Can't comment on specifics, only what I notice with myself with inside workouts. My "drift" happens exponentially. I can probably do a 150-155 effort and stay relatively constant for an hour+ but if I try to hold a 165 (95% for me) I will eventually climb to my max and exhaustion unless I slow down to keep the 165. It all seems like internal heating and elevated core temp to me, I am doing more work and heat than my body can remove off. Maybe there is more to it.
Maybe look into how fast your HR drops after stopping or cooling down and research the relationship to fitness and load?
Last edited by u235; 07-02-19 at 07:40 PM.