What ‘resting heart rate’, RHR ‘is’ is a matter of definition.
if we don’t all use the same definition, any comparison becomes meaningless.
Broadly speaking and paraphrasing there are two definitions you see widely.
One usually says something like ....’taken whilst awake but resting......’ and sometimes says how long you should rest for, sometimes says avoid coffee/alcohol. But that is all quite vague.
Heart rate recovery after exercise is fast in a well trained person - but that refers to the initial recovery from a high rate. My own observations suggest that there is a second component (scientifically, a double exponential decay with a small competent of much longer time constant) which can affect the ‘RHR’ for many hours by a few bpm. So if we use this definition, significant variation of RHR day to day is to be *expected*.
The second definition you quite often see is to take it in bed before getting up, but awake of course. This might well be more reproducible, but it’s a bit inconvenient.
however the Fitbit RHR clearly uses your *sleeping* HR in its calculation, and *NO* definition of RHR I can find does that. That doesn’t make it wrong or invalid; but it *does* mean it’s not valid to compare it to measurements made using another definition/method.
the Fitbit RHR is I think of some use tracking improvements say on at least a month average time scale. Saying ‘oh it’s gone up one bpm today’ almost certainly means nothing, it’s just measurement irreproducibility. But if it averages over a months two or three lower, that might begin to be meaningful at a guess. You would need to do proper statistics over a substantial time period.
it would be far better for Fitbit to explain how they measure it. Measurements made in a poorly defined way greatly reduce their validity.
BTW I’m a professional scientist.
