High-Intensity interval vs moderate-intensity continuous training in endurance runners: A systematic review of physiological, biochemical, physical, and biomechanical adaptations

Despite the widespread use of both High-Intensity Interval Training (HIIT) and Moderate-Intensity Continuous Training (MICT) among endurance runners, most systematic reviews over the past decade have examined heterogeneous populations, limiting the applicability of their findings to this athletic group. This systematic review specifically focuses on endurance runners and aims to synthesize current evidence on the comparative effects of HIIT and MICT across multiple adaptation domains. A total of 11 studies were analyzed, covering five key domains: physiological (e.g., oxygen uptake, heart rate variability), biochemical (e.g., lipid profiles, glucose metabolism), physical (e.g., endurance performance, pacing), biomechanical (e.g., stride length, ground contact time), and perceptual (e.g., rating of perceived exertion) adaptations. Both HIIT and MICT contribute to cardiovascular fitness, but HIIT has been associated with faster improvements in oxygen consumption, heart rate variability, and metabolic flexibility, alongside earlier gains in mitochondrial density and fat oxidation. MICT appears to promote more sustained enhancements in lipid profiles and glycemic control, supporting its role in long-term cardiovascular health. Biomechanical adaptations, such as improved running efficiency and reduced ground contact time, have been more frequently reported with HIIT, while both modalities contribute to reduced perceived exertion over time. These findings highlight the complementary benefits of HIIT and MICT for endurance runners and support their integration in periodized training programs to optimize performance adaptations.