I recommend this too. 30-45 minute steady output is a great way to find your threshold hr, “calibrate” your rpe at threshold power and see if your current FTP is set correctly. On top of that, it’s also a good workout.
If done correctly your hr raises gradually to the threshold value in ~10 minutes or so, stays in the threshold range and towards exhaustion starts slowly creeping up again. After the test, check the flat part of the hr curve and find out the average of that part. This should be very close to your threshold hr.
I’d also recommend doing this test as you’d do your normal hard workout: no tapering / rest week beforehand, no insane caffeine boost and in normal training temperature with good hydration. All of these will raise your hr relative to day-to-day training conditions.
- As a sidenote, many people don’t really know what it feels like to ride at steady threshold power for extended period of time. Your threshold power / FTP varies day to day, so it’s easy to overdo threshold workouts based on previous workout(s) thinking “I did X workout last week so I must ride at the same power today” even if you’re having a slightly bad day and your threshold is down a few watts. This easily leads to excessive fatigue and therefore to more bad days in future. Knowing what it feels like to ride at threshold will help to scale intensity up or down accordingly and nail those workouts.