How soon HR back to normal as fitness indicator?

Originally posted by Burchib

Hi all,
I was wondering if anyone had come across articles or research on a specific topic.

My question is: After a moderate to hard workout, is there any data that can indicate fitness levels based on how long it takes for HR to return to normal, or at least Z1.

For example, as a 60 year old, if I work hard up into Z4 or 5, and my workout ends with a HR at 163-165, it takes just about exactly 60 seconds for my HR to drop 30 beats to 133, and after 90 seconds it is at 115. So, just wondering if there are any articles or data that indicate (even roughly) fitness levels based on how fast HR drops and how far it drops-post work out.

Thanks!!