@Eugene_M - totally agree with your sentiments. i tried for quite some time to find out if there was any degree of consistency with my sessions and the effort of the sessions and the observed cardio load - i found nothing. hard threshold or vo2 max session ended up with cardio load scores lower than just 35 min stroll or some yard work. 3 hr z2 ride came in with lower cardio load than a 75 min gym session. the required scores to improve fitness was all over the place, regardless of what i had done the day before. taking a day off for R&R seemed to automatically create a "you slacker".
the idea to combine cardio with sleep, RHR, HRV etc is a good one but when the very tool/hardware used to capture the data (fitbit units, e.g. charge) is not very accurate (I have seen average HR for session being as much as 30bpm off with fitbit), everything else falls apart. i think trainingpeaks with their TSS, CTL, ATL has a better approach but they do not capture the rest of your day - and as we know "life happens" outside of your training sessions as well.
my 2cents' worth.
