If highly trained female handball players do intense interval workouts, take L-arginine supplements, or do both, their ability to quickly change direction and move agilely gets worse after 8 weeks.

A double-blind, placebo-controlled RCT with 4 groups: (1) high-intensity interval training only, (2) L-arginine supplementation only, (3) combination of both, and (4) placebo/control (no intervention). Participants are highly trained female handball players (n≥30 per group), assessed for agility (e.g., T-test, 5-0-5 test) at baseline and after exactly 8 weeks. Interventions are standardized in frequency, intensity, and volume. L-arginine dosage is fixed (e.g., 6g/day) and administered in identical capsules. Primary outcome: change in agility performance score.

A prospective cohort study tracking 150 highly trained female handball players over 8 weeks, assigning them to one of four pre-defined, non-randomized groups based on their actual training and supplement use: (1) HIIT only, (2) L-arginine only, (3) HIIT + L-arginine, (4) neither. Agility is measured at baseline and week 8 using the same standardized test. Covariates (sleep, nutrition, other training) are recorded and statistically adjusted. Primary outcome: change in agility performance.

A crossover trial where each of 40 highly trained female handball players undergoes all four conditions (HIIT, L-arginine, combination, control) in randomized order, separated by 2-week washout periods. Agility is tested at the end of each 8-week intervention phase. This controls for individual variability and isolates intervention effects.

A case series of 20 highly trained female handball players who all undergo the combination of HIIT and L-arginine supplementation for 8 weeks. Agility is measured before and after. While not comparing groups, it provides preliminary evidence of the effect direction under real-world conditions.

A retrospective analysis of training and supplement logs from 100 highly trained female handball players over the past 2 years, identifying those who used HIIT, L-arginine, or both for 8+ weeks. Agility performance data from pre- and post-intervention testing are extracted and compared.