The Quantified Scientist's Rob ter Horst compared the Apple Watch Series 12's heart rate sensor against a Polar H10 ECG chest strap in a new video. The revamped sensor measures heart rate every five seconds even when inactive, providing detailed insights into daily activities. Ter Horst found the Series 12 performed very well during indoor cycling with nearly identical readings to the Polar H10, and maintained a 0.98 correlation during running.

For weightlifting, traditionally challenging for wrist-based sensors, the Series 12 achieved a 0.97 correlation, placing it above 85% of devices tested. Step counting was also accurate, with only minor overcounting in manual tests. Ter Horst noted the Series 12 delivered broadly similar heart rate accuracy compared to the Series 11, though the older model performed slightly better during running, suggesting Apple may still be refining its new sensor algorithms.