The examination of Yo-Yo intermittent recovery test performance of young soccer players at different playing positions

Purpose: The aerobic fitness in soccer is an important performance component. The aerobic endurance level of players may positively effect performance of critical match activities. The examination of Yo-Yo intermittent recovery test (YYIRT) performance of young soccer players at different playing positions was aimed in this study. Material: The research group was consisted of 75 young soccer players playing at U-21 category of professional soccer teams at different leagues. The soccer players at 5 different playing positions participated in study voluntarily (centerback: n=15, fullback: midfielder: n=15, winger: n=15, forward: n=15). The YYIRT (level 1) was performed for determination aerobic endurance parameters (covered distance during test, maximum oxygen uptake (VO2max) and maximal aerobic speed (MAS) of players. The one-way ANOVA analysis was used for comparison between playing positions of YYIRT parameters. Results: It was determined that difference between YYIRT parameters of players at different playing positions was non-significant (p>0.05). However, the midfielders covered more distance (2425,33±440,42 m) than forward (2146,67±339,64 m), winger (2137,33±615,18 m), fullback (2056,00±512,43 m) and centerback (1960,00±313,51 m) players. The positional requirements of midfielders may be decisive on covered distance


Introduction
The endurance is one of the most important motoric characteristics at soccer game. The duration of soccer match is 90 minutes and the soccer players perform repeatedly various activities such as sprint, jump, standing, running with turning, intermittent running. The high level endurance assists to players during repeated performance of these activities at soccer match. The requirements of game are decisive on technical and physical activities performed at match. It was indicated that length and intensity of high and low intensity activities during soccer game vary according to various factors [1].The performance of intermittent and repeated activities during soccer game depends on aerobic endurance and recovery capacities of players. The repeated sprints and high intensity running are frequently performed at soccer match. Therefore, aerobic endurance level and recovery capacity of players may affect performance of intermittent and repeated activity during soccer match. It was indicated that the total 10-13 km distance was covered by elite soccer players at soccer match and the most of this distance was consisted of low intensity activities [2]. However, it was found that the 2-4 seconds sprint running was performed at every 90-180 seconds ORIGINAL ARTICLE of soccer match [3]. There is a dominance of aerobic endurance at soccer match. Also, the intermittent and repeated sprint running are performed throughout soccer match and these anaerobic activities may be decisive on match performance than aerobic activities.
The intermittent and repeated high intensity activities of soccer match is indicator of recovery capacities of players. However, the high level aerobic endurance is basis of recovery capacity at intermittent and repeated activities. The intermittent activity and recovery capacity performance of players is measured by various fitness tests. The Yo-Yo intermittent recovery test (YYIRT) is used for determination of aerobic endurance and recovery capacity. The YYIRT is used commonly at soccer [4]. The YYIRT is a practical, simple, inexpensive and useful test because of testing of a lot of players at single session [5]. The YYIRT includes running with turning and recovery intervals between running. It is known that the running with turning, sprint and short distance activities are available at soccer game. The soccer is a sport branch including intermittent activities and the high intensity activities are splitted with active recovery periods [6]. Therefore, YYIRT may be indicated as soccer specific test protocol including intermittent running [7]. Krustrup et al. [8] found that the distance covered with high intensity running during soccer game was correlated with YYIRT performance. Also, it was found that the post-test covered distance values of elite soccer referees increased after intermittent training period [9]. The aerobic endurance level may be followed with YYIRT test performance. The aerobic fitness level of soccer players may be measured before and after training intervention. The increase of aerobic performance may be determined after training period. There are many factors affecting aerobic endurance capacity of young soccer players such as playing position. The physical and physiological requirements of playing position at soccer may differentiate. It is expected that the soccer players increase match performance according to this discrepancy between playing positions. The performance development is important for professional soccer careers of young players. The performance of young soccer players should periodically be tracked and the essential exercises for performance development should be performed in the light of tests.
Although many studies are available about YYIRT performance, there are few studies on young soccer players in the literature. The determination of playing position effect on YYIRT performance of young soccer players may provide useful information for scientific literature. Therefore, the examination of YYIRT performance of young soccer players at different playing positions was aimed in this study.

Participants
The research group was consisted of 75 young soccer players playing at U-21 category of professional soccer teams at different leagues. The soccer players at 5 different playing positions participated in study voluntarily (n=15 -centerbacks, n= 15 -fullbacks, n=15 -midfielders, n=15 -wingers, n=15 -forwards). The descriptive data of age, body height, weight values of young soccer players was indicated in Table 1. The study was performed in line with principles of Helsinki Declaration.

Research Design Data Collection Body Height and Weight Measurements
The body height was measured by a stadiometer (Holtain, Holtain Ltd., Dyfed, UK) with 0.1 cm precision. The body height was measured as length between top point of head and feet at position of standing with bare feet. The body weight was measured by electronic body weight measurement device (Tanita 401A, Tokyo, Japan) with 0.1 kg precision at bare feet position with sports clothes.

