Cardiorespiratory and metabolic stress responses to acute high-intensity interval training anchored to critical power or maximal heart rate.

High intensity interval training (HIIT) involves repeated bouts of relatively hard work, commonly at intensities eliciting ≥ 80% of maximal heart rate (HRmax), interspersed with recovery periods. Anchoring intensity to HRmax can elicit a wide range of acute metabolic responses to exercise. Expressing intensity relative to metabolic thresholds such as critical power (CP) may reduce this variability. We therefore examined whether anchoring HIIT to CP reduced variability in change in [blood lactate] (ΔBLa−) compared to HRmax-based approach. Nineteen adults aged 23 ± 4 years completed two 4 × 4-min HIIT trials in a randomized, crossover manner at intensities equal to CP + 10% of work prime (CPHIIT) or ≥ 80% HRmax (HRHIIT). Variability in [ΔBLa−] from rest to exercise was not different between CPHIIT and HRHIIT (1.37 (0.42–1.62) vs. 1.32 (0.77–1.97) mM; p = 0.75). Workload was higher in CPHIIT vs. HRHIIT (192 ± 39 W vs. 180 ± 43 W; p = 0.001), as was exercise oxygen consumption, ventilation, respiratory exchange ratio, heart rate, and rating of perceived exertion (all p < 0.05). A CP-based HIIT protocol did not reduce variability of change in [ΔBLa−] compared to a traditional approach anchored to %HRmax. However, anchoring HIIT intensity to CP resulted in participants achieving higher workloads, eliciting higher cardiorespiratory and perceived stress which could translate to divergent training-induced responses.