There are limits to how much info I’m willing to give away for free, I like to leave people with some work/thinking to do themselves, but what I am willing to say is that I had read differing views on what types of workout would increase W/bpm, so I tried them all, and only one of them worked (for me). So even if I did tell you what I started doing lots of, it might be that a different type of workout would increase W/bpm for other people, as I find it hard to believe that the people who advocate the stuff that didn’t work for me have it so wrong that they are advocating something that doesn’t have that effect for anyone.
It does depend how fit you already are, though - there are types of training that will increase W/bpm when you are not all that fit, but they stop working before you have fully reached your potential.
you are obviously being purposely obtuse, which is fine, but since i can’t stay asleep, let’s do it for kicks. For starters, it’d be nice to know the lengths of your regimen; was this regimen over a period of a few months or over 1-2 years? Also, you’ve said nothing about which side of the equation changed more. Did you Assuming you are taking a w/bpm over a period of time and not just from a particular day. Personally, most of the time i feel much better when my HR for FTP workouts level out to 1-2 bpm above LTHR than if it were 3-4bpm below, as the latter is more often a result of fatigue than any true improvement in fitness.
To do what you did, you have to either increase the numerator or the denominator. From your saying that your LTHR is not much lower than MHR (how do you even determine this?) and assuming that it hasn’t changed much, I take it that you are probably on the more aerobically fit side of things and very likely are capable of operating at ~90% VO2max for an extended period of time. Furthermore, it means that you can probably also tolerate operating at a high blood lactate concentration.
I’ll take a stab and say that you probably eschewed from anything lower than FTP as those do more to increase enzyme activity at a muscular level than an issue of increasing O2 carrying capacity, which would lead me to think that you probably did a lot of L5 and tabata style of work so that you can squeeze out a higher raw O2 carrying capacity to increase aerobic head space.
Key benefits of L5 include a larger stroke volume, increased capilarization, and increased plasma volume. The first allows you to increase the rate of O2 delivery to the muscles at the same stroke rate (bpm). The second allows for a wider coverage (though my head is too mushy to think about effect on bpm at the moment). And the third would actually increase bpm if all the others are held constant as there’s more to circulate through the body, but is still favorable as it would mean that at the same hematocrit level, you are pushing more RBC through your body. So i say you probably went on a reverse periodization or something similar.
However, all this assumes that you did not reach the raw O2 carrying capacity dictated by your genetic potential (or genetic potential reachable given the available amount of training time). If you had, the only way to get a lower bpm is to drop weight, and the above will need to be thrown out of the window as it’s difficult to lose weight when doing a lot of L5 work.