I'm asking the question but not the naive question the answer to which is "the fastest your heart can beat (under stress)."
My question is: would you just use the highest (known, non-specious) rate you've recently seen? Or is it some other, more frequently-seen number?
More concretely: a few months ago, I came back from a six-month layoff due to an achilles injury. After my return to running, for my first runs I saw heart rates peaking at 185 or so. Saw that number multiple times in multiple runs. That's more or less what I'd expected given my past history. And that's what I've been using as my max in my GPS watch.
However, having now gotten back in reasonable shape, I'm finding that even during HIIT sessions, I never come close to hitting that 185. I'm like 13 bpm lower. And I know that for intervals often the highest heart rate is seen after you've slowed down during the "rest" parts.
My understanding was always that MHR would decline with age but that--unlike resting heart rate--it wouldn't decline as fitness improved: it was basically constant save for the slow decline with age. But now I'm wondering if fitness doesn't lower it after all, perhaps because the heart can pump more blood per beat?
TL;DR: would you use the highest rate you've recently seen (regardless of fitness) or a rate you more commonly see when your effort levels seem to be just about maxed out?