Adaptations to 4 weeks of high-intensity interval training in healthy adults with different training backgrounds

Purpose This study investigated the physical fitness and oxygen uptake kinetics (pi ?VO2) along with the exercise-onset O2 delivery (heart rate kinetics, tau HR; changes in normalized deoxyhemoglobin/?VO2 ratio, delta[HHb1/?VO2) adaptations of individuals with different physical activity (PA) backgrounds responding to 4 weeks of high-intensity interval training (HIIT), and the possible effects of skeletal muscle mass (SMM) on training-induced adaptations.Methods Twenty subjects (10 high-PA level, HIIT-H; 10 moderate-PA level, HIIT-M) engaged in 4 weeks of treadmill HIIT. Ramp-incremental (RI) test and step-transitions to moderate-intensity exercise were performed. Cardiorespiratory fitness, body composition, muscle oxygenation status, VO2 and HR kinetics were assessed at baseline and post-training.Results HIIT improved fitness status for HIIT-H (?VO2 max, + 0.26 +/- 0.07 L/min; SMM, + 0.66 +/- 0.70 kg; body fat, - 1.52 +/- 1.93 kg; pi VO2, - 7.11 +/- 1.05 s, p < 0.05) and HIIT-M ( ?VO2 max , 0.24 +/- 0.07 L/min, SMM, + 0.58 +/- 0.61 kg; body fat, - 1.64 +/- 1.37 kg; pi VO2, - 5.48 +/- 1.05 s, p < 0.05) except for visceral fat area (p = 0.293) without between-group differences (p > 0.05). Oxygenated and deoxygenated hemoglobin amplitude during the RI test increased for both groups (p < 0.05) except for total hemoglobin (p = 0.179). The delta[HHb1/?VO2 overshoot was attenuated for both groups (p < 0.05) but only eliminated in HIIT-H (1.05 +/- 0.14 to 0.92 +/- 0.11), and no change was observed in tau HR (p = 0.144). Linear mixed-effect models presented positive effects of SMM on absolute ?VO2 max (p < 0.001) and delta HHb (p = 0.034).Conclusion Four weeks of HIIT promoted positive adaptations in physical fitness and ?VO2 kinetics, with the peripheral adaptations attributing to the observed improvements. The training effects are similar between groups suggesting that HIIT is effective for reaching higher physical fitness levels.

Automated summary

This study investigated the effects of 4 weeks of high-intensity interval training (HIIT) on physical fitness and oxygen uptake kinetics of individuals with different physical activity backgrounds. The results showed that HIIT improved fitness and oxygen uptake kinetics, with peripheral adaptations contributing to the improvements. The training effects were similar between groups, indicating that HIIT is effective for reaching higher fitness levels.