Apple has published the study behind its claim that the Series 12 and Ultra 4 have the most accurate heart rate sensing in a wearable. Testing ran at five sites in the US and Malaysia over July and August, and 1,254 of the 1,460 adults Apple enrolled produced usable paired readings. Each person wore a Series 12 on one wrist and a rival on the other: a Garmin Forerunner 970, Pixel Watch 4, Huawei Watch 5, Galaxy Watch 8 or WHOOP 5. Participants also wore an Oura Ring 5 on an index finger. A Polar H10 chest strap was the reference. Across 159 comparisons, the Series 12 came out statistically better in 139.
The Mac Observer found that at launch, Apple backed the claim with a single line: more than 1,000 participants over July and August, against leading competitors it didn't name. There was no protocol and no results. The study now has both. Testing covered outdoor runs, treadmill intervals, cycling, HIIT and strength training, plus everyday stretches like desk work, cooking and walking. It also went after cadence lock, where an optical sensor mistakes your running rhythm for a pulse. It's still a study Apple ran on its own watch, and it's about reading heart rate, not spotting a heart condition.