One of the primary tenets of a polarized training program is to separately train the different energy systems by tailoring the intensity of the specific session.  If you are trying to training the Anaerobic Alactic system, then short sprints with long rests allow the athlete to produce more power in each rep and thereby increase the focus on that energy pathway.  Similarly, when doing low intensity endurance training, the idea is to keep the intensity high enough to elicit a training response, but low enough so that the metabolism of fat predominates.

The most direct way to determine the proper training intensity is to measure blood lactate levels.  This is because the metabolism of fat does not produce pyruvate, which is transformed into lactate, whereas the metabolism of carbohydrate (CHO) does end up producing lactate.  The body can use lactate as a fuel, so at a certain exercise intensity, the amount of lactate produce and consumed is at equilibrium and will stay stable over relatively long exercise sessions (60 to 80 minutes at least).

The problem is that routine measurement lactate is expensive, inconvenient and not continuous.  So it becomes desirable to find other measures that can be roughly correlated with lactate that are easier to measure, and more importantly to continuously monitor during an exercise session.

That brings us to this review paper:

The key findings are:

• Fat metabolism is maximized at a higher percentage of VO2Max for fit people than non fit people.
  • 59% to 64% of VO2Max for trained subjects
  • 47% to 64% of VO2Max for untrained subjects

• That the fat metabolism is maximized at different VO2Max percentages for different modes of exercise
  • 58% of VO2Max for walking
  • 64% of VO2Max for bicycling
  • No data about rowing 😦

• That ingesting Carbohydrates immediately before exercise reduces the amount of fat metabolism.
  • At the same percentage of V02Max, fasted subjects nearly doubles the fat metabolism of subjects that consumed CHO before exercise.

• Men maximize fat metabolism at a higher percentage of VO2Max than women

So, one thing that always trips me up is calibrating myself to VO2Max.  The best rule of thumb that I have read is that VO2Max power on an erg is roughly 2K pace or power.  So, if you can row a 7:00 2K (302W).  If maximizing fat oxidation occurs between 59% and 64% of VO@Max power, then the power range for endurance training would be 179W (2:05.0) to 194W (2:01.7).