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Considering the duration of the soccer game, aerobic metabolism is dominant. This reveals that the aerobic endurance feature is an important condition for success in soccer. However, it is not possible to talk about aerobic metabolism as the key to success in soccer. Due to the multifactorial nature of soccer, besides the development of aerobic methods for success, many features such as anaerobic structures, technical and tactical dimensions, psychological state, among others, must be at a high level. The association of recovery between high-intensity activities in the game with aerobic metabolism makes the aerobic endurance feature more important (Vanttinen, Blomqvist, Nyman, & Hakkinen, ²⁰¹¹; Hammami et al., ²⁰¹³; Sagelv et al., ²⁰¹⁹).

Physical characteristics of soccer players are known to improve with biological age and training age. It was emphasized that the soccer game is a sport that requires the fitness levels of soccer players to be constant at a high level, but the foundations of physical performance should be formed during their youth (Dragijsky, Maly, Zahalka, Kunzmann, & Hank, ²⁰¹⁷). In terms of talent development, soccer players between 15 and 18 years old are in a critical development period (Sæther & Aspvik, ²⁰¹⁴). It is known that the increases in aerobic performance with the training performed has positive improvements in combating the stress faced by soccer players during the match (Helgerud, Engen, Wisløff, & Hoff ²⁰⁰¹; Impellizzeri et al., ²⁰⁰⁶). The capacity of soccer players to gain and maintain good physiological performance throughout the season is very important (Reilly & Williams, ²⁰⁰³). It is important to determine what effects this may have on pre-season and seasonal change levels (^{Caldwell & Peters, 2009}).

Yo-Yo_{IRT 1} is a valid field test widely used in soccer. The widespread use of Yo-Yo_{IRT 1} is due to its simplicity, low cost, and the ability to test several players at the same time. (Fanchini et al., ²⁰¹⁴). Yo-Yo_{IRT 1} gives results similar to laboratory tests in the evaluation of VO_2max (Sylejmani et al., ²⁰¹⁹). Studies have shown that it is associated with the high-intensity activities during a match and it informs about competition levels throughout the season (Bangsbo, Iaia, & Krustru, ²⁰⁰⁸; Mohr & Krustrup, ²⁰¹⁴). Yo-Yo_{IRT 1} has significant correlations with the total distance (r = 0.62) and the high-intensity distance (r = 0.73) of sports athletes in the underage teams (Roe & Malone, ²⁰¹⁶).

Although there are studies investigating the seasonal changes of soccer players, there is no study investigating the aerobic levels of the players of different age groups of a professional soccer team with the same soccer culture. Knowing the changes in aerobic capacity of youth academy players in pre-season, mid-season and end-of-season will provide important information for optimal performance and enable critical periods to be determined. This study predicts that aerobic capacity increase as the soccer players’ ages increase. Within the frame of this hypothesis, we assumed that there would be differences in seasonal variation depending on the age of the teams in different categories. If looked from, it is anticipated that this research would contribute significantly to the identification of critical stages in the development of soccer players trained in youth academies. In addition, being aware of the weaknesses of the players during their youth and training for them are considered to be important in reaching the optimal performance level by eliminating these weaknesses until adulthood. Based on the above information, the purpose of this study is to investigate the seasonal changes in the aerobic levels of the youth academy players in the U14, U15, U16, U17, and U18 categories of a professional team.

Material and methods

Results

Yo-Yo_IRT1 results of all groups at different times are given in Table 2. Yo-Yo_IRT1 development percentage in the U14 age category increased by 7.47% in the mid-season compared to the beginning of the season, by 13.68% at the end of the season compared to the middle of the season, and by 22.17% at the end of the season compared to the beginning of the season. In the U15 age category, it decreased by 13.47% in the mid-season compared to the beginning of the season, increased by 49.66% at the end of the season compared to the mid-season and increased by 29.53% at the end of the season compared to the beginning of the season. In the U16 age category, it increased by 14.95% in the mid-season compared to the beginning of the season, increased by 26.20% at the end of the season compared to the mid- season and increased by 47.54% at the end of the season compared to the beginning of the season. In the U17 age category, it decreased by 6.76% in the mid-season compared to the beginning of the season, increased by 26.52% at the end of the season compared to the mid-season, and increased by 17.96% at the end of the season compared to the beginning of the season. In the U18 age category, it was observed to decrease by 5.93% in the mid-season compared to the beginning of the season, increase by 19.33% at the end of the season compared to the mid-season, and increase by 12.25% at the end of the season compared to the beginning of the season.

VO_2max results of all groups at different times are given in Table 3. Change in VO_2max in all age categories for mid-seasons, the beginning of the season and the end of the season was observed.

