A fast-start strategy, characterized by higher-intensity efforts during the initial work intervals, in high-intensity interval training (HIIT-FS) have been shown to optimize time spent at high oxygen uptake levels in endurance sports, but their effects in team sport athletes remain unexplored. This study aimed to compare the physiological (gas exchange, heart rate), perceptual responses (Rate of Perceived Exertion (RPE), and external load responses (Global Positioning System (GPS) derived distance) between a high-intensity interval training protocol using a fast-start strategy high-intensity interval training (HIIT-FS) and a constant-intensity high-intensity interval training (HIIT-C) protocol in academy rugby union players. Eight male rugby players (19.9 ± 2.2 years) from two professional French teams performed three sessions: (1) a 30-15 Intermittent Fitness Test (30-15 IFT) to determine velocity at intermittent fitness test (VIFT) and fitness assessment (V̇O_2peak), (2) a HIIT-C session: 2 x 8 intervals of 30 s at 88% VIFT with 15 s rest between intervals and 3 minutes passive rest between sets, and (3) a HIIT-FS session: 2 x 8 intervals consisting of 4 x 30 s at 98% VIFT followed by 4 x 30 s at 78% VIFT, each separated by 15 s rest and 3 minutes rest between sets. Physiological measures included time spent at or above 90% of peak oxygen uptake (T ≥ 90% V̇O_2peak) and peak heart rate (T ≥ 90% HR_peak), peak heart rate (HR_peak), mean oxygen uptake (V̇O_2mean), mean percentage of peak oxygen uptake (mean %V̇O_2peak) and peak heart rate (HR_peak), mean minute ventilation (V̇E_mean), and mean respiratory frequency (f_Rmean). External load variables were total distance, distance ≥ 7 km·h^-1, distance ≥ 16 km·h^-1, and mechanical work distance. Perceptual response was assessed using RPE. HIIT-FS significantly increased T ≥ 90% V̇O_2peak (318.8 ± 138.9 s versus 230.6 ± 115.6 s; d=0.88; p < 0.05) and mean % V̇O_2peak (86.1 ± 3% versus 82.1 ± 3.2%; d = 1.54; p < 0.05) compared to HIIT-C, primarily during the first four intervals of each set, where higher intensities (98% VIFT) were prescribed. No significant differences were observed in external load metrics between protocols. Regarding heart rate responses, only T ≥ 90% HR_peak during intervals 1-4 of set 2 was significantly greater in the HIIT-FS protocol (129.8 ± 24.0 s vs. 109.0 ± 34.0 s; d = 0.90; p < 0.05). Perceptual response (RPE) was also significantly higher following HIIT-FS (9.0 ± 0.5 vs. 7.8 ± 0.7; d = 1.80; p < 0.05). HIIT-FS increases time spent at high V̇O₂ levels in rugby players without increasing external load, making it a promising training strategy to improve aerobic capacity. However, due to its higher perceived exertion, HIIT-FS may be more appropriate when only a limited number of sets can be performed (e.g., two), and should be balanced with classic HIIT protocols when session volume allows more time. Future research should investigate its long-term adaptations and applicability in different team-sport populations.