genejockey 

Okay, so just to be clear, I've been riding varying amounts, at varying levels of intensity and fitness for about 25 years, and for a lot of that time I was using an HRM. I have been fitter than this, and lighter than this, but the whole time, RHR and LT stayed in their same respective ranges - yes, it wasn't an official LT, but rather the "can't talk" point, where you switch from deep regular breathing to rapid breathing. Since I've used that criterion for decades, it makes sense to continue using it for LT. I have experienced big fitness gains before, and what happened was some lowering of RHR, more speed at the same HR, and the ability to sustain effort at higher HRs for longer

So, I know about changes in HR that come with fitness. What I'm saying is this is qualitatively different. I'm not complaining, because I'm going faster than I have in years, which I ascribe to losing 20 lbs, putting in more time on the bike, and pushing harder while I'm on it. It's just a strange thing I haven't experienced in all the time I've been riding. Honestly it feels as if it would be a lot harder to reach, say 174bpm than it was even just 4 months ago, but I'm going faster at 164 than I was at 174. The other thing about this shift in HR zones is that it leaves me going almost entirely by perceived exertion.