Yo-Yo Intermittent Recovery Level 1 Test
The aerobic fitness level of soccer players was determined by Yo-Yo intermittent recovery level 1 (YYIRT1) test. The test was explained to young soccer players. The warm-up exercises were performed before test. After stretching exercises, the test was performed.
The tests of all players were performed at same hour of day to prevent circadian rhythm effects. The YYIRT1 was performed on a 25 m test track. The test was consisted of 2x20 m running with turning and 2x5 m jogging running with 10 seconds for active recovery [8] (Figure 1). After every 40 m (2x20 m) running bout, the 2x5 m jogging running with 10 seconds was performed for active recovery. The starting running speed of test protocol was 10 km/h. The 4x40m running with turning at 10-13 km/h speed at 0-160 m interval (a 40 m (2x20 m) at 10, 11, 12, 13 km/h speed) and 7x40m running with turning at 13.5-14 km/h speed at 160-440 m interval (3x40 m at 13.5 km/h speed and 4x40 m at 14 km/h speed) were performed. Then, the increase of running speed was0.5 km/h at every 320 m (at every bout of 8x40 m running with turning). The running speed was announced with audio signals emitted by a transportable CD player (Philips Az1030, Eindhoven, Holland). The players arrived 20 m cones of test track at every audio signal. The test was ended when the audio signal was not caught by players at 20 m cones or players left test willingly (exhaustion). The covered distance was recorded as test score. After test, the soccer players performed warm down and stretching exercises for active recovery. The indirect maximal oxygen uptake value was determined in ml/kg/min unit by formula of Bangsbo et al. [10]: VO 2max (ml/kg/min)=YYIRT1 distance (m)×0.0084+36. 4 The maximal aerobic speed parameter of players was calculated with covered distance value of test according to regression model of Heaney et al. [11]: Maximal aerobic speed (m/sec) = 0.456250 x (covered distance at YYIRT1 [km]) + 3.617444 Statistical Analysis The data of study was presented as descriptive statistics (X±SD, Min-Max). The Shapiro Wilk test was used for determination the normality of data distribution. The test parameters were compared between playing positions by one way analysis of variance (one way ANOVA). The Levene's Test was used for determination the homogeneity of variance. The pairwise comparisons of significant difference determined between playing position groups were determined by Games-Howell test. The significance level of statistical analysis was

Results
In Table1, it was seen that the midfielders had the highest Max.VO 2 , distance and MAS values than other playing positions. However, it was determined that the test values of the centerback players was the lowest of all playing positions.
According to analysis results, no significant difference was determined between young soccer players at different playing positions in terms of the VO 2max , covered distance and MAS values. The analysis results indicated that the midfielders had the highest test values than soccer players at other playing position, but this difference was not statistically significant.

