I started taking Meoprolol 20mg/day about 12 years ago. Started mtb biking again not too long after, after 20 years off the bike. I was given the Metoprolol primarily for high HR (resting in the mid 80s), but also for the benefit of helping with some arrhythmias, including PVCs. About 4 years we went down to 10mg/day because I finally complained about the light-headedness I was experiencing due to the med pulling my blood pressure down to avg 92/60 (I had normal BP before starting the Toprol), and that helped.
My riding was almost all trail riding, unstructured, so I usually had too much intensity. I did learn about HR zones, and almost always rode with chest strap, for obvious reasons, and have gotten very familiar with my HR and zones and how it reacts to varying stimuli.
I never had much limitation on my max HR. I would hit 172 a number of times a year, usually on hot, humid days climbing a steep sandy hill toward the end of a ride. My issue always seemed to be that I didn’t really have any zone 3 or 4. My HR seemed like it was always either at z2 or all out, and was really hard to keep it anywhere in between, like I had no base fitness. I ride year-round and had been at 4-6 hrs per week, so seemed like I should have had a better base than I was experiencing. I just couldn’t seem to improve beyond a certain point (but wasn’t very successful doing z2 to any degree, due to spiking HR at any hills).
I stopped taking the metoprolol a year and a half ago (doctor issues, didn’t refill my Rx), so I went with it. I had stopped for a couple weeks previously to see if I saw any improvement or detriment, and my EP didn’t seem too concerned, so I wasn’t too worried about stopping all together. I also started trying to do some consistent z2 on my stationary bike last year, starting at 30 mins a few times a week, then to 45 mins 4-5x per week, and now I’ve settled in with 1 hour at 3-4x per week (but doing another 3x of higher intensity stuff). Since I quit the Toprol, I’ve seen 174 bpm max on rides several times (so it’s up a couple BPM), and saw 176 once (I think this was an error). My resting HR was mid 70s, and is now in the upper 50s if I get good sleep, 61 if not. My power is up 30% at my preferred z2 HR. Oh, and my LTHR went up from 154 to 157. So, a lot of information, but some combination of stopping the med and building a good base have made a huge difference for me.
TLDR, max HR was 2bpm lower, LTHR is up 3 bpm, z2 power is up significantly, from some combination of stopping Metoprolol and doing more, consistent z2 training, and can’t tell how much is correlation vs causation (sorry, I can’t ever seem to isolate one change at a time).