I was asked this question 3 times in the last 3 days, so it must be a common concern. Let’s take a quick look at it.
If you have a power meter and set up your zones based on your FTP using Coggan’s zones and a heart rate monitor which you set up based on your LTHR and my zones you’ve undoubtedly found out that they don’t always agree. (You can find instructions for setting both sets of zones here.) In fact, they may never agree. This is not an issue regarding your heart, legs or devices. It’s just the way it is. Here’s why.
Heart rate zones change very little, if at all, throughout the year. On the other hand, power zones (and also pace zones for running) change significantly. As you get in better shape your FTP rises. This means all of your power zones rise also. When fitness decreases your FTP also decreases and down come your power zones. But all the while heart rate zones remain constant. So there may well be large and small overlaps over the course of a season—or no overlaps at all at some times in the year.
For some reason, this seems to concern many athletes. Look at it this way. If your power zones didn’t change you’d never get any faster at given heart rates. They must change otherwise you will never race any faster.
I hope this makes sense but I suspect there will still be questions so feel free to ask. (If you are concerned about a “high” or “low” heart rate please read this before asking me about it.)