A simple method to determine the severe-extreme intensity exercise domain boundary using the VO₂ kinetic response

The boundary separating the severe intensity exercise domain, wherein the maximal O₂ uptake ($\dot{V}$O_2max) can be attained, from the extreme intensity exercise domain, wherein $\dot{V}$O_2max cannot be attained, is not widely adopted in applied sports sciences due to the labour-intensive nature of its measurement. The aim of this study was to propose an alternative and practical approach to estimate the severe-extreme domain boundary using $\dot{V}$O₂ kinetics. Sixteen males completed an incremental exercise test and a series of constant power output exercise tests on a cycle ergometer. The relationship between time to achieve $\dot{V}$O_2max and time to task failure during severe-intensity exercise was calculated to estimate the highest power output at which $\dot{V}$O_2max was momentarily attained before task failure (P_UPPERBOUND). Separately, the highest power output that provided a time to task failure that was equal to or greater than four times the time constant (τ) describing the $\dot{V}$O₂ kinetics during severe-intensity exercise (P_4-τ) was calculated. The P_UPPERBOUND and P_4-τ occurred at similar power outputs (379 vs. 380 W; p = 0.62) and were closely related (r:0.99; bias:1 W; SEE:7 W; CV%:1.92%; LoA:-14 W to 16 W). Compared to P_UPPERBOUND, the P_4-τ method provides a simpler and more time efficient approach to identify the severe-extreme domain boundary.