SomviaWhat Apple Watch writes, and when
Apple's Cardio Fitness is an estimate of VO2 max, the volume of oxygen your body can use per kilogram per minute at maximum effort. The watch produces it from outdoor walks, runs and hikes of enough intensity and duration, using heart rate, speed, and the profile you entered. It does not update after every session, and indoor work, cycling and swimming generally do not feed it at all.
That is a real limitation for anyone whose training is mostly indoors, on a bike, or in a pool: the figure in Apple Health can sit unchanged for weeks while the training underneath it changes considerably.
The second estimate, from your own sessions
Somvia estimates VO2 max from the workouts in Apple Health, by two routes, and shows the result alongside Apple's line rather than replacing it:
- From submaximal work. Across your easy and steady running, heart rate and speed hold a relationship. Fitting that relationship and extrapolating it to your maximum heart rate gives an estimate of the speed you could hold at maximum, which converts to an oxygen cost by the standard running equations. It needs no hard session at all, which is why it is the route that usually has data.
- From a hard effort. When a session contains a genuinely hard sustained stretch, the pace and duration of that stretch imply a VO2 max directly, by the equations behind the VDOT tables. This route is more faithful when it is available, because it is built on something you actually ran.
When both exist, the effort-based number takes precedence, and it is also the only input to race predictions: a time predicted from an extrapolation has nothing behind it.
Reading the gap between them
The two lines rarely sit on top of each other, and the distance carries information:
| What you see | Usual reading |
|---|---|
| Somvia above Apple | Recent hard efforts have moved before Apple's slower estimate has caught up, or your training has moved indoors and Apple's figure is stale. |
| Somvia below Apple | The recent record is mostly easy running, so the effort-based route has nothing to work from and the submaximal fit is doing the work. |
| Both flat, training up | Fitness gains are landing somewhere other than aerobic capacity: economy, threshold, durability. The threshold estimate is the better place to look. |
None of these are measurements. A laboratory test with a gas analyser is a measurement; everything a wrist produces is a model fitted to heart rate and speed. Read the direction over months, not the decimal.
Percentile and fitness age
A number in millilitres per kilogram per minute means little without a reference. Somvia places your figure in the FRIEND reference standards for cardiorespiratory fitness, by age and sex, and reports two things: the percentile you sit at, and the age at which your value would be the median, which is what a fitness age is.
Which of the two figures it uses matters. When Apple Health holds a Cardio Fitness value, the percentile is that value's percentile, because that is the number displayed above it; only when Apple has none does the percentile come from Somvia's estimate. Two estimates a couple of points apart can sit several percentile points apart, and reading a percentile taken from one number while looking at the other would be quietly wrong.
Both rows need a date of birth and a biological sex in Apple Health, because the reference standards are specific to both. Without them Somvia shows no percentile rather than averaging the two tables, which would describe nobody.
There is a second reason the Health app and Somvia can disagree about the same figure: they are reading different editions of the same reference. Apple's published methodology cites the 2015 FRIEND standards (Kaminsky, Arena and Myers, Mayo Clinic Proceedings, 2015) for its low cardio fitness threshold. Somvia uses the 2022 update of the same registry (Kaminsky et al., Mayo Clinic Proceedings, 2022), which rests on roughly three times as many tests and adds an age band for 80 to 89. Treadmill values across the 2022 percentiles sit a little lower than in 2015, so the same VO2 max places slightly higher in the newer tables.
Either way the standards come from a large sample of American laboratory tests, so read a percentile as an anchor rather than a verdict.
Where the method comes from
The submaximal route follows the classic heart-rate-to-workload extrapolation with the ACSM running equations for the conversion; the effort route uses the Daniels and Gilbert equations behind VDOT; the reference standards are the FRIEND update published in Mayo Clinic Proceedings in 2022. Full list on how the scores work.
Questions
Why is Somvia's VO2 max different from Apple's?
They are built from different data. Apple's Cardio Fitness comes from qualifying outdoor walks, runs and hikes; Somvia's estimate is fitted to the heart rate and speed of the sessions in Apple Health, including the ones Apple does not use. Both are estimates, and Somvia draws them on one chart rather than picking a winner.
Can I get a VO2 max estimate without an Apple Watch?
You need workouts with heart rate and speed in Apple Health. Any device or app that writes those can feed the estimate. Apple's own Cardio Fitness figure requires an Apple Watch.
How accurate is a wrist VO2 max?
Both figures carry error of a few points against a laboratory test, and the error is larger for people at the extremes of fitness. Treat the trend over months as the signal and the absolute value as an approximation.
Why does the Health app show a different fitness level than Somvia's percentile?
Partly because the two can be reading different figures, and partly because they read different editions of the same reference standards. Apple's published methodology cites the 2015 FRIEND standards; Somvia uses the 2022 update, which rests on roughly three times as many tests and places the same VO2 max slightly higher.
Does cycling or swimming count?
For Somvia's estimate, sessions need speed and heart rate to be usable, which in practice means running. Cycling with a power meter feeds the critical-power estimate instead. Apple's Cardio Fitness uses outdoor walking, running and hiking only.