An improvement in RE after the HIIT program could lead to a lower energy cost during submaximalrunning bouts, which might allow the athletes to achieve higher speeds at the end of the maximal incremental treadmill test. Therefore, it seems that the main beneficial effects of the HIIT program are mediated by a reduction in the energetic cost of running. Taken together, these findings reinforce the suggestion that a HIIT program performed during 4 weeks is an effective short-term strategy to alter some physiological variables related to endurance performance.