Polarized training involves high volumes of zone 1 and zone 2 intensities when doing the easy workouts. For the hard workouts, it involves HIIT, so the ratio is about 80/20. Basically, it means running the easy miles easy enough and the hard miles hard enough. Not much time should be spent in zone 3.
By using the 180 minus age heart rate, the intensity should be closer to the border of zones 2 and 3. That should raise the training stress score for a given duration so that you need less time for the same training effect.
At a heart rate of 180 minus age or less, you should still be in the aerobic zone unless you fall into certain exceptions.
What are the possible disadvantages of staying close to but below the 180 minus age HR?
Update: So far, I see some advantages of going further below 180 minus age.
One might develop the cold or flu and suddenly fall into into one of the exceptions. Staying lower can minimize the risk of overtraining and speed up recovery. Getting sick may be too risky for the athlete's career.
Using a lower HR can increase fat burning which means increased endurance.
- It's easier, more fun, and less stressful for the rider to have a wider margin of error of around 30 BPM than 10 BPM especially if the gradient changes frequently.
- Since heart rate lags power, we might overshoot our effort and burn more glycogen when the gradient changes.
- Our bodies may produce less cortisol at zone 1 or 2 than at the MAF HR so recovery should be better.