Training programs for young soccer players are designed to optimize physical development, including body composition and sport-specific performance. Modern soccer requires players not only to demonstrate advanced technical and tactical abilities but also to possess high levels of physical fitness to cope with the increasing demands of the game (Sarmento et al., 2018; Slimani & Nikolaidis, 2018). Therefore, the development of structured and evidence-based training programs during adolescence is essential to ensure optimal athletic progression. Scientific monitoring and continuous evaluation during the growth and maturation period are particularly important, as these phases represent critical windows for physical and physiological development (Lloyd et al., 2016; Moran et al., 2017).

Soccer is characterized by complex, dynamic movement patterns that involve both cyclic and acyclic activities, such as sprinting, jumping, turning, and tackling. These movements require integrating multiple physical components, including strength, endurance, and coordination (Stolen et al., 2005). During adolescence, players experience rapid growth and maturation, which significantly influence body composition and performance capabilities. Previous research has shown that changes in body composition, particularly increases in lean mass and reductions in fat mass, are closely associated with improved physical performance in youth athletes (Deprez et al., 2015; Malina et al., 2015). Moreover, longitudinal monitoring of these variables is essential for understanding the interaction between growth, training, and performance development in young soccer players.

According to the Youth Physical Development Model, training responsiveness during adolescence is strongly influenced by biological maturation and the interaction between growth-related physiological changes and training stimuli (Lloyd & Oliver, 2012). As athletes progress through puberty, hormonal adaptations, neuromuscular development, and increases in muscle mass may enhance their capacity to respond to strength, power, and agility training. Consequently, players of different age groups may exhibit distinct patterns of adaptation despite being exposed to similar training programs.

Physical performance variables such as muscular strength and explosive power are considered key determinants of success in soccer. Strength development, particularly in the lower limbs, is crucial for executing fundamental actions such as sprinting, jumping, and kicking (Suchomel et al., 2016). Vertical jump performance, including squat jump (SJ) and countermovement jump (CMJ), is commonly used as an indicator of explosive power and neuromuscular function (Claudino et al., 2017). In addition, isokinetic strength assessments provide valuable insights into muscle function and imbalances, which are important for both performance enhancement and injury prevention (Croisier et al., 2008). Therefore, evaluating these variables is essential to understanding the effectiveness of training programs in youth soccer players.

Previous research has demonstrated that growth, maturation, and systematic soccer training play important roles in the development of physical performance in youth players. Philippaerts et al. (2006) reported that the greatest improvements in speed, explosive strength, agility, and endurance occur around peak height velocity, highlighting the strong influence of biological maturation on athletic development in youth soccer players. Furthermore, Hammami et al. (2013) found that a competitive soccer season resulted in significant improvements in anthropometric characteristics and physical fitness parameters, including jumping ability, sprint performance, and aerobic capacity, compared with age-matched controls. More recently, Stastny et al. (2018) emphasized the importance of isokinetic assessment for evaluating knee flexor and extensor strength adaptations in athletic populations and highlighted its value for monitoring training-related neuromuscular changes. Similarly, Sermaxhaj et al. (2024) demonstrated that a seasonal soccer training program improved body composition, jumping performance, and isokinetic strength, particularly in U-17 players, suggesting age-related differences in training responsiveness.

Despite these contributions, several important gaps remain. First, previous studies have generally examined body composition, jumping performance, sprint ability, or strength separately. In contrast, limited research has simultaneously evaluated body composition, isokinetic strength, and agility within the same seasonal training framework. Second, although agility is recognized as a key determinant of soccer performance due to the frequent changes of direction required during match play, longitudinal evidence regarding agility adaptations across different adolescent age categories remains limited. Third, while age- and maturation-related differences have been acknowledged in youth soccer development, few studies have directly compared seasonal training adaptations between U-15 and U-17 players using a comprehensive set of body composition and neuromuscular performance indicators. Therefore, it remains unclear whether players at different stages of adolescence exhibit distinct responses to the same seasonal training program, particularly regarding agility development.

