Introduction:
High-intensity interval training (HIIT) is widely used in team sports, particularly in soccer. Aim of study.

Aim of the study:
This study aimed to examine the effects of a short-term, progressive overload HIIT program on the performance of under-15 soccer players.

Material and methods:
The sample consisted of 30 adolescent soccer players, aged 14, who were randomly assigned to either the control group (CG, n = 15) or the intervention group (EG, n = 15), the latter completing additional HIIT sessions. The intervention program lasted six weeks, with progressive overload implemented every two weeks. Performance indicators, including speed over 10 and 30 meters, change of direction (COD) to the right and left (505 right and left), maximal oxygen uptake (30-15IFT), and repeated-sprint ability (RSA) (RSAtotal, RSAmean, RSAdecr), were measured before and after the training program. Two-way (group × time) ANOVA with repeated measures was used for statistical analysis.

Results:
Significant interactions were observed in the intervention group in 10-m sprint, RSAtotal and RSAmean. Improvements were also recorded in the experimental group in the COD performance (505 right and left) and in the 30-15IFT.

Conclusions:
The findings of the present study suggest that a short-term, progressively overloaded HIIT program can enhance speed, COD ability, aerobic capacity, and repeated-sprint performance.