I have been thinking recently about Mark's suggestions to do low level aerobic training. For the most part, I believe it to be a spot on recommendation that is perfect for most people. There is an inherent problem within in relating to the guess work. The point of low level aerobic training is to use a greater percentage of stored fat for fuel as opposed to stored sugar or glycogen. How can one know when their heart rate is optimal for fat burning without the use of a heart rate monitor. Dr. Phil Maffetone, a long time staple in the endurance world, devised a formula and simple progress measuring system using a heart rate monitor. One simply subtracts their age from 180 and then makes slight adjustments based on health and training. More in depth info can be found at his website, Phil Maffetone, www.philmaffetone.com - Home. It has helped champions win Ironmans and ultramarathons and many regular people have gotten remarkable endurance and fat loss. With this system, one can truly know when fat burn is happening, to what degree, and can better avoid overtraining. With progress, one can be particularly fast at running and still stay aerobic while burning mass amounts of fat for energy and avoiding overtraining. Any thoughts on the topic? Anyone here who has used HR monitors in this way?