The calculation of heart rates seems very unreliable. The same applies to the calculation of zone minutes. There have been times when they have stated that I have over 300 zone minutes in a day when it is impossible as I have not exercised on that particular day. I think the concept is very good and initially I gave a very good review on Amazon. However, upon using the software for a while, I do have to say that the numbers are very suspect. Hopefully they will be able to improve the accuracy