Associations between internal training load and changes in sprint endurance and aerobic fitness during a 4 − week off−season HIIT program in adolescent female football players

: This single-group observational study investigated, in female football players, the associations between internal training load (ITL) metrics and changes in sprint endurance and aerobic fitness following a 4-week off-season training program combining small-sided games (SSGs) and repeated-sprint training (RST). Fifteen adolescent female football players (age: 14.9 ± 1.6 years) completed two SSG and two RST sessions per week, for a total of 16 sessions. Sprint endurance was assessed by the 30-s all-out test (30-s test), while aerobic fitness was evaluated using the Yo-Yo Intermittent Recovery Test Level 1 (YYIR1), both administered pre- and post-intervention. ITL was quantified using heart rate-derived measures, including time spent at ≥ 90% of peak heart rate (T ≥ 90%HRpeak), training impulse (TRIMP), and session rating of perceived exertion training load (sRPE-TL). Significant changes were observed in both sprint endurance (30-s test: p = 0.004, d = 1.88) and aerobic fitness (YYIR1: p < 0.001, d = 1.21). Weekly TRIMP and T ≥ 90%HRpeak showed significant increases throughout the intervention period (p < 0.01), whereas sRPE-TL remained relatively stable. Correlation analyses revealed significant associations (p ≤ 0.008) between ITL metrics and performance changes: sRPE-TL was correlated with changes in the YYIR1 (ρ = 0.94), TRIMP with the 30-s test (ρ = 0.66), and T ≥ 90%HRpeak with the 30-s test (r = 0.87) and YYIR1 (r = 0.67). These findings suggest that, in adolescent female football players, ITL metrics, particularly T ≥ 90%HRpeak, are associated with changes in sprint endurance and aerobic fitness during a short-term, HIIT off-season program. Moreover, sRPE-TL and TRIMP may provide practical indicators for monitoring individual training responses in short off-season periods. However, our findings should be interpreted with caution due to the absence of a control group.