Therefore, the present study aimed to examine the effects of a seasonal soccer training program on body composition, strength, and agility in U-15 and U-17 soccer players. By simultaneously evaluating these performance domains and quantifying the magnitude of change through effect size analysis, this study seeks to provide a more comprehensive understanding of age-related training adaptations during adolescence. By comparing U-15 and U-17 players, this study also aims to provide deeper insights into age-related differences in responsiveness not only in strength and power, but also in agility performance, which remains underexplored in the existing literature.

Based on these considerations, an effective training program requires appropriate periodization to ensure optimal adaptation throughout the preparation and competition phases. Coaches must design training interventions that align with players' biological and developmental characteristics to maximize performance outcomes (Issurin, 2016). However, despite growing interest in youth training, there is limited evidence on how structured soccer training programs differentially affect players at different stages of adolescence, particularly between the U-15 and U-17 age groups.

In contrast, the U-17 group demonstrated significant improvements in body composition, with increased lean mass and decreased fat mass (p < 0.05). No statistically significant changes were found in body height or body mass (p > 0.05).

Mean = arithmetic mean; SD = standard deviation; F = F-value; p = significance level. *Significant at p < 0.05

Mean = arithmetic mean; SD = standard deviation; F = F-value; p = significance level; d = Cohen’s d Effect size interpretation: trivial (<0.2), small (0.2–0.49), moderate (0.5–0.79), large (≥0.8) p < 0.05; ** p < 0.01

In contrast, the U-17 group demonstrated significant improvements in most physical performance variables, including squat jump, countermovement jump, maximal countermovement jump, agility, and knee flexion peak torque (p < 0.05). However, no statistically significant change was observed in knee extension peak torque (p > 0.05).

Furthermore, effect size analysis indicated that changes in the U-15 group were predominantly small; however, a moderate effect was observed in agility, while large effects were found in isokinetic strength variables. In contrast, the U-17 group exhibited large effect sizes across most performance variables, indicating substantial practical improvements following the training program.

This study examined the effects of a seasonal soccer training program on body composition, strength, jump performance, and agility in U-15 and U-17 players, representing two critical stages of biological development. During adolescence, players experience rapid physiological changes, including increases in muscle mass, neuromuscular coordination, and overall physical performance (Malina et al., 2004; Philippaerts et al., 2006). Structured training during this period is therefore essential to optimize body composition and enhance sport-specific performance capacities (Helgerud et al., 2001; Lloyd & Oliver, 2012).

In the U-15 group, results indicated improvements across several physical performance variables; however, statistically significant changes were observed only in isokinetic strength variables, particularly peak torque at knee flexion and extension. No significant changes were found in body composition, jumping performance, or agility. These findings are consistent with previous studies suggesting that younger athletes often demonstrate limited adaptations in explosive performance despite regular training (Philippaerts et al., 2006; Sperlich et al., 2011). The relatively small effect sizes observed in this group further support the notion that the magnitude of training-induced adaptations in U-15 players remains modest, particularly for neuromuscular performance variables.

The lack of significant improvement in jumping and agility performance among U-15 players may be explained by the concept of “windows of trainability,” in which certain physical capacities are more responsive to training at specific stages of maturation (Ford et al., 2011; Lloyd & Oliver, 2012). At this stage, the neuromuscular system is still developing, which may limit the effectiveness of training programs aimed at improving explosive power and change-of-direction speed. Therefore, although strength gains were observed, these did not translate into significant improvements in functional performance such as jumping and agility. This finding suggests that training interventions for U-15 players may need to emphasize coordination, technique, and movement efficiency rather than maximal power output.

