Where your numbers come from

1

Where do your numbers come from?

Mostly Apple Health, where your watch and apps save readings. A Polar link, imported files and a heart-rate strap add a few more.

Apple Health is the place on your iPhone where watches and apps save their readings. Aera asks your permission to read from it. The list under What Aera reads in Settings names sleep, heart rate, HRV, steps, workouts and blood pressure, and Aera reads other readings too, such as blood oxygen, breathing and wrist temperature.

Add a device, a page under Devices in Settings, lists Apple Watch, Garmin, Polar and Another device. A watch that comes only through Apple Health says Arrives through Apple Health. If it has sent nothing, its page says Nothing from it yet, and for a Garmin it adds: In Garmin Connect, turn on Apple Health.

So a Garmin watch reaches Aera the way every other watch does, through Apple Health. Aera sees what Apple Health holds, plus what comes in by the routes below. A reading that stays inside another app never arrives.

2

What does a Polar link add?

Nights and workouts straight from your Polar account, on devices that record them. You sign in once on Polar's own page.

A Polar link is a direct line from your Polar account to Aera. You sign in once on Polar's page, and new nights and workouts then come in on their own. Only some Polar devices record sleep, among them the Pacer, Vantage, Grit X and Ignite watches and the Loop and Unite bands. For an older Polar, the link brings workouts only, and Aera says your sleep keeps coming from Apple Health.

The sign-in goes through a small login helper that only swaps your sign-in code for keys. Your readings go from Polar to your phone without passing through an Aera server. The article What stays on your phone tells that in full.

What arrives stays on your phone. Nights are kept in a file in the app's storage, and workouts are saved into Apple Health beside your other workouts. When you unlink, Aera asks whether to keep them. Unlink and remove workouts takes out the workouts, routes and nights Polar brought in, also from Apple Health.

3

How do old workouts and heart-rate straps fit in?

Settings can import old workouts from files. When you start a workout on your phone, the Heart row names who reads your heart.

Import workouts in Settings takes Strava, Garmin or Polar export zips, or FIT, GPX and TCX files. Aera adds only what Apple Health does not have. If Apple Health holds a workout without a GPS track, Aera keeps the track from the file.

On a workout you start on your phone, the Heart row says who will read your heart. The watch, when the run is on the watch. Otherwise a Bluetooth heart-rate strap that is connected, and otherwise AirPods. Aera asks about Bluetooth only when you tap the Heart row to look for a strap.

A chest strap is worth naming for a reason. Cleveland Clinic researchers had 50 healthy adults exercise on a treadmill, a stationary bike and an elliptical trainer, wearing a chest strap and wrist monitors, with each compared against an ECG. Across all the exercises, the strap agreed with the ECG almost perfectly, scoring 0.996. The Apple Watch scored 0.92 and the Garmin Forerunner 235 scored 0.81. The authors advised chest monitors where accuracy is imperative.¹

The Heart row lists a connected strap ahead of AirPods. That order is Aera's choice, and the test above did not include AirPods. It compared a strap with wrist watches, in a study published in 2017, so newer watches may do better. How a wrist watch reads your pulse is told in How a watch measures you.

4

What happens when two sources report the same thing?

Aera shows one copy: workouts go by heart-rate evidence, then source, then detail, and resting heart rate and HRV take one source a day.

If more than one device recorded your run, it is still one run in Aera. The Devices page says it under Also here: Shared runs count once. Aera picks the copy to show in a fixed order.

First comes measured heart-rate evidence. A copy with a full trace of heart samples beats one with a handful of stray ones. Aera compares this only once it has counted the samples in every copy. Next comes the source. Apple and Aera rank first. Makers of watches and sensors such as Garmin, Polar, Oura and Coros come next, then Nike and Peloton, then sources Aera does not know. Last come apps that copy a workout someone else recorded, such as Strava. If a tie is left, the copy with more detail wins, with heart rate counting most. A fixed tiebreak settles the rest.

The ranks and the weights are Aera's choice, not taken from a study. Because the heart-rate count can be missing for a copy that Aera has not read yet, a workout can change copy after a later sync.

Resting heart rate and HRV work differently. For each day, Aera groups that day's readings by who wrote them and keeps the group from the highest-ranked source, using the same ranking as above. If sources tie, the one with more readings wins. Apps can measure HRV in different ways, and mixing them within a day had been throwing Recovery off.

A Polar night joins Apple Health's sleep records as one more source. A watch that records stages itself still wins the night on trust. Polar's heart and breathing rates fill the night only where Apple Health has none. Polar's HRV stays out and is not used, because it is a different kind of HRV from Apple's.

5

When does a number show nothing?

When no source has sent it. Aera can only read what Apple Health holds, or what Polar or an import has brought in.

A device you have just set up often has nothing to show. The Devices page says Nothing from it yet. A Polar strap such as the H10 cannot be linked, because its heart rate already goes through Apple Health, so the page points you to Polar Flow: In Polar Flow, turn on Apple Health.

The Heart row can come up empty too. With a phone workout and no watch, no strap and no AirPods, nothing will read your heart. The start sheet then says: Without heart rate the run still saves in full.

6

What can't it tell you?

Which device is right, or whether a reading is accurate. Aera chooses by its own rules and cannot check a copy against your body.

A source's rank is a judgment about its maker, not a measurement of your workout. Aera cannot know which copy is closer to what your body did. Heart-rate evidence only shows which copy holds more heart samples.

Whatever the source, a number is only as good as the sensor behind it. If a reading worries you or you feel unwell, talk to a doctor.