International Journal of Clinical Case Reports, 2026, Vol.16, No.2, 84-91 http://medscipublisher.com/index.php/ijccr 86 Both groups trained three times per week under supervision by certified strength and conditioning coaches. Testing occurred one week before (baseline) and one week after the intervention. 2.2 Participants Forty adolescent athletes (22 males, 18 females) aged 14-17 years were recruited from local sports academies (soccer, basketball, athletics, and rugby). Inclusion criteria were: Minimum 2 years of organized sports participation. Currently training at least 3 times per week. Medical clearance for high-intensity exercise. Exclusion criteria included: history of cardiovascular or respiratory disease, musculoskeletal injuries within the past 6 months, or participation in structured HIIT programs in the previous 3 months. Parental consent and athlete assent were obtained in compliance with ethical guidelines for research involving minors. 2.3 Training interventions 2.3.1 HIIT training program The HIIT protocol was designed to progressively increase intensity and volume over 8 weeks as shown in Table 1. Training sessions lasted 25-30 minutes, excluding warm-up and cool-down. Table 1 Training Regime for 8 weeks Week Frequency Work Interval Reps Recovery Mode 1-2 3×/week 15 sec @ 90% HRmax 10 45 sec jog Running-based sprints 3-4 3×/week 20 sec @ 90-92% HRmax 12 40 sec jog Running-based sprints 5-6 3×/week 25 sec @ 92-94% HRmax 14 35 sec jog Shuttle sprints+hill sprints 7-8 3×/week 30 sec @ 94-95% HRmax 16 30 sec jog Sprint repeats+resisted runs Warm-Up (10 minutes): Dynamic stretches, mobility drills, and light jogging. Main Set: Short, maximal or near-maximal efforts (85-95% HRmax) interspersed with active recovery periods. Cool-Down (5 minutes): Static stretching and light jogging. Intensity monitored using heart rate monitors and rate of perceived exertion (RPE≥8/10). Progressive overload (more resistance, shorter rest periods, and longer work) increased the weekly load. Average session duration: ~25-30 minutes (excluding warm-up/cool-down). 2.3.2 Control training program The control group performed traditional training consisting of moderate-intensity continuous running (65-75% HRmax) and short sprint efforts (6-8×60 m), reflecting commonly used conditioning practices in adolescent athletes (Seiler, 2010; Buchheit and Laursen, 2013). Although the total session duration was longer than that of the HIIT group, this difference reflects the time-efficient nature of HIIT protocols (Tomlin and Wenger, 2001; Gibala et al., 2012). Sessions lasted (40-50 minutes) This discrepancy in training volume should be considered when interpreting the results. Warm-Up (10 minutes): Drills for dynamic mobility. Main Set: Endurance: Continuous running at 65-75% HRmax for 20-25 minutes. Speed: 6-8×60-meter sprints with full 2-3 min passive recovery. Cool-Down (5 minutes): Light jogging and static stretching.
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