Originally Posted by prabbit
I don't have an answer to your question...exactly. I do have a response however. I have the Polar S510 and I found that the Fitness Test sets my max heart rate to 187 (I'm 29 and in decent shape). I've had my heart rate up beyond 187 and to 192 or so. So, it's possible that the max heart rate as determined by the Fitness Test is a bit low.
Also, was this your first Fitness Test? I believe the manual says that you should do several tests in the first couple of weeks and then every other week or once a month thereafter.
I had my first experience with a HRM (Polar s210) this morning and had the same question. I am 57 years old and the Fitness Test pegged my max at 169 but I did 173 during several hill climbs this morning. It appears that Polar uses an age-based formula explained in the attached:
http://www.cptips.com/maxhrsc.htm