V.O2

Stripped of Jack Daniels' name and authority, the claim amounts to a classic endurance-training principle: an aerobic base built at easy intensity (65-79% of max heart rate per Daniels' formula) precedes and supports harder quality sessions. This is consistent with the polarized training model, where even elite athletes spend roughly 80% of their volume at low intensity. The mechanisms invoked (mitochondrial density, capillarization, cardiac strength) are plausible and documented in exercise physiology, but the Storoschuk et al. (2025) review notes that evidence for this zone's specific superiority over other approaches at matched volume remains limited and mixed. The practical advice — don't skip the aerobic base before jumping into hard sessions — is solid and widely agreed upon among coaches, but presenting one precise percentage of easy training as a fixed scientific rule would go beyond what the literature supports.