To assess the physiological demands of including high-intensity efforts during continuous exercise, we designed a randomized crossover study, in which 12 physically active young males executed 3 different exercises in random order: FATmax - continuous exercise at the highest fat oxidation zone; 2min-130% - FATmax interspersed by a 2-minute bout at 130% of the maximal oxygen uptake associated intensity (iV̇O2max); and 20s:10s-170% - FATmax interspersed by four 20-s bouts at 170%iV̇O2max interpolated by 10s of passive recovery. We measured oxygen uptake (V̇O2), blood lactate concentration ([LAC]), respiratory exchange rate (RER), and fat and carbohydrate (CHO) oxidation. For statistical analyses, repeated-measures ANOVA was applied. Although no differences were found for average V̇O2 or carbohydrate oxidation rate, the post-exercise fat oxidation rate was 37.5% and 50% higher during the 2min-130% and 20s:10s-170% protocols, respectively, compared with the FATmax protocol, which also presented lower values of RER during exercise compared with 2min-130% and 20s:10s-170% (p < 0.001 in both), and higher values post-exercise (p = 0.04 and p = 0.002, respectively). [LAC] was higher during exercise when high-intensity bouts were applied (p < 0.001 for both) and was higher at post-exercise during the intermittent bouts compared with FATmax (p = 0.016). The inclusion of high-intensity efforts during moderate-intensity continuous exercise promoted higher physiological demands and post-exercise fat oxidation. Novelty: The inclusion of 2-minute efforts modifies continuous exercise demands. Maximal efforts can increase post-exercise fat oxidation. 2-minute maximal efforts, continuous or intermittent, presents similar demands.
Keywords: HIIT; calorimétrie indirecte; energy substrates; indirect calorimetry; perte de poids; substrats énergétiques; weight loss.