In the U14 age category, maximum oxygen uptake increased from 48.77 ml*kg^-1*min^-1 at the beginning of the season to 49.71 ml*kg^-1*min^-1 in mid-season and to 51.52 ml*kg^-1*min^-1 at the end of the season. In the U15 age category, it decreased from 49.15 ml*kg^-1*min^-1 at the beginning of the season to 47.43 ml*kg^-1*min^-1 in mid-season, but it increased to 52.93 ml*kg^-1*min^-1 at the end of the season. In the U16 age category, there was an increase from 50.07 ml*kg^-1*min^-1 at the beginning of the season to 52.13 ml*kg^-1*min^-1 in mid-season, and then up to 56.21 ml*kg^-1*min^-1 at the end of the season. In the U17 age category, estimated maximum oxygen uptake decreased from 55.34 ml*kg^-1*min^-1 at the beginning of the season to 54.07 ml*kg^-1*min^-1 in mid-season, but then increased to 58.74 ml*kg^-1*min^-1 at the end of the season. In the U18 age category, it decreased from 53.43 ml*kg^-1*min^-1 at the beginning of the season to 52.39 ml*kg^-1*min^-1 in mid- season, but increased to 55.49 ml*kg^-1*min^-1 at the end of the season.

Discussion

The purpose of this study was to investigate the seasonal changes in the aerobic levels of the youth academy players of a professional soccer team in the U14, U15, U16, U17, and U18 categories.

The aerobic system and the anaerobic system are considered to be very important in soccer game. However, during a 90-minute soccer match, the main source of Adenosine Triphospate (ATP) production is regarded as the aerobic system. The maximum aerobic power determined by the maximum oxygen intake can vary depending on the league and the positions in which the soccer players participate (Vanttinen et al., ²⁰¹¹; Teplan et al., ²⁰¹²).

In the present study, aerobic development increased statistically significantly at the end of the season compared to the mid-season and similarly at the end of the season compared to the beginning of the season in all age categories (p<0.05). At the end of the season, the highest development was in the U16 category (47.54%), followed by the U15 (29.53%), U14 (22.17%), U17 (17.96%) and U18 (12.25%) categories. In U15, U17, and U18 categories, a decreasing percentage was observed in mid-season compared to the beginning of the season, but these rates were not statistically significant (p<0.05). All teams reached their highest running distance at the end of the season. While the percentage of development at the end of the season is higher in U16 and the younger age groups, the running distance is higher in U16 and older age groups (Table 2). Besides, VO_2max increased at the end of the season compared to the beginning of the season in all age categories (U14 from 48.77 ml*kg^-1*min^-1 to 51.52 ml*kg^-1*min^-1, U15 from 49.15 ml*kg^- 1*min^-1 to 52.93 ml*kg^-1*min^-1, U16 from 50.07 ml*kg^-1*min^-1 to 56.21 ml*kg^-1*min^-1, U17 from 55.34 ml*kg^-1*min^-1 to 58.74 ml*kg^-1*min^-1, U18 from 53.43 ml*kg^-1*min^-1 to 55.49 ml*kg^-1*min^-1) (Table 3). Standard deviation in Yo-Yo_{IRT 1} and VO_2max parameters of athletes differ in terms of the data obtained at the beginning, in the middle and at the end of the season. This change shows that all of the athletes were affected disparately from the training.

The increase in aerobic capacity is expected to be the usual effect of pre-season training (^{Jastrzębski, Dargiewicz, et al., 2011}). Nevertheless, there are studies revealing that VO_2max increases at the end of the preparation period (McMillan et al., ²⁰⁰⁵; ^{Jastrzębski, Barnat, Konieczna, Rompa, & Radzimiński, et al., 2011}; ^{Charalampos, Zisis, Asterios & Nikolaos, 2013}). Other studies asserting that VO_2max decreases at the end of the competition period (Mohr, Krustrup, & Bangsbo et al., ²⁰⁰²; ^{Mohr, Krustrup, & Bangsbo, 2003}, Dragijsky et al., ²⁰¹⁷). Besides, evidence shows that VO_2max increases during the entire competition period (Edwards, Clark, & Macfadyen, ²⁰⁰³). When the literature is reviewed, it is possible to come across some contradictory findings.

Dragijsky et al. (²⁰¹⁷) reported that in their study carried out with the participation of 28 players from the U13 category of a professional team in the Czech League, the average Yo-Yo_IRT 1 values of the players gradually increased at a statistically significant level compared to the beginning of the season and decreased at the end of the competition period (t1-beginning of the pre-season training period 625.26m; t2-start of the competitive period 858.95m; t3-middle of the competitive period 953.68m; t4-end of the competitive period 924.21m).

Multi-stage Fitness Test and Yo-Yo_{IRT 1} Test can be interchangeably used to predict VO_2max (Thomas, Dawson, & Goodman, ²⁰⁰⁶). Caldwell & Peters also found the VO_2max value (56 ml_*kg^- 1*min^-1) at the beginning of the season lower than other periods of the season using the Multi- stage Fitness Test (^{Caldwell & Peters, 2009}). Similar to the U14 and U16 age group results in the present study; Silva et al. (²⁰¹⁵), in their study with elite young soccer players aged 17, reported that the running distance increased (35%) according to the Yo-Yo_{IRT 1} test results obtained at the end of the week 5 of a 7-week preparation period. In the study where the development of the male soccer players in the U16 category was followed for an 8-week period, Hammami et al. (²⁰¹³) reported an increase in Yo-Yo_{IRT 1} (VO_{2 max} from 47.9 to 55.7 ml_*kg^-1 _*min^-1) performances of the players. Vanttinen et al. (²⁰¹¹) in their study with the (Yo-Yo_{IRT 1}) 14(d=0.29-small), 15(d=0.21- small), and 16(d=0.03-trivial)-year-old finnish soccer players in the a-year follow-up in response to cardiovascular endurance reported identical findings to the current study.

