Vol. 7, Issue 1, Part A (2025)

Background: Soccer demands robust aerobic and anaerobic capacities to sustain high-intensity running, sprinting, and change of direction (COD) actions, particularly for youth players developing competitive fitness. Enhancing these capacities in-season is critical for performance and progression in amateur settings. 
Aim: This study investigated the effects of an 8-week high-intensity interval training (HIIT) protocol with COD on aerobic (Yo-Yo Intermittent Recovery Test Level 1 [YYIR1]) and anaerobic (Running-based Anaerobic Sprint Test [RAST]) capacities in U-17 male soccer players. 
Methods: Twenty-five male players (age: 16.0±0.5 years) from the “Albanët” Football Team, Albanian Football Federation, participated. Pre- and post-intervention assessments included anthropometric measurements (height, weight, BMI), YYIR1 (VO2max, distance), and RAST (anaerobic capacity: sum of six sprint peak power outputs [PPOs]). The 8-week intervention (2 sessions/week, 70 s at 105% maximal aerobic speed, 1:5 work-to-rest ratio, 840 m/session) incorporated 45° and 90° COD on a FIFA-standard artificial pitch. Paired-sample t-tests assessed changes (p<0.05), with Cohen’s d for effect sizes (small: 0.2, medium: 0.5, large: 0.8). 
Results: VO2max increased significantly (44.6±2.1 to 47.8±2.4 ml/kg/min, +7.2%, p<0.001, d=1.42), as did YYIR1 distance (966.0±235.7 to 1314.0±265.6 m, +36.0%, p<0.001, d=1.37) and RAST anaerobic capacity (3055.7±324.5 to 3303.8±346.8 W, +8.1%, p<0.001, d=0.74). Anthropometrics remained stable (p>0.05). 
Conclusions: The 8-week HIIT protocol with COD significantly enhanced aerobic and anaerobic capacities in U-17 soccer players, offering a practical, low-volume strategy for in-season training in soccer academies. These improvements support enhanced match performance, particularly in high-intensity running and repeated sprints.