This is a complicated question.
I will address 2 questions.
- is a fast HR safe.
- why we think it is fast based upon heart rate zones.
One reason it is complicated is the previously recognized heart rate zones based on age are very inaccurate.
(220-age) may be a good start for a rough estimate of what your max heart rate is but it’s usefulness ends there.
Your max heart rate is what you measured your heart rate to be.
Whether that is safe or not depends on the circumstance and not the number.
The “circumstance” is - is your heart rate still coupled to your power output.
People who have a tachyarrhythmia (fast pulse due to a conduction issue in the heart) are short of breath and feel weak, frequently even at rest.
If, on your bike, when you put out a hard effort and your pulse goes up, that’s good and normal.
As referenced above, when you back off, your pulse should come down.
Your still coupled and this is safe.
The typical HR zones are garbage.
90,80,70,60 percent of your max HR means nothing and are generally much lower than an individual’s actual zones.
These low numbers cause worry because people feel the are over exerting.
You should take a percentage of your variable or adjustable heart rate.
This is (Max HR- resting HR) X percentage (90,80 etc) + resting HR.
Your HR never goes below your resting HR.
When you take 90% of your max HR, you are subtracting a percentage of your resting HR.
You can never drop your HR to Zero (I know, I see it too!) so your HR zones should only be for range of HR between your resting HR and your max HR. You then have to add the resting HR back in.