HR decreasing during work interval

Long time reader, first time poster. Love all your work everyone (well most of it)

Another question on HR.

Have just come back following a “recovery” week which was as follows:

Monday - Off (beers)
Tuesday - Off (beers)
Wednesday - 45 mins Z2. HR elevated as usual after rest (low plasma etc)
Thursday - Off
Friday - 45 mins sweet spot. HR elevated again
Saturday - 4hr easy endurance outside. Felt like I had actually ridden a bike before after about hour 3
Sunday - Red Lake +8 Felt fine, fueled and cooled well and all business as usual however noticed my HR came up normally during the warm up but then decreased during the work intervals. Did the same for three intervals, rising back up again during the rest periods. Felt fine but skipped remainder and finished with easy spin.

Has anyone experienced this? Is this just signs of fatigue? Why does HR raise normally during the rest and decrease during the work interval?

Or do I just need a new HR monitor?

For reference
ATL 74
CTL 76

That’s either a bad HR monitor or a bad trainer. I assume that the work intervals felt about as hard as they should? If so, I’d blame the monitor.

Something seriously wrong with the readings here as your HR shouldn’t be lower in an endurance period than a threshold one. Depends what you’re using to measure it.
What’s your max heart rate? You should be getting near that on the VO2 max intervals I’d think - especially over that length of interval.

Make sure the skin under the HR strap is moist. Bad contact can cause strange readings.

Yeah thanks guys. That was my thoughts exactly. Intervals definitely felt unpleasant enough. And had never seen or read anything like this happening in any literature.

Must be HR monitor issue. Buying a new one today. This will be my third Wahoo Tickr in 12-18 months…

It’s not exactly the same situation but something that I often experience is decreasing heart rate from one interval to the next s long as I have proper cooling. Using sweet spot intervals as an example, my HR might peak at 155, then 153, then 150, and so on. I’ve also noticed this during long races- my heart rate fell during the course of a marathon I ran last fall even though I held very even pacing (even when I was grunting and trying to get everything out of my legs in the last mile, I couldn’t crack 160HR after I ran the first few miles at 162). Seen the same thing at the end of gran fondos where riding threshold power has a significantly lower heart rate than in the first hour.

Within a long enough interval (20min +), it might happen but at most will be a few bpm. Also I would have to be riding sub threshold- threshold and above on the trainer my heart rate will typically plateau (unless we’re talking really high VO2, where it keeps going up until my legs can’t hold the power). My heart rate is weird so very much N=1.