Purpose:
High-intensity interval training (HIIT) can effectively improve health-related physical fitness as a preventive measure against central obesity risk factors in adolescents. This study aimed to examine the effects of a 12-week HIIT program on health-related physical fitness in adolescents by comparing pre- and post-training measurements.

Methods:
The sample consisted of 12 students from Nakhon Phanom University who were enrolled in the applied physical fitness course. The participants, aged 18–20 years, were selected purposively. The training program lasted for 12 weeks, with training sessions conducted 3 times per week (Monday, Wednesday, and Friday) for 80 min per session. The program consisted of running at maximum speed for 40 s, alternating with jogging for 60 s, for 16 rounds on a 400-metre track. The exercise intensity was controlled at 80–90% of the target heart rate, calculated using the Karvonen formula, and the heart rate was monitored with a smartwatch throughout the training.

Results:
The results of the study showed that the effects of HIIT on anthropometric variables – body weight, body mass index (BMI), and percentage of fat (%Fat) – were non-significantly decreased (p > 0.05), while the maximum oxygen consumption (VO₂max) was significantly increased after 12 weeks of HIIT training compared to baseline (p < 0.05). However, the arm and leg muscle strength was not significantly increased compared to baseline after 12 weeks of training (p > 0.05).

Conclusions:
Twelve weeks of HIIT resulted in physiological adaptations that improved body composition, even though there were no statistically significant changes in overall anthropometric measurements. Cardiorespiratory fitness was significantly enhanced, as shown by increased VO₂max in both male and female participants. These improvements are likely due to multilevel haemodynamic and respiratory adaptations, including an increased heart rate, higher cardiac output, and improved lung ventilation, along with elevated mitochondrial density in muscle cells, which enhances the fat utilisation efficiency. Although high-intensity interval training does not significantly improve muscle strength, it fosters neurological adaptations that enhance motor unit function, muscle coordination, and contraction efficiency, thereby improving overall muscle performance.