Estimated starting in 2016 and refined year after since. Part of my methodology here:
It’s not my VT1 or LT1 because I’ve no plans to get tested. It’s my pragmatic lower aerobic threshold where I can go out and do long 8-16 hour rides, wake up the next day and feel normal, and two days later continue training as normal.
Like here, 14 hours into a double century:
That’s about 3-4bpm below my estimated HR at lower aerobic threshold. Easy but not too easy pace. That was on a Saturday, went back to normal training and on Tue or Wed hit a short power PR.
And yes, Xert LTP was relatively close when I used it in 2017.
The heart rate (small range) at that lower threshold hasn’t appeared to change over the last 7 years.