The impact of work-matched interval training on VO_2peak and VO₂ kinetics: Diminishing returns with increasing intensity

High-intensity interval training (HIIT) improves peak oxygen uptake (Vf O_2peak) and oxygen uptake (Vf O₂) kinetics, however, it is unknown whether an optimal intensity of HIIT exists for eliciting improvements in these measures of whole-body oxidative metabolism. The purpose of this study was to (i) investigate the effect of interval intensity on training-induced adaptations in Vf O_2peak and Vf O₂ kinetics, and (ii) examine the impact of interval intensity on the frequency of nonresponders in Vf O_2peak. Thirty-six healthy men and women completed 3 weeks of cycle ergometer HIIT, consisting of intervals targeting 80% (LO), 115% (MID), or 150% (HI) of peak aerobic power. Total work performed per training session was matched across groups. A main effect of training (p < 0.05) and a significant interaction effect was observed for Vf O_2peak, with the change in Vf O_2peak being greater (p < 0.05) in the MID group than the LO group; however, no differences were observed between the HI group and either the MID or LO groups (ΔVf O_2peak; LO, 2.7 ± 0.7 mL·kg–1·min–1; MID, 5.8 ± 0.7; HI, 4.2 ± 1.0). The greatest proportion of responders was observed in the MID group (LO, 8/12; MID, 12/13; HI, 9/11). A nonsignificant relationship (p = 0.26; r² = 0.04) was found between the changes in Vf O_2peak and_τVf O₂. These results suggest that training at intensities around Vf O_2peak may represent a threshold intensity above which further increases in training intensity provide no additional adaptive benefit. The dissociation between changes in Vf O_2peak and Vf O₂ kinetics also reflects the different underlying mechanisms regulating these adaptations.