In contrast, the U-17 group demonstrated significant improvements in body composition, including increases in lean mass and reductions in fat mass. These findings align with previous research indicating that late adolescence is characterized by more pronounced adaptations in body composition due to hormonal changes and increased training responsiveness (Milanese et al., 2013; Vänttinen et al., 2011). The observed improvements in body composition are likely the result of both systematic training and the natural progression of biological maturation, which enhances anabolic processes and muscle development. Importantly, the significant improvement in agility observed in U-17 players highlights the importance of including change-of-direction ability as a key outcome in youth soccer training studies, which has been underreported in previous research. From an applied perspective, the large effect size observed for agility (d = 1.18) indicates a substantial performance enhancement that is likely meaningful for match performance, even beyond statistical significance. Coaches should interpret effect sizes alongside p-values, as meaningful performance changes may occur even when statistical significance is not achieved. The observed improvements are likely the result of both systematic training and the natural progression of biological maturation, which enhances anabolic processes and muscle development. Because all players participated in the same seasonal program without a non-training control group, some observed improvements may reflect normal developmental changes occurring during adolescence rather than training effects alone.

From a physiological perspective, the greater responsiveness observed in U-17 players can be attributed to more advanced neuromuscular development and hormonal status, particularly increased testosterone levels, which facilitate muscle hypertrophy and strength gains (Lloyd & Oliver, 2012; Malina et al., 2004). This supports the idea that training programs are more effective during and after puberty, when athletes are better able to adapt to higher training loads and intensities. Consequently, the training program used in this study appears more suitable for athletes at this stage of development.

Regarding physical performance, the U-17 group showed significant improvements in jump performance (SJ, CMJ, and CMJmax), agility, and knee flexion peak torque, whereas knee extension peak torque did not change significantly. The absence of significant changes in knee extension torque may indicate that the training stimulus was insufficiently specific to target quadriceps strength development. Another possible explanation is the higher baseline values observed in the U-17 group, resulting in a ceiling effect. These results are consistent with previous studies demonstrating that improvements in explosive performance are closely related to increases in muscle mass and neuromuscular efficiency (Lehnert et al., 2014; Markovic & Mikulic, 2010). Importantly, agility performance also improved significantly among U-17 players, indicating enhanced change-of-direction ability, a key determinant of soccer performance (Sheppard & Young, 2006). This highlights the added value of including agility as a variable in the present study.

The effect size analysis provided additional insight into the practical significance of the findings. While the U-15 group demonstrated predominantly small to moderate effect sizes, the U-17 group exhibited large effect sizes across most variables, particularly in CMJmax, agility, and flexion peak torque. This indicates that, beyond statistical significance, the magnitude of training adaptations was substantially greater in older players. The inclusion of effect size analysis strengthens the interpretation of results, enabling a more comprehensive understanding of the training program’s effectiveness (Batterham & Hopkins, 2006; Cohen, 2013).

Coaches working with U-15 players should prioritize movement competency, coordination, and progressive strength development rather than expecting substantial gains in agility and explosive performance over a single season. For U-17 players, incorporating structured strength and change-of-direction training may yield meaningful improvements in agility, jump performance, and lower-limb strength. Age-specific training prescription should therefore be considered when designing long-term player development programs.

The present study demonstrated that a seasonal soccer training program elicited different adaptations in U-15 and U-17 players. In the U-15 group, training improved strength variables; however, changes in body composition, jumping performance, and agility were not statistically significant, suggesting limited responsiveness at this stage of development. In contrast, U-17 players showed significant improvements in body composition, jump performance, agility, and knee flexion strength, reflecting a greater capacity to adapt to training stimuli during later stages of maturation. Furthermore, effect-size analysis revealed that the magnitude of change was generally small in U-15 players but large in U-17 players, underscoring the practical relevance of training adaptations in older athletes. These findings suggest that age-related developmental factors contribute to differences in training responsiveness between U-15 and U-17 players. However, the role of biological maturity should be confirmed through direct maturity assessment in future studies.

The authors would like to express their sincere gratitude to all players who voluntarily participated in this study, as well as to their coaches and supporting staff from the Netherlands football league for their cooperation and assistance throughout the research process. The authors also acknowledge Maastricht University for providing ethical approval and institutional support. This research did not receive any external funding. The authors declare no conflicts of interest, financial or otherwise.

The data supporting this study's findings are available on request from the corresponding author. The data are not publicly available because they contain information that could compromise the privacy of research participants

This research does not receive external funding.

The author hereby declares that this research is free from conflicts of interest with any party.