Day 7

I am Ninja!  This morning was Kenpo X,  I really like this workout the only thing is I dont feel like it is as brutal as the others.  While this is good in theory you have to take a step back and remember what we are doing this for.  The next time we workout I am going to use my heart rate monitor to make sure I am in the zone.  Which makes me wonder what is the zone anyway, I am sure I missed it somewhere in the p90x book, but does anyone know how to calculate your optimal heart rate?