IMM Training Plans: % of What?

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Those registered for the Monumental Marathon or Half before July 1 were offered free training plans for their chosen race.  A question I have received from Daniel, Missy and others is in regard to the percentages assigned to the quality workouts.  An example workout is mile repeats at 85%.  The basic question was percentage of what?  The basic answer is percentage of effort.
We know that at specific percentages of maximal oxygen consumption (VO2 max) different training effects occur.  Because oxygen consumption is a very elaborate and equipment intense thing to measure we can rely on heart rate due to the very close correlation.
Now I know I just lost many of you because you tried the heart rate monitor thing and hated it because you felt like you were always running too hard (or too easy).  Without going into more detail than the scope of this post I will bet you were using bad predictive numbers based on a bell curve formula.  If we know your actual maximum heart rate and plug it into the Karvonen formula we can get you some useful numbers.  For example a maximum heart of 185 and resting HR of 45 equals a reserve of 140.  If we multiply this number by 85% we get 119 and by adding back the resting 45 beats per minute we get a target HR of 164 and a target zone of 159-169.
Using the right maximum number and this formula help us arrive at a heart rate correlating to VO2.  It is also worth mentioning that significant dehydration will also drive up heart rate numbers due to the decreased blood volume level requiring more beats per minute to deliver oxygen.
I have used heart rate in my own training since 1988 and in my coaching since 1991 and have a great deal of confidence in the benefits.  However, I like it best as part of my "Effort Dashboard" along with pace, perceived exertion, aerobic pressure, and muscular tension.
Used in this context the issues with each of these measurement tools are overcome by their collective strength.