Ventilatory thresholds are measurable proxies for metabolic transitions. VT1 corresponds closely to the aerobic threshold and the upper end of Zone 2 — you can still hold a conversation. VT2 lines up with the lactate threshold / FTP — full sentences become impossible. Lab tests identify VT1 and VT2 from expired gas analysis; in field settings, the talk test is a serviceable proxy. VT1 is the most important boundary for base training — riding above it turns easy aerobic work into moderate work and reduces adaptation.