Apple Watch Series 12 shifts its update toward continuous health sensing.

Apple introduced Watch Series 12 with a new Health Sensing System and S11 chip. The watch measures heart rate every five seconds, adds more frequent heart-rate-variability readings and produces a 0-to-10 readiness score based on activity, training load, vitals and sleep. It starts at $399 in the United States and is due in stores September 18.

Apple says a study of more than 1,000 participants found its new heart-rate sensing more accurate than leading wearables, but that is a manufacturer claim and not a medical diagnosis guarantee. Several software features arrive later, and regional availability differs. The watch remains a connected wearable rather than a replacement for clinical assessment.