Introduction

Soccer necessitates high-intensity efforts interspersed with low-intensity activities (Gil et al., 2007). The escalating demands, intensity and speed of the game require soccer players to have highly developed aerobic and anaerobic mechanisms (Chena et al., 2022; Stølen et al., 2005). This demand for a high level of aerobic capacity is a key factor differentiating among elite soccer players (Slimani et al., 2019).

Elevated aerobic capacity enables the maintenance of peak performance throughout the season and expedites recovery following intense efforts. In developing athletes, it permits tolerance of numerous training units (Deprez et al., 2015), facilitating greater high-intensity sprint and run frequencies as well as increased ball contact (Buchheit et al., 2010; Clemente et al., 2020).

Furthermore, superior aerobic capacity correlates with enhanced cognitive function, including internal inhibition, attention, and memory—crucial for skill acquisition (Scudder et al., 2014). As indicated in studies on the subject, better-performing young athletes exhibit improved reaction times, selective attention, executive control and accuracy (Pontifex et al., 2011; Voss et al., 2011). Therefore, monitoring aerobic capacity, even in early training stages, is crucial. This emphasis on physiological development, alongside technical skill development, is paramount in optimising training efficacy among youth soccer players.

Comprehensive assessment of aerobic capacity, in addition to evaluating the maximum values of a given indicator, should include evaluation of variables at the anaerobic threshold level. Maximal aerobic speed (MAS) should also be considered, a variable often overlooked in the limited research on this developmental period (Clemente et al., 2020; Mendez-Villanueva et al., 2010). This is necessary to better understand the relationships among ontogenetic development, physiological indices and regular sports participation in childhood.

For such assessments, MAS is a key variable, as it determines the threshold running speed beyond which uncompensated metabolic acidosis begins, making it a good indicator of the fitness level and conditioning (Baker and Heaney, 2015). However, there are few studies on the effects of training loads among children in the first stages of training, or the effects of training on changes in individual aerobic capacity indices.

Therefore, the aim of this study was to determine developmental changes in aerobic capacity of boys aged 7–12 participating in soccer training. An additional objective was to assess the influence of applied training loads on changes in aerobic capacity.

Methods

Results

The athletes exhibited a characteristic increase in body height and mass across the studied age period. These differences were statistically significant in all age groups. A similar trend was observed for lean body mass, with significant gains differentiating among all age groups. The percentage of body fat (%F) decreased with age, although these differences were not statistically significant. Group-related differences in fat mass were significant between 7- and 9-year-olds, 7- and 10-year-olds, as well as 8- and 9-year-olds. Significant differences were also noted in Group 2 between 10- and 11-year-olds, as well as 10- and 12-year-olds.

Analysis of birthdates revealed over-representation of athletes born in the first two quarters of the year in both groups (Table 1).

Aerobic capacity indices measured during maximal exercise significantly differed among the age groups in most cases (Tables 2 and 3). The most pronounced differences were observed in Group 1 within the initial two years of training.

The results obtained during exercise at anaerobic threshold intensities showed significant differences in oxygen uptake (VO₂/AT) across all age groups except for participants between the age of 9 and 10 (Table 4). Significant differences, although not universally observed, were also found in minute ventilation (VE/AT) and the percentage of maximum oxygen uptake (%VO_2max) (Table 5).

Total work time (per metabolic zone) demonstrated a steady increase with age, except for the 10- to 11-year-olds in Group 2. The "intake" group began training in September, with less than three months prior to the first assessment point, meaning the training period lasted only four months. Thus, the lowest training loads were observed in the 7-year-olds. From 9 years onward, annual training loads were influenced not only by regular training sessions but also by sports camps. Pearson's r correlation analysis did not reveal a significant effect of the applied training loads on the level of aerobic capacity indices, with the exception of %HR_max and %VO_2max in 8-year-old boys (Table 6).

Discussion

In this longitudinal study, dynamics of aerobic capacity indices were examined, relative to training loads. Body composition was also assessed in 7- to 12-year-old prepubertal boys participating in soccer training. The longitudinal design, with repeated measurements performed for the same individuals, allowed to note long-term training effects. Importantly, the inclusion of a group beginning training at age 7 provided a unique opportunity to observe the initial impact of training on physiological adaptations. This is particularly relevant given the limited research on early training adaptations in children and the distinct physiological responses to exercise in this population compared to adults (Clemente et al., 2020).

The findings of the present study allow to demonstrate a gradual and systematic increase in absolute VO_2max (l/min) among the participants. This increase, driven by muscle tissue growth and training experience, reflects developmental functional transformations occurring during childhood. Furthermore, relative VO_2max (ml/kg/min) also increased progressively from early school years, likely due to the combined effects of regular training and age-related decreases in the body fat percentage. The most pronounced increases in VO_2max were observed between the age of 7 and 10, with a 74% increase in absolute VO_2max and a 29% increase in relative VO_2max. This period highlights the synergistic influence of training and biological maturation on aerobic capacity development. Between 10 and 12 years of age, the increases were less pronounced (50% and 18% for absolute and relative VO_2max, respectively).

