Use test evidence first
Anchor custom boundaries to recent tests, not historical assumptions from a different fitness phase.
When Custom Zones are the better choice
Choose custom zones when model defaults drift from your field tests or actual workout response.
Align zones with block objective
Base periods, build phases, and race prep often require different emphasis even with similar thresholds.
Keep naming and labels consistent
Consistent labels make workout prescriptions and post-session reviews easier to interpret.
Recheck after major adaptation
When threshold and execution quality shift materially, recalibrate zones to avoid stale targets.
Should I always use model-based zones?
No. Model zones are a good start, but custom zones can fit better when you have reliable test data.
How often should I update custom zones?
Usually per block or after a meaningful threshold change, not after every isolated workout.
Can I mix custom zones across sports?
Yes. Running and cycling can use different zone boundaries when their threshold behavior differs.