Originally Posted by
WhyFi
Nah, Apple almost certainly calculates it independently, as does Fitbit; neither would have the feature if it relied on users to have peripheral gear like a power meter.
Yup. I believe most wearables use an algorithm based on HR response to exercise and a set of population tables for age and sex. They derive a relationship and estimate the max, based on submaximal data. Probably accurate within a few points in most people.