Background/objectives Hypoxia compromises training quality, yet high-intensity interval training (HIIT) performed under hypoxia may elicit greater benefits than does normoxic HIIT. However, its effects on pacing and perceived exertion remain underexplored, particularly in moderate-altitude distance runners. This study aimed to compare the efficacy of HIIT performed at low (~1220 m) versus moderate (~2850 m) altitudes on pacing and rated perceived exertion (RPE) during a 5000 m race. Methods Forty-two moderate-altitude inhabitant runners (men/women, 23/19) were randomly allocated into the HIIT groups at ~2850 m (HIIT2850m, n = 14) and at ~1220 m (HIIT1220m, n = 14), and the control group at ~2850 m (CG2850m, n = 14). The HIIT intervention was completed by the HIIT2850m group at moderate altitude and the HIIT1220m group, which travelled to low altitude for 8 weeks with 2 sessions.wk-1. Each session consisted of 4 × 4 min intervals at 100% velocity at maximum oxygen consumption (vVO2 max, determined at their respective training sites), with 3 min recovery at 70% vVO2 max intensity. Before and after the intervention, 5000 m races were performed in a matched group on a 400 m track at ~2850 m and every km speed and RPE were recorded. Results The results indicated that the participants used a similar parabolic reversed-J shaped strategy, with significant 5000-m time improvements observed in both the HIIT1220m (Delta: 20.1 ± 23.7 s, p = 0.007) and the HIIT2850m (Delta: 16.7 ± 25.9 s, p = 0.003) groups. A significant main effect of time (except at the 2nd km, p = 0.693) and a main effect of group (at the 1st km, p = 0.034 and 5th km, p = 0.044) were observed in pace, along with a significant group-by-time interaction in all km segments. Compared with the CG2850m, both interventions demonstrated significantly greater speeds at the 1st and 5th km and lower RPE, with greater changes in the HIIT1220m group. Conclusion The results suggest that HIIT at both altitudes improved performance and RPE, with quality training at low altitudes producing greater benefits, offering strategic insights for athletes and coaches.
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