Stress-Induced “Immediate” Lactate (iBLC) Response Differences in Pubertal and Young Adult Soccer Players

<b>Background</b>: High-intensity physical activity elicits acute physiological responses across the metabolic, cardiovascular, respiratory, endocrine, immune, and neuromuscular systems. During recovery, multiple processes act to restore homeostasis and functional capacity. The aim of this study was to examine age- and position-related differences in cardiovascular and metabolic responses among youth soccer players. <b>Methods</b>: A total of 147 male academy soccer players were assessed, divided into four age groups (U15, U16, U17, U18/U19) and four playing positions (defenders, midfielders, forwards, goalkeepers). <b>Results</b>: Significant anthropometric and physiological differences were observed between age groups. Body height and weight increased progressively with age (<i>p</i> < 0.05), while body fat percentage was lower in younger compared to older cohorts (<i>p</i> < 0.001). Relative muscle mass did not differ significantly between groups. Absolute aerobic capacity (VO₂max) was higher in U17-U19 compared to U15-U16 (<i>p</i> < 0.001). Heart rate at anaerobic threshold (HRAT) and maximal heart rate were greater in the younger groups (<i>p</i> ≤ 0.005). Immediate blood lactate (iBLC) and ΔiBLC were significantly higher in U15-U16 compared to U17-U19 (both <i>p</i> < 0.001). Position-specific analysis revealed higher iBLC in U15 defenders compared to U18/U19 defenders (<i>p</i> < 0.01), whereas no positional differences were observed in relative VO₂max. <b>Conclusions</b>: These results are due to a carefully designed training program and frequent individual training sessions.