After some research, here is some data on the steps based upon a couple of my current PL’s:
Sprint PL = 1.0
- Achievable = 1.0
- Productive = 1.1 to 2.0 = (0.9 delta)
- Stretch = 2.1 to 3.5 = (1.4 delta)
- Breakthrough = 3.6 to 5.5 = (1.9 delta)
Anaerobic PL = 3.9
- Achievable = 1.0 to 3.9 = (1.0 to current PL delta)
- Productive = 4.0 to 4.9 = (0.9 delta)
- Stretch = 5.0 to 6.4 = (1.4 delta)
- Breakthrough = 6.5 to 8.4 = (1.9 delta)
Endurance PL = 7.6
- Achievable = 1.0 to 7.6 = (1.0 to current PL delta)
- Productive = 7.7 to 8.6 = (0.9 delta)
- Stretch = 8.7 to 10.1 = (1.4 delta)
- Breakthrough = 10.2 to 12.1 = (1.9 delta)
I got these values by using the TR Android App, loading a workout in that zone, then using Alternates and looking at the Difficulty options. It list the actual ranges shared above for any workout relative to that zone.
There are some general trends to see but I wouldn’t be surprised if zones had different steps between the Difficulty Levels, not to mention it might be relative to your PL at that moment. Hard to say more with such a small capture of data, but it gets the general trend at least.