Apple says the Series 12 and Ultra 4 have the most accurate heart rate sensor on the market, based on its own study with a Polar H10 chest strap as the reference. Tom's Guide senior writer Nick Harris-Fry tested the claim by wearing the Series 12 alongside a Garmin HRM 600 strap for more than 100 miles of running, a half marathon, an indoor ride, two strength sessions and a bodyweight HIIT circuit. On runs the watch stayed closely in line with the strap once it locked on, and on the bike its average heart rate of 109.97 landed within a fraction of the strap's 109.89.
The watch did gray out its reading briefly at the start of some runs. On a treadmill run it added a few minor errors that nudged its average up. In the gym it lagged the strap and missed one small jump, though it caught most spikes and every HIIT interval. Harris-Fry says it performed well but isn't a marked improvement over other watches he has used, and that recent Garmins with the Elevate v5 sensor are at least as good. If you train by heart rate, especially in HIIT or strength sessions, he says a chest strap may still be worth buying.