OK here is a thread that we did a month ago on this subject. The issue is simply this don't get hooked on a number. if you are trying to become more fit then make a good periodization schedule and stick to it. Balance is good take this easy schedule-one day do aerobic and the next anaerobic take a day off and then go again. karvonens is not the ideal method and only a doctor given stress test is the best way to use precentages to find that hr+intensity number that is valid enough to say I am working at 156 and that is 70% of my max HR- Question is at that point -due to what you did a couple of days before- is that 156 feeling great or is it like your heart is coming out of your chest?
http://www.bikeforums.net/showthread.php?t=103000


Velocity