I have been using the low-volume half distance try plan for training. On my saturday/sunday long rides(2-3.5H) I have been hitting the high end of my target watt range, but by my HR has been hanging around 115-120.
Should I be increasing my watts during the ride to increase my HR, but still stay in the Aerobic zone? I know I don’t want my HR to get into Z3/Z4+
An unexpectedly low HR during exercise could be related to fatigue (over training/under recovery).
For an endurance ride it seems like a heartrate firmly in Z2 is a good thing, it’s also seems normal (for me) for my HR to be lower outside or at least higher power being easier to produce at specific heart rates.
Have you updated your ftp lately? Might just be that your cardio has progressed.
Its hard to say @jlawn if you are comfortably hitting power outside it could be a sign you are just getting fittter, yet on the other hand it could be a sign that your worn out and cant raise it. Then again it might just mean something else. A trained coach might be able to take a look at your training history and take a more educated guess.