Zone 4 intervals

Zone 4 intervals are something that I've recently started doing, based on Joe Freil's Heart Rate Training book.  I'm sure most of you are wondering what the big deal is, but I've got to admit this this is something new for me.

I can see how these are effective.  I just finished a 45 minute workout with 4 6 minute work intervals at my Zone 4 heart rate.  I don't know yet if these workouts are improving my fitness, but if the phrase "no pain, no gain" is even partially true, I'll be getting a lot stronger soon.