First some facts about Heart Rate (HR) and how to use the Heart Rate Monitor (HRM) to train.
When you train with your HR above 70% of your Maximum Heart Rate (MHR) you improve your AEROBIC (with air) capacity and your systems ability to store glycogen (muscle fuel).
As your HR approaches 90% of MHR, you begin to go ANAEROBIC (without air). Training at the 90% level of MHR stretches your ability to ride through the pain of lactic acid buildup. Reaching and staying at the anaerobic threshold is a delicate balancing act. The use of a HRM can keep you from going over the edge.
In general:
Training below 70% of MHR = no aerobic improvement.
The ZONE = 70% to 90% of MHR. This is where you want to be to improve your aerobic capacity.
EASY = 60% to 70% of MHR.
MID = 80% of MHR
MHR = 220 - age
Using this calculation for myself, I get:
My estimated MHR = 220 - 30 = 190 bpm
My actual MHR = 197 (07/12/96 on Genoa)
Note: I determined my actual MHR while climbing that NASTY hill in Genoa. It's got to be close to actual, because I feel like I'm gonna die every time I climb this hill.
Therefore (for me):
ZONE = 138 bpm to 177 bpm
EASY = 118 bpm to 138 bpm
MID = 158 bpm