i tried for quite some time to find some logic and reason to the cardio load, the ranges it suggested as targets, what it could be based on. was it a combination of HRV, RHR, sleep score, CL the previous days, readiness score etc. I never seemed to find any consistency to what it suggested as a range. one day it will tell you aim for 220 - 280 to improve your fitness. when i fail to do so i would just as often get "you are at risk for overtraining" as i were to get "you are maintaining". or if i exceeded the target of x to y, i would just as often "you are risk of undertraning" or maintaining as i were to get "you are at risk of overtraining".
but, the biggest issue, which goes to the heart of what Fitbit is trying to do with these scores, the unit is just not accurate at all. i can see 20 to 30bpm difference in avg HR between HRM on chest vs the fitbit unit. when the gap is this big, it will severely underestimate the cardio load and everything else that follows. it is sometimes said that write-readers are ok at rest or very light movement but useless at more vigorous efforts. i am not sure i can trust it at lower ranges either, say 40 - 80 - 100bpm.
only good use for fitbit as i see it now are the alarms (wake-up) and notifications. cardio load, HRV, readiness score etc seem to be junk science right now. and Google a couple of years ago stated that their strategy was to make an inroad into the "health", health monitoring, where they could leverage all the data they collect from fitbit etc but when the baseline data is erroneous, then there is no foundation to build from there.
I have cancelled my subscription. i will use the fitbit alerts so i wake up at the right time but otherwise - back to my trusted old mechanical Swiss watch now.
