Hi there, without going into a rabbit hole of why I’m training fasted whilst out on a 2 hour fasted ride the other day I noticed my heart rate was a fair bit lower than it usually would be, one particular stand out moment in the ride is on a hard 5min climb my heart rate stayed a good 30bpm lower than what I would expect it to get to, has. Anyone else noticed low heart rate with fasted riding and if so why does this occur? I should also add I was pretty. Beat heading into the ride so it could also be that I was just wasted before I even went out?
Do you have a power meter and was the output the same as your normal “hard” for that climb?
If yes then I think your HR monitor was having issues.
If no then you were probably just fatigued and/or depleted and couldn’t hit your normal power output.
No power meter on the bike I was on. But my mate I was riding with was at around 450w an we were both in our lowest gear, I’d second guess I was similar power wise - looking back thru strava segments I’ve been up the hill a few times within 5or so seconds of the other day and hr was higher on the other days, I’d like to think my judgement is good when I say I believe my heart rate monitor wasn’t having any issues that day, but yes I’d agree I was fatigued and possibly put it all down to that?
Yes, have noticed it, though not to the extent of 30bpm. More like 5-10. And the delta seems to increase as the workout goes on. I do some longish (~2 hours) easy social/coffee rides fasted. Generally the first 30-45 minutes seem quite hard, with elevated RPE for the intensity. Like my body is trying to warn me that I don’t have a lot of carb supplies. HR can be a little low during that period but not much. Then at some point things start to feel easier and both RPE and HR drop, I guess because the body is kicking over into fat burning mode. RPE drops to what would be normal for a carb-fuelled workout, HR was already normal to low and drops lower still.
But 30bpm seems extreme. I would guess either your HRM is playing up (mine sometimes gets stuck on low HR when the battery is low) or you simply couldn’t put out a lot of power on the climb because your carb stores were so low. I don’t do hard climbs fasted, easy z1-2 only for me!
30bpm was an extreme example on one particular section, what you a have described is exactly how my ride panned out it was what I’d class as a coffee stop ride 
