably because of game rivalry, and resulted in a significant maximal oxygen uptake increase in the SSGG after the 8-week regime. Both groups showed relevant increases in V O 2 and HR at the AT. The significant differences in all of the indices identifying aerobic capacity in the tested soccer players support our hypothesis. Moreover, our assumption that this type of training has limited effects on running speed in young soccer players was confirmed. However, contrary to our assumption, no significant soccer skill development was observed in any of the tested groups. Small-sided games in the form of 7 x 3 min proved as effective as other training programs available in the literature and could be successfully applied by the coaches.

10 1178 Soccer-Specific High-Intensity Interval Training in Young Soccer Players REFERENCES 1. Hill-Haas, S.V., Coutts, A.J., Rowsell, G.J. and Dawson, BT., Generic Versus Small-Sided Games Training in Soccer, International Journal of Sports Medicine, 2009, 30(9), Impellizzeri, F., Marcora, S., Castagna., C, Reilly, T., Sassi, A., Iaia, F.M. and Rampinini, E., Physiological and Performance Effects of Generic versus Specific Aerobic Training in Soccer Players, International Journal of Sports Medicine, 2006, 27(6), Iaia, F.M., Rampinini, E. and Bangsbo, J., High-Intensity Training in Football, International Journal of Sports Physiology & Performance, 2009, 4, Rampinini, E., Impellizzeri, F.M., Castagna, C., Abt, G., Chamari, K., Sassi, A. and Marcora, S.M., Factors Influencing Physiological Responses to Small-Sided Soccer Games, Journal of Sports Science, 2007, 25(6), Dellal, A., Lago-Penas, C., Wong, D.P. and Chamari, K., Effect of The Number of Ball Contacts Within Bouts of 4 vs. 4 Small-Sided Soccer Games, International Journal of Sports Physiology & Performance, 2011, 6(3), Koklu, Y., A Comparison of Physiological Responses to Various Intermittent and Continuous Small-Sided Games in Young Soccer Players, Journal of Human Kinetics, 2012, 31, Little, T. and Williams, A.G., Measures of Exercise Intensity During Soccer Training Drills with Professional Soccer Players, Journal of Strength and Conditioning Research, 2007, 21(2), Dupont, G., Akakpo, K. and Berthoin, S., The Effect of In-Season, High-Intensity Interval Training in Soccer Players, Journal of Strength and Conditioning Research, 2004, 18(3), Medbo, J.I., Gramvik, P. and Jebens, E., Aerobic and Anaerobic Energy Release During 10 and 30 s Bicycle Sprints, Acta Kinesiol University Tartuensis, 1999, 4, Gaitanos, G.C., Williams, C., Boobis, L.H. and Brooks, S., Human Muscle Metabolism During Intermittent Maximal Exercise, Journal of Applied Physiology, 1993, 75(2), Spencer, M., Bishop, D., Dawson, B. and Goodman, C., Physiological and Metabolic Responses of Repeated-Sprint Activities, Sports Medicine, 2005, 35(12), Bogdanis, G.C., Nevill, M.E., Boobis, L.H. and Lakomy, H.K.A., Contribution of Phosphocreatine and Aerobic Metabolism to Energy Supply During Repeated Sprint Exercise, Journal of Applied Physiology, 1996, 80(3), Owen, A.L., Wong, D.P., McKenna, M. and Dellal, A., Heart Rate Responses and Technical Comparison Between Small vs. Large Sided Games in Elite Professional Soccer, Journal of Strength and Conditioning Research, 2011, 25(8), Platt, D., Maxwell, A., Horn, R., Williams, M. and Reilly, T., Physiological and Technical Analysis of 3 v 3 and 5 v 5 Youth Football Matches, Insight FACA J, 2001, 4(4), Katis, A. and Kellis, E., Effect of Small-Sided Games on Physical Conditioning and Performance in Young Soccer Players, Journal of Sports Science & Medicine, 2009, 8, Jastrzebski, Z., Barnat, W., Konieczna, A., Rompa, P. and Radziminski, Ł., Changes of Physical Capacity and Soccer-Related Skills in Young Soccer Players within One-Year Training Period, Baltic Journal of Health and Physical Activity, 2011, 3(4), Beaver, W.L., Wassermann, K., and Whipp, B.J., A New Method for Detecting Anaerobic Threshold by Gas Exchange, Journal of Applied Physiology, 1986, 60, Chamari, K., Hachana, Y., Kaouech, F., Jeddi, R., Moussa-Chamari, I. and Wisloff, U., Endurance Training and Testing with The Ball in Young Elite Soccer Players, British Journal of Sports Medicine, 2005, 39(1), Hoff, J., Wisloff, U., Engen, L.C., Kemi, O.J. and Helgerud, J., Soccer Specific Aerobic Endurance Training, British Journal of Sports Medicine, 2002, 36, McMillan, K., Helgerud, J., Macdonald, R. and Hoff, J., Physiological Adaptations to Soccer Specific Endurance Training in Professional Youth Soccer Players. British Journal of Sports Medicine, 2005, 39(5),

11 International Journal of Sports Science & Coaching Volume 9 Number Ferrari Bravo, D., Impellizzeri, F.M., Rampinini, E., Castagna, C., Bishop, D. and Wisloff, U., Sprint vs. Interval Training in Football, International Journal of Sports Medicine, 2008, 29(8), Dellal, A., Chamari, K., Pintus, A., Girard, O., Cotte, T. and Keller, D., Heart Rate Responses During Small- Sided Games and Short Intermittent Running Training in Elite Soccer Players: A Comparative Study, Journal of Strength and Conditioning Research, 2008, 22(5), Balsom, P.D., Precision Football, Polar Electro Oy, Kempele, Finland, Dupont, G. and Berthoin, S., Time Spent at a High Percentage of V O 2 max for Short Intermittent Runs: Active Recovery versus Passive Recovery, Canadian Journal of Applied Physiology, 2004, 29, Fanchini, M., Azzalin, A., Castagna, C., Schena, F., McCall, A. and Impellizzeri, F.M., Effect of Bout Duration on Exercise Intensity and Technical Performance of Small-Sided Games in Soccer, Journal of Strength and Conditioning Research, 2011, 25(2), Helgerud, J., Maximal Oxygen Uptake, Anaerobic Threshold and Running Performance in Woman and Men with Similar Performances Levels in Marathons, European Journal of Applied Physiology, 1994, 68, Helgerud, J., Engen, L.C., Wisloff, U. and Hoff, J., Aerobic Endurance Training Improves Soccer Performance, Medicine and Science in Sports and Exercise, 2001, 33(11),

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