While the findings regarding increases in relative VO_2max align with those from previous reports (Baquet et al., 2003), the timing of the largest annual increments differs from some studies. For instance, it has been suggested that the most significant increases occur between 11 and 13 years of age, coinciding with the pubertal growth spurt (Lemura et al., 1999). In contrast, we observed the largest inter-annual changes between 7 and 8 years of age and the 10–11-year-old age group. This discrepancy underscores the potential for individual variation in the timing of aerobic capacity development and suggests that factors beyond pubertal maturation may influence these adaptations in youth athletes. Further research is needed to elucidate the complex interplay of biological and training-related factors contributing to aerobic capacity development in young athletes.

It is widely accepted that high VO_2max levels are crucial for success in soccer (Mohr et al., 2003; Ortiz et al., 2024; Teplan et al., 2012). Elite soccer players in the Spanish Primera Division, for example, exhibit average VO_2max of 66.4 ± 7.6 ml/kg/min (Casajús, 2001). The findings of this study allow to suggest that the studied boys possessed an adequate physiological predisposition for soccer. Their VO_2max levels exceeded those noted for untrained peers and were comparable to those of trained peers from other studies (Strøyer et al., 2004).

The observed high VO_2max level is undoubtedly the cumulative effect of exceptional genetic predispositions regarding aerobic capacity and early training adaptation. This aligns with research confirming the possibility of VO_2max exceeding 55 ml/kg/min in children with high levels of physical activity, particularly in endurance and team sports (Aandstad et al., 2006). Our observations are also consistent with the findings of a recent study on the impact of intensive game-based training which is the model used in leading soccer academies on the fitness levels of young soccer players (Jastrzębski et al., 2025).

It is quite interesting that despite a higher frequency of training sessions (Bunc et al., 2002), the 10-year-old soccer players in our study did not demonstrate a strong influence of training on aerobic capacity. This observation is further supported by the lack of correlation between training loads and VO_2max levels (both absolute and relative), except between the ages of 7 and 8. These findings suggest that differences in VO_2max between groups may be primarily attributed to biological development and innate predisposition, rather than training or pubertal status.

The impact of training on VO_2max in pre-adolescent children remains a subject of debate. While in some studies long-term benefits are reported of childhood training on adult aerobic capacity (Baquet et al., 2003; Janz et al., 2002), in others, improvement is demonstrated in VO_2max following short-term vigorous training interventions (Baquet et al., 2002; Clemente et al., 2020). However, these findings are not universally supported (Williams et al., 2000).

Given the inconclusive nature of training effects on VO_2max, a broader perspective on performance assessment is warranted. Therefore, in our study, we also incorporated the assessment of threshold values, such as MAS. These are known to be highly sensitive to training adaptations and commonly used in training programmes for both adult and junior soccer players. However, research on MAS in children and adolescents remains limited (Baker and Heaney, 2015; Clemente et al., 2020).

Although our study was not focused on the relative age effect (RAE), we observed over-representation of players born in the first two quarters of the year in both groups. This suggests that RAE should be considered during players’ recruitment, selection and evaluation, especially given its prevalence in soccer at various training levels (Helsen et al., 2005; Yagüe et al., 2020).

Monitoring and analysing changes in aerobic capacity from initial training stages is of significant predictive value for talent identification, performance prediction and training optimisation (Coppola and Raiola, 2019; Raiola and D'isanto, 2016). While using it solely for selection has limitations due to the multifaceted nature of athletic development, understanding a young player's aerobic potential, especially VO_2max with its strong genetic influence, can be a valuable tool for identifying promising talent (Calandro et al., 2020; Ceruso et al., 2019). This is particularly relevant considering the demands of professional soccer and the importance of appropriately managing training loads (Mohr et al., 2003; Sargienko and Starosta, 2003).

One limitation of our study is the small sample size, which may affect the generalizability of our findings. Moreover, there are no precise methods for measuring training loads in young children. The child's body reactions to the same training stimuli may vary significantly. This requires further research and analysis.

Conclusions

The findings of the present study allow to indicate that between the age of 8 and 12, physical training is not the primary factor influencing aerobic capacity. This is evidenced by the lack of correlations between applied training loads and VO_2max levels, as well as no significant impact on the anaerobic threshold. However, monitoring initial VO_2max levels remains valuable for identifying individuals with high aerobic potential, often linked to success at elite levels of soccer.

A comprehensive approach is essential, including factors such as MAS, which showed sensitivity to training in this study, highlighting its potential in optimizing training for this age group. Coaches should avoid focusing on increasing aerobic exercise volume and instead prioritize speed, agility, and other high-intensity activities. Developing technical skills through high-intensity exercises is more beneficial during this stage of development.