This study aimed to investigate the effect of eight weeks of high-intensity interval training (HIIT) plus L-arginine supplementation on performance of highly trained female handball players. Thirty-two female handball athletes (age: 20.69 ± 0.45 years, body height: 169.38 ± 0.57 cm, body mass: 66.49 ± 1.06 kg) were randomly assigned to a placebo (n = 8), a L-Arg (n = 8), a HIIT+placebo (n = 8) or a HIIT+L-Arg (n = 8) group. HIIT was performed 2 days/week for 8 weeks and consisted of running at 90 to 95% of maximum aerobic speed with 15 s of active recovery, with all training sessions performed on a handball court. The L-arginine supplementation was 0.1 g/kg on training days and 0.05 g/kg on rest days. Performance was assessed using a comprehensive battery of tests, including the 20-m sprint test, the T-agility test, the Cooper test, and the running-based anaerobic sprint test (RAST). Both high-intensity interval training (HIIT) and L arginine (L-Arg) supplementation led to significant improvements in anaerobic power and 20-m sprint speed (p < 0.05). However, combining HIIT with L-Arg resulted in improvements solely in anaerobic power, without yielding any additional benefits compared to HIIT alone. Notably, all intervention groups (L-Arg, HIIT, and HIIT + L-Arg) experienced significant declines in agility performance (p < 0.05). None of the strategies improved performance during Cooper test. These findings suggest that L-Arg supplementation during HIIT does not confer additional performance benefits and may even exert detrimental effects. Therefore, HIIT alone appears to be sufficient for enhancing anaerobic capacity in highly trained female handball players, and the use of L-Arg supplementation may be unnecessary or counterproductive in this context.
