Originally Posted by Carbonfiberboy

It's called "arousal." That's absent indoors or at least usually. Really loud music is helpful as is having some goals and staying on the rollers or trainer for over an hour. Indoors, warm up for 15', do three 1.5' max efforts, then take it up into Z4. It's not that hard. Once you get it up there, you'll cruise right along. Try holding a cadence well over 90. If you don't need a towel on the floor to absorb all the sweat, you're dogging it. That's why folks are saying usually lower power indoors because of overheating. Two box fans and cycling glasses should be necessary.

The thing is that HR comes up with arousal. Hormonal releases are what drive HR up. That's easy to get outdoors, not so easy indoors until you figure out how to do it.