Are you on any meds?
I used to train with a guy who was USPro champion in the 90’s, we’d be riding and I’d be slaying myself at 165 or 175 beats a minute, and I’d look over at his heart rate monitor and he’d be at like 130. Some people just have a lower heart rate.
I used to have problems getting my heart rate above 160. That was with efforts that felt like I was going to heave. Now I’m taking blood pressure meds, and my HR rarely goes above 145-ish. I asked my doc about it, and he (rather glibly IMHO) said it was because it wasn’t pushing as hard. I haven’t noticed any problems with performance on these meds, so for now I’m not going to worry about it.
I have given this a lot of thought since I began to receive this council from other TR people. I believe a proper course of action needs to include a doctor that can give me a stress test and provide the medical side of things. However, I should also add to the conversation that perhaps I have not been following Rule 5 (HTFU). I put the bit in my mouth and put the hammer down for 15 minutes last night and got the HR up into the high 130’s. I will try the RAMP Test again and see if I can HTFU and stay pushing the pedals until my body fails and not my comfort level…
As an added insight into the medical side of things - I do an annual checkup which includes a stress test (ECG under effort on a treadmill), and I never reach max HR on that thing. The warmup period is too short (overall test is approx 15 mins), and it’s early in the morning - both factors which keep me away from my max. I do a 20 mins warmup before doing a ramp test, that helps me get going - and I’ve hit pretty much my max every time.
My understanding is that the test only requires you to reach ~90% HRmax.
I was 60% HRmax @10:30min (before aborting); my brother reached his 90% @12min; my 80-year old father get up to 9min before having to stop due to other physical limitations, but never achieves the 90% HR.
The way I understood it was that you go until you can’t anymore - sort of like a ramp test. But I’m sure I get above 90% every time. They don’t stop me then.
I did it once in a hospital, with an ultrasound right after (my doc wanted to check something specific that bothered him), and the Dr overseeing the test was really annoyed at how long it took before I blew up - was used to elderly patients for whom the test is over in minutes. He wanted me out of there really badly.
What are you using to measure your HR while riding? Wrist HRM or chest strap?
If you are certain that your HR is as low as you say during exercise, I would suggest seeing your doctor. If your HR is accurately as low as you say after very strenuous exercise, the last thing I would be suggesting until you get checked over is very strenuous exercise.
I current use the Wahoo Fitness TICKR Heart Rate Monitor. I have used the Garmin Chest Strap for a long time, and decided to try this one a few months back. Another factor that could influence this is I do all of my riding inside. And I do mean all. I rode a double century (Seattle to Portland) in 2016 and when I rode it again in 2017 I had not been on the road bike since the last year’s ride.
It seems like it could be the result of getting to comfortable on the spin bike (Powertap Pedals) and Zwift. Doing a RAMP Test until I decide to stop rather than when my legs shut down might be an issue as well. Going to kick it up a notch and see if the upper end HR will increase as a result.
Unless you really are giving up ridiculously early on a ramp test, a max HR of 125 IS NOT a normal exercise response for a 58 year old with a resting HR in the 50’s. My advise again I’d see a doctor before applying rule 5.
Open link to your strava account would be helpful
If your max HR is, say, 138 from your recent HTFU ride, and your threshold HR is about 115, that would put your threshold HR at 83% of max.
As a comparison, my threshold HR is about 90% of max.
Maybe your FTP is higher than your current setting e.g. if you were to do a HTFU ramp test?
A couple of things to note about pushing yourself to the limit is that it feels awful (in a good way 
), and could potentially be a health risk - so proceed carefully.
Perhaps try another heart rate monitor. I’ve had previous older ones report low - either faulty or low battery.