Apple Watch Series 12’s new heart rate sensor tracked almost exactly with a Polar H10 chest strap in the first round of testing by Rob ter Horst, who runs The Quantified Scientist YouTube channel. In his own testing, the watch scored correlations of a rounded 1.0 for indoor cycling, 0.98 for running and 0.97 for weightlifting, the exercise that usually trips up wrist sensors.
It is an independent check on Apple’s headline claim. Apple says Series 12 “offers the most accurate heart rate sensing in a wearable,” based on its own study of more than 1,000 participants. His test is one person over one day, as he stresses, but he has put a long list of wearables through the same method.
How he tested it
Ter Horst, who describes himself as a post-doctoral scientist specializing in biological data analysis, wore the watch on his right wrist and used a Polar H10 ECG chest strap as his reference. His video, posted September 18, covers heart rate, step counting and GPS. He calls it “very initial but systematic testing” and plans a long-term review.
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A correlation of 1.0 means the watch and the strap moved in perfect step. Anything in the high 0.9s is very close.
The heart rate results
| Exercise | Correlation with chest strap |
|---|---|
| Indoor cycling | 1.0 (rounded) |
| Running | 0.98 |
| Weightlifting | 0.97 |
Against every device he has tested, he said Series 12 beat 92 percent of them for indoor cycling and 85 percent for weightlifting. It ranked noticeably lower for running.
Indoor cycling was close to perfect. The watch “tracked all my intervals correctly,” he said.
Running still looked good to him, with minor deviations in the higher heart rate range near the start of his run. He noted that Apple Watches have traditionally struggled with running on his body.
Weightlifting was the pleasant surprise. Many wrist sensors read too low during a set, when the wrist is under tension; he saw no sign of that here. Older Apple Watches tended instead to drop readings at heart rate peaks. On Series 12, during bench press and dips, most peaks were largely caught, though not always in full.
Not clearly better than Series 11
This is the part that tempers Apple’s pitch. Comparing the new results with his long-term data on Apple Watch Series 11, ter Horst said they are “very similar” in terms of correlation, and that Series 11 did a little better at running.
His possible explanation: Apple had years to tune its algorithm for the old sensor and may still be adjusting it for the new one. His impression is that Apple is aiming less at exercise accuracy than at making heart rate sensing efficient enough to run all day. Apple’s announcement fits that reading. It says the Series 12’s larger, power-efficient green LEDs now measure heart rate every five seconds, all day long. He still says he can recommend Series 12 for heart rate tracking.
Steps and GPS
Apple also claims the most accurate step tracking of any smartwatch, and his count backs that up so far. He tallied roughly 5,000 steps by hand in segments of about 1,000. The watch was spot-on or slightly high, with a median difference of about five steps per 1,000 and a worst segment 14 steps over. He has not yet checked for phantom steps during cycling or desk work.
GPS was the weakest result. On a repeated loop he rated it “not great, but also not terrible,” with rounded corners and a track that cut through bushes, especially early in the run and next to a large wall.
What it means if you just bought one
If you bought a Series 12 for training, this early evidence says its heart rate data holds up for cycling, running and even lifting. If you own a Series 11 and wonder whether the new sensor alone justifies an upgrade for workout accuracy, this test does not make that case yet.
For choosing a model, see which Apple Watch suits a runner. If you are waiting on the new Readiness score, it needs several nights of data first; our guide covers what to check if it isn’t showing up.
