Muscular morphological adaptations of two whole-body high intensity interval training configurations

BACKGROUND: High-intensity intermittent training (HIIT) has increased in popularity due to being time-efficient mode of exercise. Previous HIIT studies have mainly focused on percentage of fat loss, fat mass loss, and weight loss. However, enhancing muscle protein synthesis induced by HIIT that results in muscular morphological adaptations is a potential benefit of HIIT. This study compared the effects of two HIIT protocols on muscular morphological adaptations. METHODS: Thirty-four recreationally active participants were randomly assigned to 10-5-HIIT and 20-10-HIIT to complete 6 sets of 6 intervals. The 10-5-HIIT and 20-10-HIIT protocols were performed with 10s:5s and 20s:10s exercise-to-rest ratios and provided with 1- and 2-min recovery periods between sets, respectively. Muscle cross-sectional area (mCSA) and echo intensity (EI) of the rectus femoris (RF) and vastus lateralis (VL) were assessed via B-mode ultrasonography before and after intervention. Two-way mixed factorial ANOVAs were used for analyses. RESULTS: The 10-5-HIIT and 20-10-HIIT groups significantly (P<0.05) increased RF mCSA (change (Delta)=0.4 +/- 0.8 cm(2), 8.0%; Delta=0.5 +/- 0.8 cm(2), 5.5%) and VL mCSA (Delta=1.2 +/- 1.6 cm(2), 9.0%; Delta=2.20 +/- 1.4 cm(2), 10.4%), respectively. No significant (P>0.05) change was observed for the EI of the RF and VL. CONCLUSIONS: Whole-body HIIT can be a time-efficient exercise modality to elicit muscular morphological adaptations in the RF and VL muscles. The 10-5-HIIT protocol induced benefits comparable to those of the 20-10-HIIT, while it reduced the total exercise time by 50%.