Heart can't keep up...
 

I've been tapering off slightly on my riding lately, down to 75miles/week down from 100+. But I've also been taking Whey protein powder the day after every ride. Long story short, my legs are getting stronger and I'm faster than I was this summer. I'm climbing hillls at 10-14mph and doing 20-28mph on the flats. This is in the windy, hilly San Francisco area, mind you.

The problem is that I hit a new record high HR today, 109% of threshold! I was breathing hard but not about to die. In fact, my legs felt better than ever. My HR has been running into the red zone more than usual and is starting to worry me. I don't think my threshold HR has changed more than 2BPM up or down this year.

Is my strength/speed training outpacing my heart capacity? After months of building both heart conditioning and body strength and resilience, I don't understand why my heart suddenly is falling behind.

Any ideas?

Yeah, your muscles get stronger and more efficient faster than your cardio. Do more tempo workouts and long hillclimbs 20-30 minutes. :)

Sounds like you also need to retest your MHR.