Mental Fatigue and Performance in Young Soccer Players

The study employed a randomized, parallel-group, between-subjects design in which participants were assigned to either the MF+ or MF- condition and completed a single testing session accordingly. Players were recommended to sleep for at least 7-8 hours, refrain from exercising at moderate to high intensity, and avoid consuming alcoholic or caffeinated beverages during the 24 hours before the intervention. Indoor and outdoor temperatures (25-28°C) and humidity levels (35-40%) were kept consistent across all intervention sessions. All test sessions were conducted on synthetic turf pitches and comprised three stages of testing, which included assessments of aerobic and anaerobic endurance, jumping, agility, and balance. During the familiarization session, players were briefed on the study procedures, practiced the main tasks, and became accustomed to the equipment and environment used during the intervention. During the MF+ session, prior to each physical testing stage, participants completed a 30-minute Color-word Stroop task (CST) to induce MF. Four color words (red, blue, green, and yellow) were randomly printed, and participants were required to verbally name the color of each word. During the control (MF-) session, prior to each physical testing stage, participants watched emotionally neutral documentaries for 30 minutes. Each testing session began with a standardized 15-minute warm-up, consisting of 5 minutes of low- to moderate-intensity running followed by 10 minutes of upper- and lower-extremity-specific dynamic stretching, mobility exercises, and change-of-direction drills. To minimize external influence, players were not prompted or verbally encouraged during testing. Immediately, after each session, ratings of perceived exertion (RPE 6--20) were recorded using the category-level Borg scale, and enjoyment was assessed using the exercise enjoyment scale, a bipolar instrument consisting of eight items scored on a 1-7 Likert scale.