Physiological and functional adaptations of high-intensity interval training at two altitudes in moderate-altitude endurance runners: a randomized controlled trial

Abstract

High-intensity interval training (HIIT) under hypoxia has been demonstrated to increase sealevel athletes’ performance, but limited evidence exists regarding moderate-altitude runners. This study aimed to examine 8-week HIIT at low (~1,220 m) and moderate (~2,850 m) altitudes on physiological (maximal aerobic capacity (VO₂max) and velocity associated with VO₂max (vVO₂max)) and functional (5,000 m running time (5kRT)) adaptations. Forty-two trained long-distance runners were randomly assigned to one of three groups: HIIT at 1,220 m (HIIT1220m, n=14), HIIT at 2,850 m (HIIT2850m, n=14) or control group at 2,850 m (CG2850m, n=14). While all participants maintained regular training at ~2,850 m, the HIIT groups completed two weekly sessions (4x4-minute work interval at 100% vVO₂max, 3-minute recovery at 70% vVO₂max). Both interventions had significant improvements over time (p < 0.05) in vVO₂max, VO₂max, and 5kRT, with greater gains in the HIIT1220m (+1.2%, +1%, –1.8%) than HIIT2850m (+0.6%, +0.8%, –1.4%), respectively. In addition, a strong correlation was observed in the HIIT2850m between changes in 5kRT and vVO₂max (r=-0.64, p=0.013) and VO₂max (r=0.74, p=0.002). In conclusion, HIIT at low altitudes induced greater functional improvements than it did at moderate altitudes. Thus, moderate-altitude long-distance athletes and coaches may benefit more when HIIT is strategically managed at lower altitudes.