I always thought that, barring some weird heart rhythm, that your max heart rate was the highest HR you can generate. Maybe I'm missing something.
Most people find obtaining their max HR to be an unpleasant process, which is why we don't obtain it very often. When I feel like blowing chow, tossing my cookies, and puking my guts out, I know I'm closing in on it.
__________________
Momento mori, amor fati.