Oleg,
Great question.
In regards to the intensity balance of 80/20 training, you just have to be aware that all 3 of these tools change your zones. Your Zone 2 with paddles is going to be different than Zone 2 without. To accomodate this, you technically should perform 3 separate threshold tests to come up with three separate zones (one without equipment, one with paddles and buoy, and a third with fins).
This is sort of ridiculous, and instead I would advise that you limit your fin use to the Zone 1 warmup/drills and use perceived effort. Further, don’t separate your paddles from buoy use: keep them together and reserve them for Zone 2 use only, where you will also have to use perceived effort unless you create a separate paddles set of zones, which you can and should do if you plan on using paddles for Zone 3 (which is a high-risk use).
No harm in using fins for 100% of your Zone 1 and paddles for 50% of your Zone 2. Both are effective tools if used in this limited fashion.
David
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