Seems like they tested every other factor though: https://support.apple.com/en-us/HT204666 https://support.apple.com/en-us/HT204666 They do explicitly note tha

Seems like they tested every other factor though: https://support.apple.com/en-us/HT204666 https://support.apple.com/en-us/HT204666 They do explicitly note that you may need an external heart rate sensor band in some conditions if the pulse oximeter isn't working for some reason.