Purpose:
This study examined the variations in selected aerobic fitness parameters, namely maximum oxygen consumption (VO₂max), running velocity at VO₂max (vVO₂max), running velocity at ventilatory threshold (vAnT) and 1-minute post-exercise heart rate recovery (%HRrec) as well as height, body mass and percentage of body fat (%BF) in high-level adolescent soccer players. The relationship between %BF and VO₂max values was also analyzed.

Methods:
25 athletes (mean age: 14.5 ± 1.3 years) were measured at the second week of the preseason period (1st period) and at the same corresponding week after a 12-month period (2nd period).

Results:
VO₂max, vVO₂max and vAnT increased significantly by 6.6, 6.5 and 3.27%, respectively, from the 1st to 2nd period (p ≤ 0.01 - 0.05; effect size (ES) = -0.35 - 0.66). In contrast, %HRrec remained relatively unchanged (-0.5%) during the same period (p > 0.05; effect size (ES) = 0.01). %BF showed a negative trend, but no significant correlation with VO₂max was found during the same period (r = -0.331; p > 0.05). Significant variations were found for height and body mass (2.64 and 11.17%; p ≤ 0.01; effect size (ES) = -0.56 and -0.72, respectively), but not for %BF (1.34%; p > 0.05; effect size (ES) = -0.27).

Conclusions:
This data can provide useful information regarding physiological aerobic adaptations and changes in physical characteristics as a result of a year of soccer training, including growth and maturations processes, during the specific periods tested.