I've improved my fitness dramatically over the past year, reducing my resting HR by something like 30bpm. I've certainly also improved the power I can put out without pushing my HR up, but when I do push hard, I've set new HR records. My max is at least 10 beats higher than I had thought it was a year ago. When I'm unable to push my HR up, it is always the results of fatigue: either towards the end of long hard ride/race, after a series of hard days, or from a lack of nutrition and rest. That last one really bit me hard once - I was flat out unable to complete a workout. I couldn't get my HR up no matter what; my body just would not respond. Hard lesson on eating and sleeping!

I've learned that, when Hermes offers an opinion, it's a good idea to give it a lot of weight. Of course, he's probably just passing on his uber-fast wife's thoughts. :-)

I'd love to hear what your HR does in response to an intense intervals session. If it zooms right up there, you generate great results, and it pops right back down, then you are not fatigued. If, however, your HR doesn't respond to the intervals, by getting close to the max you've seen in the past, then you know something is "off". I've learned to pay attention anytime my HR doesn't do what I expect it to do. Either direction can be a sign of fatigue, and there is a fine line between maximizing your training, and crossing over into 'dead-cyclist-riding' land.