Aerobic capacity and V ˙ O 2 kinetics adaptive responses to short-term high-intensity interval training...

This study investigated the physical fitness and oxygen uptake kinetics (τ$\dot{\text{V}}{\text{O}}_{2p}$) along with the O₂ delivery and utilization (heart rate kinetics, τHR; deoxyhemoglobin/$\dot{\text{V}}{\text{O}}_2$ ratio, ∆[HHb]/$\dot{\text{V}}{\text{O}}_2$) adaptations of untrained female participants responding to 4 weeks of high-intensity interval training (HIIT) and 2 weeks of detraining.

Participants were randomly assigned to HIIT (n = 11, 4 × 4 protocol) or nonexercising control (n = 9) groups. Exercising group engaged 4 weeks of treadmill HIIT followed by 2 weeks of detraining while maintaining daily activity level. Ramp-incremental (RI) tests and step-transitions to moderate-intensity exercise were performed. Aerobic capacity and performance (maximal oxygen uptake, $\dot{\text{V}}{\text{O}}_{2max}$; gas-exchange threshold, GET; power output, PO), body composition (skeletal muscle mass, SMM; body fat percentage, BF%), muscle oxygenation status (∆[HHb]), $\dot{\text{V}}{\text{O}}_2$, and HR kinetics were assessed.

HIIT elicited improvements in aerobic capacity ($\dot{\text{V}}{\text{O}}_{2max}$, + 0.17 ± 0.04 L/min; GET, + 0.18 ± 0.05 L/min, P < 0.01; PO-$\dot{\text{V}}{\text{O}}_{2max}$, ± 23.36 ± 8.37 W; PO-GET, + 17.18 ± 3.07 W, P < 0.05), body composition (SMM, + 0.92 ± 0.17 kg; BF%, - 3.08% ± 0.58%, P < 0.001), and speed up the τ$\dot{\text{V}}{\text{O}}_{2p}$ (- 8.04 ± 1.57 s, P < 0.001) significantly, extending to better ∆[HHb]/$\dot{\text{V}}{\text{O}}_2$ ratio (1.18 ± 0.08 to 1.05 ± 0.14). After a period of detraining, the adaptation in body composition and aerobic capacity, as well as the accelerated τ$\dot{\text{V}}{\text{O}}_{2p}$ were maintained in the HIIT group, but the PO-$\dot{\text{V}}{\text{O}}_{2max}$ and PO-GET declined below the post-training level (P < 0.05), whereas no changes were reported in controls (P > 0.05). Four weeks of HIIT induced widespread physiological adaptations in females, and the majority of improvements were preserved after 2 weeks of detraining except for power output corresponding to $\dot{\text{V}}{\text{O}}_{2max}$ and GET.