Figure 3. Panel A. The ventilatory equivalents for oxygen (V̇E/V̇O2) and carbon dioxide (V̇E/V̇CO2). V̇O2θ, the first threshold, is identified by the dual criteria of a systematic increase in V̇E/V̇O2 without an increase in V̇E/V̇CO2. V̇O2θ, the second threshold, is identified by a systematic increase in V̇E/V̇CO2. Panel B. End-tidal gas tensions for oxygen (PETO2) and carbon dioxide (PETCO2). V̇O2θ, the first threshold, corresponds with the requirement that PETO2 systematically increases while PETCO2 does not decrease. Note that V̇O2θ is identified at V̇O2 = 1.40 L/min. V̇O2θ, the second threshold, corresponds with the requirement that PETCO2 systematically decreases.