Many fitness wearables now display an estimated VO2 max number, but few users understand what it represents or how reliable the estimate actually is.
VO2 max measures the maximum rate of oxygen your body can use during intense exercise, and is a well-established marker of cardiovascular fitness in sports science, traditionally measured precisely in a lab using specialized equipment.
Wearable estimates use heart rate and pace data during your normal workouts to approximate this number algorithmically, rather than measuring it directly — meaning the figure is a reasonable estimate, not a clinical-grade measurement, and can shift somewhat based on factors unrelated to actual fitness changes, like outdoor temperature or hydration during a given run.
The trend over weeks and months is more meaningful than any single day’s number — a gradual upward trend across many sessions is a more trustworthy signal of improving fitness than one isolated reading.