Originally Posted by Heathpack

@Carbonfiberboy is correct. It's not the amount of time at 90% max heart rate, it's the amt of time over lactate threshold. The problem with training with HR (as opposed to power), is that heart rate changes slowly, there is a physiologic "damper" on HR, it doesn't change instantly even though your power output might.

Here is an example from some VO2max type intervals that I did this morning. These were 3 min intervals at 115% FTP, followed by 2 minutes at FTP, for a total of 20 min.