Kirk McElhearn is concerned about changes that Apple has made to the way it describes how the Apple Watch’s heart rate sensor works, specifically that it “won’t record [your heart rate] when you’re in motion or your arm is moving.”
While knowing your resting heart rate is useful, and is a good way of determining your cardiac fitness, you don’t need to do it every ten minutes when you’re not moving; in fact, that is simply foolish in terms of battery life. You do, however, need to record the heart rate when someone is walking, even if they’re not doing a workout, in order to have an idea how active they are. If someone is walking to work, they’re not going to bother to start a workout; and they shouldn’t. But their activity should be counted as more than just steps. For now, that doesn’t seem to be the case.
This seems to have been a change made in the Apple Watch OS 1.0.1 update back in mid-May, as some users have reported noticing more sporadic heartbeat measurements, but Apple only updated the technical document about heart rate measurements this past weekend. It’s unclear why this behavior changed, however, though it’s largely believed to be a matter of conserving battery life. (If you start a session with the Workout app, the sensor will actively record your heart rate.)
While I’m not quite ready to haul out the “bait and switch” moniker, it does seem like this is an issue that needs to be addressed if the Apple Watch is actually intended to be used to track heart rates outside of workouts: How, for example, does the Activity app track Exercise without using the heart rate sensor? Via just the accelerometer and gyroscope? That seems potentially inaccurate, since there are any number of non-exercise activities that could trigger false readings. (This also potentially meshes with some of the head-scratching Exercise readings I got while traveling with the Apple Watch.)
It’s also possible that this is merely a temporary change as Apple refines the heart rate sensor’s behavior in future software updates. Still, it seems like a change that should be better communicated to Apple Watch owners.
I’ve reached out to Apple for a response on this matter, and will update with further information as it becomes available.