Many times I have an athlete tell me, "I had a great workout and pushed real hard on my ride except I couldn't get my Heart Rate up". My first thought is...that's an oxymoron if I ever heard one. After all, how can you push your body/heart real hard and not get your heart rate up? Besides, define "real hard"? Real hard for you...or real hard for the masses? And, what does, "couldn't get my HR up" mean? Up to what? Up past your Tempo zone (L3)? Up past your Lactate Threshold zone (L4) or up to your HRmax?
It's not that I'm doubting these athletes workout...after all, I get to see most of their power files when they're finished so I know they're pushing BIG watts. So, what's up? It turns out that your body is a VERY efficient adaptive learning machine. If you do the same "tough" workout, day-in day-out, or from week-to-week, your body learns to adapt. It's called the "General Adaptation Syndrome". Over time your body actually learns how to ride that favorite hour long route of yours MORE efficiently..without getting stronger. Instead of building stronger muscles you're actually building better neural pathways to allow you to do the same work with less muscle.
So, what's the solution? Stress your body differently, rest it, then stress it again. By altering your workout/ride, your forcing your muscles to grow in new ways. If you have a power meter, look at a previous ride/file and see where you can possibly go harder on a given section. Or, instead of hitting the hill at 70 rpm in a high gear, gear down and spin up the hill at 80 rpm. Just change it up. You can still ride hard/fast...just do it differently at different sections of the ride. Instead of coasting on the downhills and recovering...go HARD downhill and recover on the flats instead. You'll be surprised how much your HR will deviate from previous rides. Your HR will go up...I'll betcha a beer on it. And, I'll bet you that both your cardiovascular system (and pulmonary system) along with your muscles will build up stronger.
Change it up...vary the workout...and get stronger!