Different categories of VO₂ kinetics in the ‘extreme’ exercise intensity domain

The aim of this study was to classify potential sub-zones within the extreme exercise domain. Eight well-trained male cyclists participated in this study. The upper boundary of the severe exercise domain (P_upper-bound) was estimated by constant-work-rate tests. Then three further extreme-work-rate tests were performed in discrete regions within the extreme domain: extreme-1) at a work-rate greater than the P_upper-bound providing an 80–110-s time to task failure; extreme-2) a 30-s maximal sprint; and extreme-3) a 4-s maximal sprint. Different functions were used to describe the behaviour of the V˙O₂V˙O₂ kinetics over time. V˙O₂V˙O₂ on-kinetics during extreme-1 exercise was best described by a single-exponential model (R² ≥ 0.97; SEE ≤ 0.10; p < 0.001), and recovery V˙O₂V˙O₂ decreased immediately after the termination of exercise. In contrast, V˙O₂V˙O₂ on-kinetics during extreme-2 exercise was best fitted by a linear function (R² ≥ 0.96; SEE ≤ 0.16; p < 0.001), and V˙O₂V˙O₂ responses continued to increase during the first 10–20 s of recovery. During the extreme-3 exercise, V˙O₂V˙O₂ could not be modelled due to inadequate data, and there was an M-shape recovery V˙O₂V˙O₂ response with an exponential decay at the end. The V˙O₂V˙O₂ response to exercise across the extreme exercise domain has distinct features and must therefore be characterised with different fitting strategies in order to describe the responses accurately.