# Zone 2 Training and Cardiorespiratory Fitness

Aerobic training improves fitness in many settings; evidence for an exclusive below-first-threshold prescription remains limited by exposure definitions and mixed results.

The registered primary-subclaim rule returns Γ from SC2. General aerobic training can improve fitness and the older study has a real positive result. However, its study-defined lactate threshold cannot yet be equated with the registered first threshold. The modern six-week randomized moderate-domain comparison is null. An unverified positive exposure and a verified null cannot establish context-specific causality. The63 originals include intensity comparators, shared-cohort physiology, prognosis and descriptive device validation; they are not63 exclusive below-LT1 trials. Weakest-link returns Β; primary-subclaim and scope-matched return Γ.

An older-adult eight-week positive and a young-adult six-week individualized-domain null map experimental contexts, not a demonstrated age interaction: baseline fitness, dose and horizon differ. All context qualifiers were defined before outcomes. Percentage-HR heart-failure and brief-stroke nulls remain related boundaries with exposure or horizon mismatch; responders are never removed from eligibility.