What does it mean when your MHR drops 15-20 beats when you're in shape?

It takes as an input my training history broken down by intensity level and outputs a predicted 10K race time. 11 parameters. I fit to race data using simulated annealing. Chi^2/dof of around 1 using reasonable race time uncertainties. I can email you more details but I don't want to bore everyone here.

the reason your heart rate was not at max is because you were not running as hard as you could...maybe that is why you ran bad?

they say max heart rate is 220- age. 180-175 sounds about right for you.

question asker wrote:

You're right that there can be many problems that limit performance and therefore heart rate, e.g., insufficient carbs, tired legs, etc. After all, speed and heart rate are very nearly linearly related. The question is then why my resting rate plummets after too much speedwork and is always followed by low race MHR. Cutting out the fast stuff for a couple of months tends to give me a better shot at a high race MHR but I'm absolutely convinced that it's these workouts over the years that have provided all my long-term improvement (I have a very fancy model that tells me so unambiguously--I'm a physics/math dork). So I can improve over the long haul but race crappy, or race well but never better... Thanks for the info about your long run pace. And congrats on that 1:50.

Many coaches and athletes have used heart rates as ONE of the variables to help monitor condition. Normally it's the HR at the end of repetitions that is watched closely....If you do 10 x 400 in 62, jogging 200's and the HR's are 180 and 130 at the end of the 400's and 200's....If those rates GO TO 160 and 140 in a workout, chances are you are exhausted. For you it appears that your HR stays low even while at rest, when you're overtrained. Why not reduce the repetition workouts whenever your RHR lowers more than 10% in a short period of time. After it starts increasing again, increase the repetition training again. Normally you want to limit your HR during rep training to no more than 85 or 90% of your MHR (this is at the end of the fast reps). Also make sure your HR recovers to 70% or so, before starting the next fast rep.... I'm under the impression that you are a 5000/10000m racer?

Yeah, 10K is right around my sweet spot. I've tried, as you suggest, reducing the fast stuff when my RHR starts dropping, but I'm usually too close to a race for it to recover. It's probably better to try to avoid it in the first place, but I find that even one hard session/week will eventually bring it on. You make a good point about the max intensity during hard training and if my MHR is 185, say, then I'm definitely overdoing it (getting above 90% of MHR at the end of the session). The problem is that I don't even know how to define my MHR. Trained up, I can't even reach 185. Out of shape, I've hit 205! I'll assume it's 185 and try reducing my intensity to see if I can avoid the RHR slump. The other thing I need to try is doing my mileage very slowly--I tend to go 7-7:15 pace and I should probably go more like 8 (my masters PRs are 16:18/33:35). Very grateful for the advice--thanks.