Interval Training and HR

[JeniferF]

When interval training, how do you monitor getting "anaerobic" and then resting in between. I use a heart rate monitor, but am not sure how anaerobic translates to what % of maximum heart rate. I've been told you want to be above 85% or even 90% max HR during intervals. How low do you let your HR get between intervals?

I'm an intermediate level exerciser, trying to improve my cardiovascular fitness level (and love using Cathe tapes to do it

Thanks!

Jen