What VO₂ max is—and what wearables are estimating

The one-sentence answer

Why this came up here

My Garmin is about to start reporting a VO₂ max estimate, and it will land on the Dashboard looking like a precise, single, comparable number. It isn't, quite. Before that number shows up and gets treated as more solid than it is, it's worth being clear about what it actually represents.

The intuitive model

Think of VO₂ max as the combined capacity of a whole supply chain, not one machine: lungs bringing in oxygen, the heart and blood pumping and carrying it, and muscles pulling it out of the blood to make energy. Your maximum aerobic output is limited by whichever link in that chain is weakest under hard effort — usually the heart's ability to pump enough oxygen-rich blood to the working muscles, though muscle-level factors also matter.

What the evidence says

Two things are well established in the exercise-science literature. First, VO₂ max is trainable: structured aerobic training, especially at higher intensities, reliably raises it in previously untrained or moderately trained people, largely through improvements in stroke volume (how much blood the heart pumps per beat) (Bassett & Howley, 2000). Second, cardiorespiratory fitness measured this way is one of the stronger predictors of all-cause and cardiovascular mortality risk across large population studies — strong enough that a 2016 American Heart Association scientific statement argued it should be treated as a routine clinical vital sign (Ross et al., 2016; Kodama et al., 2009). That's part of why this project treats it as a core marker rather than a vanity number.

What a smartwatch reports is not that lab measurement. Garmin's estimate (via the Firstbeat algorithm) infers VO₂ max from the relationship between your pace and heart rate during runs, without ever making you breathe into a mask. It's a legitimately useful trend signal — it moves in broadly the right direction as fitness changes — but it's a model built on assumptions about running economy and heart-rate response that don't hold equally well for everyone, and it tends to be least reliable at the fitness extremes and in people whose running form or heart-rate response is unusual.

What remains uncertain or contested

How closely any given wearable estimate tracks an individual's true lab-measured VO₂ max varies by device, algorithm version, and person — there's no single "the watch is off by X%" number that applies universally. There's also ongoing debate in the literature about how much of VO₂ max's predictive power for health outcomes is really about the heart-and-lungs capacity itself versus what it correlates with (regular physical activity, body composition, and other lifestyle factors). This site won't resolve that debate; it just won't pretend the watch number settles it either.

What's being observed in this project

Garmin's estimate will be tracked over the four months as a trend, clearly labeled as a device estimate on every reading. If a laboratory test gets scheduled, its result will be shown as a separate, clearly marked series — not blended into the Garmin trend line as if the two numbers were measuring the same thing the same way. See the VO₂ max page for the live status, and the measurement methodology for how estimates and measurements are handled sitewide.