How strong is the evidence for VO₂max Training?

Cardiorespiratory fitness (VO2max) is one of the most extensively validated predictors of mortality in the scientific literature, supported by multiple large meta-analyses spanning tens of millions of observations and hundreds of cohort studies, including a scientific statement from the American Heart Association recommending it be tracked as a clinical vital sign. That evidence is currently rated strong (90/100), built on 6 published studies, including 3 meta-analyses -- enough for real confidence in the effect.