Quantified Scientist: Apple Watch Series 12 heart rate nearly matches Polar H10

Independent tester Rob ter Horst (The Quantified Scientist) put the Apple Watch Series 12’s revamped heart-rate sensor through indoor cycling, running, and weightlifting — with a Polar H10 ECG chest strap as the reference. 9to5Mac summarized the results.

This is a different job from feature hands-ons: it asks how accurate the new sensor is in workouts, not only what Readiness and 5-second sampling unlock in the Health stack. For the feature tour, see our Series 12 hands-on on health sensing.

Headline correlations

• Indoor cycling: near-identical to the Polar H10 (correlation reported as essentially perfect / ~1.0).
• Running: strong at 0.98, with some minor deviations at higher heart rates.
• Weightlifting: 0.97 — traditionally hard for wrist optical sensors — still described as “quite good,” above ~85% of devices in his weightlifting database.

Series 12 samples heart rate as often as every five seconds even when you are not in a workout, feeding the Health Sensing System and Readiness score.

Vs Series 11 — and caveats

Against his longer-term Series 11 testing, ter Horst said Series 12 was broadly similar overall; the older watch was slightly better during running in his sessions. He floated that Apple may still be tuning algorithms for the new hardware.

He also spot-checked steps (~5,000 manually counted): Series 12 was generally spot-on or only slightly overcounting.

Caveats: one tester’s protocols, not a multi-lab clinical study; wrist fit, skin tone, motion artifact, and firmware can all move results. Treat the correlations as early independent signal, not a warranty.

Bottom line: Early Quantified Scientist data backs Series 12’s new heart sensor as highly competitive with a chest strap in cycling and strong in running and lifting — with accuracy looking evolutionary vs Series 11, not a night-and-day leap.

Sources: 9to5Mac — Marcus Mendes (September 18, 2026); The Quantified Scientist YouTube (via 9to5Mac / MacMagazine).