Background: High-intensity interval training (HIIT) is a time-efficient strategy for improving physical readiness; however, the supportive role of melatonin in exercise-induced muscular adaptations remains unclear. This study examined isolated and combined effects of four weeks of HIIT and melatonin supplementation on skeletal muscle mass in military cadets.
Methods: In a quasi-experimental pretest-posttest study, 31 physically active male military cadets were allocated into Melatonin (n=10), HIIT (n=10), and HIIT+Melatonin (n=11) groups. Running-based HIIT group was three times weekly for four weeks, consisted of 30-second sprint intervals at 85-95% maximum heart rate followed by 60-second active recovery periods. Supplementation group received 5 mg melatonin nightly. Skeletal muscle mass was assessed using multi-frequency bioelectrical impedance analysis.
Results: Skeletal muscle mass did not significantly change in the Melatonin group. Significant increases were observed in both the HIIT group (p=0.001) and the HIIT+Melatonin group (p<0.001). ANCOVA provided baseline-adjusted estimates of group differences (p= 0.001); however, these findings were interpreted cautiously because some model assumptions were not fully satisfied. Bonferroni-adjusted comparisons indicated a significant adjusted difference between HIIT+Melatonin and Melatonin groups, while the adjusted difference between HIIT+Melatonin and HIIT did not remain statistically significant.
Conclusion: A four-week HIIT intervention was associated with increased skeletal muscle mass in military cadets. Melatonin supplementation showed no significant anabolic effect. Although the combined intervention produced the largest numerical increase, these findings should be interpreted cautiously because of the small sample size, baseline group differences, absence of a true control group, lack of placebo supplementation, and absence of participant blinding.