Physiological Correspondence Between Different Indexes of High-Intensity Endurance Exercise in Young Male Runners

Critical speed (CS), the respiratory compensation point (RCP), and the midpoint between gas exchange threshold and maxial oxygen uptake (VO_2max) (i.e., 50%Δ) have been considered indexes able to demarcate the boundary between the heavy and severe exercise domains. However, the agreement between these indexes-and therefore the validity of using them reciprocally-remains to be reported in running. The current study analyzed the agreement between RCP, 50%Δ, and CS. Twelve young runners performed an incremental test to assess VO_2max, RCP, and 50%Δ, with CS estimated by the linear model of time-limited trials at 90, 95, and 110% of the speed corresponding to VO₂max. One-way ANOVA showed no differences when comparing VO₂ and running speed at CS vs. 50%Δ vs. RCP (47.5 ± 4.4 vs. 46.6 ± 4.4 vs. 47.8 ± 4.5 mLO₂∙kg^-1∙min^-1; and 13.9 ± 1.3 vs. 13.7 ± 1.3 vs. 14.0 ± 1.4 km∙h^-1; p > 0.05 for all comparisons). The bias for 50%Δ vs. CS was -0.82 ± 1.55 mLO₂∙kg^-1∙min^-1 and -0.23 ± 0.55 km∙h^-1, and for RCP vs. CS, it was 0.36 ± 1.21 mLO₂∙kg^-1∙min^-1 and 0.05 ± 0.46 km∙h^-1. Therefore, the agreement between RCP, 50%Δ, and CS in estimating VO₂ responses and running speed did not preclude their reciprocal similarity in exercise intensity, although the observed individual variability in physiological variables is a constraint on considering these indexes interchangeable.