Effects of a 4-Week Off-Season High-Intensity Training Program on Aerobic Performance and Sprint Endurance Ability in Adolescent Female Football Players: A Pilot Study

: Background: The off-season is often characterized by a significant decrease or even a complete cessation of training. If this reduction is not planned properly, it can result in detraining. Despite numerous studies examining the effects of HIIT in football players, its specific role in mitigating detraining and maintaining aerobic and anaerobic performance during the off-season in adolescent female football players remains underexplored. Therefore, this study evaluated the effects of a 4-week off-season high-intensity training (HIIT) program on aerobic performance level and sprint endurance ability in Under-15 (U-15) female football players. Methods: Fifteen U-15 female football players from a professional club completed an experimental protocol consisting of two HIIT formats: Small-Sided Games (SSGs) and Repeated Sprint Training (RST), each performed twice weekly. Before and after the intervention period, participants performed the Yo-Yo Intermittent Recovery Level 1 (YYIR1) test to gauge aerobic performance and the 30-seconds sprint test to assess sprint endurance. The internal training load was monitored via heart rate (HR) and blood lactate concentration ([La]+), while external training load metrics included the total distance (TD), moderate-speed distance (MSD), high-speed distance (HSD), acceleration distance (≥3 m·s-2; ACC), and deceleration distance (≤-3 m·s-2; DEC). Results: YYIR1 improved by 57% (p = 0.0001; d = 1.12; 95% CI: 121.94-224.71) and the 30-s test performance increased by 13% (p = 0.004; d = 0.91; and 95% CI: 14.46-25.53) following the intervention period. A very large correlation between time spent at 90-95% HRmax and the 30-s test (r = 0.90, p = 0.0001) and YYIR1 (r = 0.81, p = 0.0001) performance was observed. Very large and nearly perfect correlations between DHS and YYIR1 (r = 0.82, p = 0.0001) and the 30-s test performance (r = 0.94, p = 0.0001), respectively, were found. Conclusions: In U-15 female football players, a four-week off-season HIIT program improved both aerobic performance and sprint endurance ability, indicating that a HIIT regime attenuates the off-season detraining, thus supporting a better-conditioned return to play. Coaches may implement 4-week high-intensity off-season programs to enhance aerobic performance and start the pre-season with a satisfactory level of aerobic fitness and sprint endurance.