In soccer-related studies, it was emphasized that the short off-season (8 weeks) was associated with a decrease in aerobic fitness level (Reilly & Williams, ²⁰⁰³) and an increase in body fat percentage (Hoshikawa et al., ²⁰⁰⁵). In the studies which showed that the level of aerobic fitness in soccer players increased in the mid-season compared to the pre-season and decreased at the end of the season, the reason for the increase in VO_2max in the mid-season compared to the beginning of the season stemmed from the training and competitions, and this situation was associated with the decrease in body fat percentage. They emphasized that the decrease in VO_2max in the second half of the season might be caused by overload/overtraining and fatigue at the end of the season (Haritonidis, Koutlianos, Koudi, Haritonidou, & Deligiannis, ²⁰⁰⁴; ^{Caldwell & Peters, 2009}).

Aerobic performance development in young adolescents is related to the performance in the matches. The development of aerobic performance is affected by the process of biological maturation and weekly training volume (Teixeira et al., ²⁰¹⁴). The most important factor that can affect the sportive condition of the player and the whole team in a comprehensive way is the training load (^{Jastrzębski, Dargiewicz, et al., 2011}). Studies show that exposure to training loads among elite young soccer players increased between the ages of 11 and 19 (Baxter-Jones & Helms, ¹⁹⁹⁶; Brito et al., ²⁰¹²; Malina et al., ²⁰⁰⁰). “Practice makes perfect” is a well-known expression in most sports, including soccer. However, severe training and match load increase the risk of injury, weariness and burnout (Sæther & Aspvik, ²⁰¹⁴). The pressure to be successful for talented players is felt at earlier ages (Hill, ²⁰¹³).

Researchers have suggested that such players, who had a low level of condition at the start of the season, had to work extremely hard to reach the high level they required. They also argue that to start the season in such a short time may be associated with a potentially high catabolic condition, causing mental and physical fatigue later in the season (Kraemer et al., ²⁰⁰⁴). Pressure to be successful can lead to a potential lack of motivation and burnout (Sæther & Aspvik, ²⁰¹⁴). Hill (²⁰¹³) stated that one out of four 13-16 years-old players selected for a British professional club experienced burnout at least once in their career (Hill, 2013).

According to the research findings of young soccer players, the period in which growth / development and individual differences are most evident is between 11 and 16 years old. On the other hand, there is limited information about late adolescent soccer players aged 17-19 (Sylejmani et al.^{, 2019}).

Furthermore, it should be noted that the training process in young players has an important role in their development and positively affects the development of individual components of their athletic performance (Dragijsky et al., ²⁰¹⁷) because systematic young academy soccer education increases physical and physiological development in young soccer players, but maturity status is affected by these processes in different ways (Morris et al., ²⁰¹⁸).

In addition to the positive adaptations from training, changes in performance in youth athletes will also reflect the effects of normal growth and maturation, therefore any conclusions regarding the magnitude of performance improvement to specific training must account for changes in maturation. As such this highlights the importance of considering physical testing results in the context of maturation status when evaluating the performance of youth soccer players (Emmonds, Sawczuk, Scantlebury, Till, & Jones, ²⁰²⁰).

Moreover, to the beneficial improvements of training, changes in the success of young athletes will likely demonstrate the results of natural development and maturation, thus all assumptions on the extent of skill adjustment of individual preparation will account for the changes in maturation. This emphasizes the importance of considering the results of physical testing in the context of maturation status when evaluating the performance of young soccer players (Emmonds et al., ²⁰²⁰).

Conclusions

The present study aimed to examine the seasonal changes of the young elite soccer players in the U14, U15, U16, U17, and U18 categories. The findings show that the aerobic performance of U14 and U16 age groups increased from the beginning of the season to the end of the season. When we look at the mean values the aerobic performance of the U15, U17, U18 age groups, we can state that there is a decrease in the middle of the season and increase at the end of the season. While the increase is statistically significant, the decrease is not statistically significant. However, this decrease is worthy of notice. It is thought that some differences in the responses to the training loads during maturation can be observed (Roe & Malone, ²⁰¹⁶). Significant increase at the end of the season compared to the beginning of the season in all groups may be due to the fact that the adaptation of the athletes to the training programs was positively affected by the development characteristics according to the age categories. For this reason, carefully selected trainings should be performed in all age categories according to the developmental characteristics of the athletes. Furthermore, the development of the athletes should be tested at short intervals.