I <3 Free VO2max testing:
MaxHR is 197bpm, Latic threashold is 188bpm, 90% of MaxHR is 178 bpm. I can ride all day there

until cardiovasular drift happens.
Actaully, to be honest, I've never really tried to see how long I could hold it there. According to tests and research, I should be able to hold that HR until glucose = no mo but, like i said, never tried.