Glenn, this is addressed in our document Intensity Guidelines for Triathlon but I have included the relevant section below for your convenience:
Using Scheduled RT, CT, and STT Workouts to Verify Zones
Because your fitness level and lactate threshold can change quickly, it’s important to keep your zones current throughout the training process by retesting your lactate threshold every few weeks. Repeating your chosen field test in every recovery week (recovery weeks fall ever third or fourth week in our 80/20 Triathlon plans) is the theoretical ideal. As a practical matter, however, this is onerous for many athletes.
Fortunately, your 80/20 training plan includes Swim Time Trial (STT), Cycling Tempo (CT), and Running Tempo (RT) workouts that may serve as zone testing sessions. Most of these sessions feature Zone 3 effort that are less than 30 minutes in duration. Advanced athletes can replace these with the full 30- or alternative 20-minute time trials described above. Another option is to use the “backing in” method of verifying running threshold pace or cycling or running threshold power. Because LTHR changes less than TP and rFTP over the course of a training plan, you can retest either of these variables in the context of CT and RT workouts featuring Zone 3 efforts as short as 10 minutes by adjusting your effort until your heart rate levels off at your previously determined LTHR and observing the pace or wattage that corresponds to it.
Note that CT and RT sessions occur less frequently in the L2 and L3 plans because 1) the high volume of these plans makes frequent high-intensity/high-duration testing risky, 2) we assume advanced athletes have a longer training history and are already confident in their lactate threshold, and 3) advanced athletes tend to experience smaller changes in lactate threshold than do beginner athletes. But if you ever feel you’re “outgrowing” your zones, feel free to insert one of the easier testing options into your next recovery week if it does not already contain a CT or RT session.
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