Discussion
It was determined that difference between YYIRT1 performance of soccer players at different playing positions was statistically non-significant. The young soccer players playing at under 21 age categories joined to this study. These soccer players were candidates to professionalism in their teams. The YYIRT1 performance of midfielders was higher than other soccer players at other playing positions but this difference was not statistically significant. This difference may cause from workload of midfielder players. Cihan et al. [12] reported that professional midfielders had more covered distance and higher VO 2max values at YYIRT1 test than defence, forward players and goalkeepers. Although there was no significant difference between test values of young soccer players at different playing positions, there was a similarity between results of two study in terms of better test values of midfielders. The professional soccer players may have higher test values and the results of mentioned study may arise from this factor. Similarly, Mohr et al. [13] found that midfielders and fullback players covered more distance than forward and defence players duringYYIRT1. The midfielders have important role on game tactic and structure of team. Therefore, the performance of midfielders is decisive of performance at soccer match. It was determined that covered distance and VO 2max values at Yo-Yo intermittent recovery level 2 (YYIRT2) test of defence and midfield players were higher than forward players and goalkeepers at study performed on young soccer players of Slovakia U-21 (under 21 age) national team [14]. Although the loading intensity of the YYIRT2 test was higher than YYIRT1, it was seen that the covered distance and VO 2max values of midfielders during test was higher than other playing positions similarly to findings of our study. Markovic and Mikulic [15] determined that the covered distance values during YYIRT1 of young midfielders and wingers was higher than other playing positions but this difference was not statistically significant. Similarly, Mohr and Krustrup [16] reported that semi-professional male wingers covered significantly more distance during YYIRT1 than fullback and attacker players. Also, the covered distance during test of central midfielders was higher than centerback, fullback and attacker players in mentioned study. In study performed on male soccer players aged 23 years, it was found that midfielders covered more distance than defenders and less distance than attacker players [17]. It was remarkable that the findings of our study were similar to findings of mentioned studies.
Rago et al. [18] indicated that defenders covered more distance than midfielder and attacker players in sub-elite soccer players at under 19 age. The defender players were divided to two categories as centerback and fullback in our study such as most studies. The YYIRT1 values of centerback and fullback was the lowest of all playing positions. The game tactics and strategies may vary between soccer teams. The defensive and offensive game strategies are performed during soccer matches at different forms and levels according to various factors (such as features of own and opponent's players, match status, pitch condition). Therefore, the discrepancy between our study and mentioned study may arise from these factors. On the other hand, the significant difference between covered distance values duringYYIRT1 of goalkeepers, defenders, midfielders and attackers was not determined [19]. Although the physical and conditional features of male and female soccer players is different, the results of our study draw parallelism to findings of mentioned study. The values of covered distance during YYIRT1 in our study was higher than test values of Milanović et al [19] but no significant difference was determined between playing positions at both two studies. The physical and conditional features of soccer players at different playing positions may be similar. The parallelism between two studies may arise from this similarity.
The YYIRT1 performance is correlated with soccer match performance. This relationship was emphasized by many studies. Dupont et al. [20] found a correlation between YYIRT1 peak velocity and maximal aerobic velocity of Montreal Track test (r=0.79, p˂0.01). Also, Krustrup et al. [8] determined that the YYIRT1 performance was significantly correlated with the covered distance with high intensity running during soccer match (r=0.71, p˂0.05). Similarly, it was found that the covered distance during YYIRT1 was significantly correlated with soccer match performance (covered distance with high intensity activity (r=0.77) and total covered distance (r=0.65) during soccer match) [21]. In another study, Castagna et al. [22] determined moderate and high correlations between covered distance during YYIRT1 and soccer match activities with high intensity (covered distance with high intensity activity (r=0.73), covered distance with high intensity running (r=0.65) and sprint distance (r=0.76)). The VO 2max value is an important indicator of aerobic endurance. It was indicated that the YYIRT1 performance is strongly related to VO 2max value in many studies. Aslan et al. [23] reported high correlation between covered distance during YYIRT1 and VO 2max values (r=0.89). Wong et al. [24] performed a study on U-14 (under 14 age) and found a correlation between covered distance during YYIRT1 and Hoff dribbling test (specific soccer test) performance and VO 2max value (r=0,71 and r=0.63, respectively). Rampinini et al. [25] determined that YYIRT1 distance value was correlated with Max.VO 2 value. Similarly, it was found that YYIRT2 test performance is related to covered distance during soccer match. Alonso et al. [26] performed a study on young soccer players and found a correlation between covered distance during YYIRT2 test and covered distance with middle intensity and high intensity and total covered distance (r=0.456, r=0.451, r=0.581, respectively). In young soccer players aged 14-17, the covered distance during YYIRT1 was correlated with covered distance with high intensity running (r=0.56), high intensity activity (r=0.56), sprint distance (r=0.63) and sprint time percentage at last 13.3 minutes (r=0.63) during soccer match [27]. The running with turning, changes of direction, short distance sprints, jumps and 239 acceleration are performed during soccer matches. The YYIRT1 includes these activities. Therefore, the test may indicate soccer performance such as findings of mentioned studies.
It is known that the YYIRT1 is a soccer specific test including physical activities used frequently at soccer matches. The YYIRT1 may discriminate aerobic fitness of soccer players. The levels of aerobic endurance and physical condition may be determined by YYIRT1. The many studies examined aerobic fitness level and indicated difference between soccer players at different aerobic fitness level. Veale et al. [28] performed YYIRT1 on soccer players and determined that elite junior soccer players covered more distance than sub-elite and nonathletic general healthy soccer players. Similarly, Deprez et al. [29] found that the covered distance values during YYIRT1 of elite young soccer players were higher than test values of sub-elite young players in study performed on young players at U-13, U-15 and U-17 (under 13, 15 and 17 age) categories. Also, it was determined that the YYIRT1 distance of professional soccer players was higher than test distance of amateur soccer players [25]. In another study performed on young soccer players and active sedentary individuals, the YYIRT1 distance of soccer players at 12-13 age category was higher than test distance of sedentary individuals [30]. In study of Teplan et al. [31], it was found that the YYIRT1 distance was higher at soccer players at U-17 category (under 17 age) than players at U-16 category (under 16 age). The age and maturation effect on physical fitness may decisive on YYIRT1 performance of young soccer players at mentioned study. The results of mentioned studies show that the YYIRT1 performance is an indicator of match performance. The aerobic fitness level of soccer players may be distinctively determined by YYIRT1.
There are different findings in literature about examination ofYYIRT1 performance according to playing position. In our study, it was determined that the difference of YYIRT1 performances between playing positions was statistically non-significant. However, the midfielders had higher YYIRT1 performance than players at other playing positions. The inconsistency of YYIRT1 performance findings in literature may be arisen from aerobic fitness level and individual features of subjects at studies.

Conclusion
Consequently, it was seen that the young soccer players at different playing positions at U-21 (under 21 age) category had similar YYIRT performance. However, young midfield players may cover more distance in YYIRT1 because of requirements of playing position. It may be performed more studies including young soccer players at different league levels for more comprehensive findings about YYIRT1 performance.