Does anyone know what formula is used to calculate % HR for the Polar HRMs?

I mean, after you enter in your max HR, does it calculate percentage straight from that number to get your training zones, or does it use the Karvonen formula ((MaxHR * %) + RHR) or something else.

I think it uses the Karvonen formula, but can't say for sure.

I've put in the question to Polar, but wanted to see if anyone around here knew.

Thanks in advance!