I was looking up polarised training to see if it had been done with a threshold day instead of a VO2 day, though yes I do get that that would be a bit of a contradiction of the term polarised.
Still I want to know if it can be done and what the improvements would be, what the workload and intervals might look like etc.
Saw this swimming study testing the models using zones 1 to 3. Anyone know what those 3 zones are for swimmers? Not up for reading all that on my phone 
When I was swimming seriously (training for a 10km swim) I used Swimsmooth. What mattered was CSS (Critical Swim Speed), your ability to hold that speed over distance, and stroke rates (I increased mine and it made an enormous difference).
I can’t imagine what these zones are but suspect that a 50m sprinter is working in a different zone to a 200m long course, to a 400-1500m and an open water 5-10km. I would suspect threshold is ability to hold pace for 1500m which is a typical standard distance tri or long course indoor longest swim.
If you want to understand this look at the swimsmooth videos, they are really good.
