## 1 Introduction  
Elite youth soccer players must meet substantial physical demands. In a typical game, adolescent players cover \(\sim\)8-9 km, with roughly 20-22% of that distance at high speed (13-18 km/h) and frequent sprints [1]. To sustain such intensity, both aerobic and anaerobic energy systems are taxed continuously throughout the match. Consequently, comprehensive conditioning is required to develop endurance, repeated-sprint ability, speed, and power in concert. High-intensity interval training (HIIT) has become a cornerstone of soccer conditioning for this reason. As [PERSON] and [PERSON] (2013) explain, HIIT (or HIIT) comprises repeated bouts of high-intensity exercise (from brief all-out sprints of \(<\)45 s to longer 2-4 min intervals) interspersed with recovery [2]. An optimal HIIT session is designed to accumulate several minutes above \(\sim\)90% of VO\({}_{2}\)max (\"red zone\") per session, since this high-intensity stimulus elicits both central (cardiac output) and peripheral (muscle oxidative) adaptations [3]. High-level youth soccer demands the development of a broad range of skills and fitness qualities, often with limited training time and busy schedules. Empirical data support its effectiveness in adolescents: compared to alternative training, HIIT interventions typically produce larger gains in aerobic fitness (e.g., mean VO\({}_{2}\)max improvements of \(\approx\)7% vs. \(\approx\)4%), even though the total training volume may be lower [4]. Notably, studies on junior players have found that small-sided game drills (sport-specific intervals) and generic running HIIT elicit comparable improvements in aerobic capacity [5], suggesting that the HIIT principle (high intensity with recovery) can be delivered in multiple forms. Recent trials also report that even short-term, progressively overloaded HIIT programs can enhance the change-of-direction speed, the repeated-sprint ability, and the intermittent shuttle-run capacity of U-15 players [6]. Despite this progress, most training studies examine outcomes one metric at a time (e.g., VO\({}_{2}\)max, sprint time, lactate). In reality, adaptation is integrative: improvements in cardiovascular, metabolic, neuromuscular, and motor domains occur simultaneously and interact. Recent advances in exercise physiology propose that performance should be understood through a systems-level lens, emphasizing coordination across metabolic, cardiovascular, neuromuscular, and cognitive processes. Emerging frameworks like network physiology highlight that physiological states arise from nested interactions across systems [7; 8]. In line with this, we sought to assess not only univariate improvements but also how key performance variables interact and reorganize under HIIT conditions. To capture multidimensional adaptation, this study evaluated a broad set of physiological and cognitive parameters previously shown to respond to high-intensity interval training (HIIT). HIIT has been reported to reduce blood lactate accumulation and elevate lactate threshold speed, reflecting improved glycolytic efficiency and buffering capacity [9; 10]. Related markers such as base excess and pH recovery also improve post-HIIT, indicating enhanced acid-base regulation, though evidence in youth populations remains limited [3; 11]. Heart rate recovery and kinetics are often positively affected and are linked to improved autonomic control [12]. While speed reserve and anaerobic work capacity (D\({}^{\prime}\)) have not been widely studied in HIIT contexts, they are expected to improve due to enhanced neuromuscular conditioning [13; 14]. Emerging evidence suggests that HIIT may improve cognitive outcomes, including reduced perceived exertion and enhanced executive functioning. Specifically, it may benefit inhibitory control, cognitive flexibility, working memory, and processing speed--key skills for decision-making and tactical responses in soccer.