Ironboom asked: "On Friday you describe two runs: the first run at MAF; and the second run at low MAF. I was under the impression that MAF was a ceiling and not a range. If the latter, how would you go about calculating the range? Would you mind elaborating? Thanks."
First the short answer: MAF is best utilized if you use a ~10 beat HR range.. MAF minus 10 beats is the range. Working within 10 beats (below) is still very effective in building the correct fitness.
Now the long answer: Your aerobic base is your ability (or lack thereof) to burn body fat as a source of fuel during exercise. By working out below maximum aerobic heart rate (or MAF) you will exploit the time that you train more effectively in regards to building your base.
Here is how we figure the number that represents the upper limit of your effort during your base period.
1. Take 180 and subtract your age.
2. Take that number and correct it by the amount next to the statement that best describes your level of fitness:
a. Subtract five beats if you are recovering from a major illness or injury that has kept you from training for six months or more.
b. Leave the number where it is if you have been working out about two to three days per week for at least a year.
c. Add five beats if you have been working out more than three days per week for at least a year.
d. Add 10 beats if you have been working out more than five days per week for at least five years without recurring colds, illnesses, injuries or long periods of burnout.
e. If you are older than 55 years old or younger than 25 years old, add an additional five beats to whatever number you have right now.
The number that I use is 180-36=144... +10 beats because of the number of years I have trained consistently. All of my runs during my base period should be done at a heart rate that is at or below this number. Instead of trying to run exactly at 154 HR which would be impossible- I use a HR range and cap my HR at 154 (note: during my initial "transition" or "adaptation" phase I go easier and use a max HR of 150). The rule of thumb is a 10 beat range so my minimum HR for a run would be 140.. When I say "low MAF " I was running at the low end of the 10 beat range.
MAF- Why this number?
If you go over your maximum aerobic heart rate you switch off your aerobic development (your fat-burning metabolism) and turn on anaerobic metabolism, which uses your stored, and limited supply, of carbohydrates. During base training this is not a good thing to do consistently.
First, once you turn off your fat-burning system it stays shut off for seven to nine hours, even after your heart rate has dropped back down to resting. If you do this consistently you will limit your ability to efficiently burn fat as your primary fuel.
Second, going over your aerobic maximum heart rate causes you to turn on your adrenal system. This is the system that enables people to deal with stress. A little stress is good because it strengthens your entire body and helps your whole system function better. However, if you turn on your adrenal system consistently—which is what happens when you go over your maximum aerobic heart rate—you end up reducing your ability to respond to stress; you build up fat in your cardiovascular system; you reduce the output of DHEA, which is the hormone that enables you to look at life as a good thing; you can become depressed and lose motivation (not a good thing for being consistent with workouts); you can become unable to sleep deeply in the way that is needed to recover your body; and ultimately you will end up not having the reserve necessary when it comes time to race. In other words, you will get burned out and actually lose fitness with regular anaerobic high heart rate training over time. There is a time for the speed work, but the base period is not that time.