In one sentence
The most oxygen your body can use in a minute, per kilogram of you, which is the standard measure of aerobic fitness and the one number in VT's fitness area.
Why it matters
Of all the measures a person can change, cardiorespiratory fitness has some of the strongest outcome data behind it. In a meta-analysis of 33 cohort studies, each additional MET of fitness (about 3.5 mL/kg/min) was associated with 13 percent lower all-cause mortality, and the least fit group had 70 percent higher mortality than the most fit (Kodama 2009). It is also the measure that aerobic training moves most reliably over months, which is why Vitality Tracker gives it the whole of the fitness area of the Vitality Score.
What moves it
Upward over months: aerobic training, both steady work and intervals. Downward: detraining, weight gain (the figure is per kilogram, so the same lungs and heart carrying more weight score lower), and age across populations. On a given day the watch's estimate also moves with heat, hills, wind, a cold, and how hard you happened to run, none of which changes your fitness.
How to measure it well
A laboratory test on a treadmill or bike with a gas mask is the measurement; everything else is an estimate from it. A watch estimates it from how your heart rate relates to your pace or power during outdoor runs, walks or rides, so it needs sessions with GPS and a good heart-rate signal, and it improves with more of them on the same device. Type a laboratory result in when you have one: VT marks it as typed so a sync never overwrites it, and the score uses it until a newer value arrives.
How Vitality Tracker uses it
The fitness area of the Vitality Score, 20 percent of the whole, scored as a cardio-fitness level against age- and sex-specific norms (the ACSM table, after Shvartz and Reibold) for ages 20 to 79. A value counts in full for 30 days and is still used for up to 60, after which the area is left out of the score until a fresh value arrives. It also helps the energy-expenditure engine decide whether to treat you as an athlete.
How far to trust the number
Estimate, on a device; Laboratory, from a test. A device's figure tracks your fitness over months on the same device, and that is what the score needs. Its level can sit some way from a laboratory test for an individual, and two makers' estimates for the same person differ, so VT never compares a value from one device with a value from another, and it never compares yours with anyone else's.
Sources
- Kodama S, et al. Cardiorespiratory fitness as a quantitative predictor of all-cause mortality and cardiovascular events in healthy men and women: a meta-analysis. JAMA 2009;301:2024–35. doi:10.1001/jama.2009.681
- Vitality Tracker, What VT measures (catalogue of 15 October 2026).