## 2 Materials and Methods  
**Sampling Protocol and Schedule**: This randomized controlled trial was conducted across five distinct phases from February to June 2024, including baseline testing, a four-week intervention, post-intervention evaluation, standard training continuation, and final assessments. Ethical approval for the study was granted by the Commission for Research Ethics at the University of Physical Education in Wroclaw. Written informed consent was obtained from all participants and their legal guardians prior to enrollment, in accordance with the Declaration of Helsinki and institutional guidelines. This study focused on multidomain adaptations in elite youth soccer players and employed a comprehensive array of physiological, cognitive, and biochemical assessments. Participants were randomly assigned to either a HIIT intervention group (n = 12) or a control group (n = 11). A total of 23 elite male soccer players (HIIT: n = 12; control: n = 11), aged 17 to 20 years (mean \(\pm\) SD: 19.3 \(\pm\) 0.7 years), completed physiological and cognitive testing across three phases (R1, R2, R3). A G*Power 3.1 analysis was conducted to estimate the required sample size for detecting a medium group \(\times\) time interaction effect (f = 0.25) in a 2 \(\times\) 3 mixed-design ANOVA, assuming \(\alpha\) = 0.05, power = 0.80, correlation among repeated measures = 0.5, and \(\varepsilon\) = 0.75. This yielded a required total sample of 28-36 participants. Our sample (n = 23) was slightly below this threshold but remained adequately powered to detect large effects, such as those observed in the Anaerobic Fatigue Index (\(\eta^{2}\) = 0.21, p = 0.035). Goalkeepers were excluded from the analysis due to the distinct physiological and tactical demands of their position; thus, only outfield players were included. The semi-professional participants followed a structured microcycle consisting of five on-field training sessions, two gym-based strength sessions (conducted on Tuesdays and Fridays), and one competitive match held on Saturdays. This study comprised three laboratory testing sessions (pre-, post-, and final), along with ongoing field-based and wellness monitoring. Daily subjective wellness questionnaires tracked sleep quality, fatigue, muscle soreness, mood, and perceived exertion. Athletes were instructed to maintain regular hydration habits, and all laboratory testing sessions were conducted in a euhydrated state,. Recovery was standardized by requiring 48 h without intense physical activity before each testing session. During the four-week intervention, the HIIT group performed twice weekly 30 min motor intermissions consisting of three specific long intervals during which the athletes ran at an intensity between their anaerobic threshold and the psychomotor fatigue threshold determined in the first laboratory session (Figure 1). The anaerobic threshold (ANT) was identified using fingertip blood lactate sampling and is defined as the exercise intensity corresponding to the individual anaerobic threshold, determined using lactate curve analysis following the protocol by [PERSON] et al. (2022) [9]. The psychomotor fatigue threshold (PFT) was determined using choice reaction time (CRT) assessments performed in the last minute of each 3 min treadmill stage. CRT deterioration, as described by [PERSON] et al. (2010) [15], was operationally defined as the first intensity at which a statistically significant increase in response latency or the error rate was observed compared to previous stages. The control group followedstandard preseason training with a similar session frequency but without high-intensity interval exposure. **Training Load Monitoring**: Training loads were continuously monitored using Catapult Vector (Melberne, Australia) GPS systems and heart rate telemetry, capturing internal and external workload indices such as total distance, heart rate exertion, and player load. Each player's session data were phase-coded (R1-R3), enabling within- and between-group comparisons [2,3].