Coupling of and kinetics: insights from multiple exercise transitions below the estimated lactate threshold

During a step-change in exercise power output (PO), ventilation ($$) increases with a similar time course to the rate of carbon dioxide delivery to the lungs ($$. To test the strength of this coupling, we compared $$ and $$ kinetics from ten independent exercise transitions performed within the moderate-intensity domain. Thirteen males completed 3-5 repetitions of ∆40 W step transitions initiated from 20, 40, 60, 80, 100, and 120 W on a cycle ergometer. Preceding the ∆40 W step transitions from 60, 80, 100, and 120 W was a 6 min bout of 20 W cycling from which the transitions of variable ∆PO were examined. Gas exchange ($$ and oxygen uptake, $$) and $$ were measured by mass spectrometry and volume turbine. The kinetics of the responses were characterized by the time constant (τ) and amplitude (Δ$$/Δ$$). Overall, $$ kinetics were consistently slower than $$ kinetics (by ~ 45%) and τ$$ rose progressively with increasing baseline PO and with heightened ∆PO from a common baseline. Compared to τ$$, τ$$ was on average slightly greater (by ~ 4 s). Repeated-measures analysis of variance revealed that there was no interaction between τ$$ and τ$$ in either the variable baseline (p = 0.49) and constant baseline (p = 0.56) conditions indicating that each changed in unison. Additionally, for Δ$$/Δ$$, there was no effect of either variable baseline PO (p = 0.05) or increasing ΔPO (p = 0.16). These data provide further evidence that, within the moderate-intensity domain, both the temporal- and amplitude-based characteristics of V̇_E kinetics are inextricably linked to those of $$.