Time-in-Zone
Sources and review status
Review pending — this text hasn’t been signed off by a reviewer yet.
See this lesson’s sources0% read · 2 min left
The minutes you spend in each intensity zone — the metric that verifies whether your easy days were easy and your hard days hard.
Detail
Time-in-zone (TIZ) is the breakdown of a session (or week) into minutes spent in each HR zone (see Heart-Rate Zones (5-Zone Model)) or power zone (see Power and FTP). It is the data-layer expression of intensity distribution — the single most important thing to get right in endurance training (see Training-Intensity Distribution (Polarized / Threshold / Pyramidal)).
Why it matters:
- Most endurance programs want the bulk of weekly time easy (zone 1–2, below threshold) with a smaller dose hard — "polarized" or "pyramidal" distributions. TIZ is how you confirm you aren't living in the unproductive moderate "grey zone."
- Aggregating TIZ per week is more meaningful than per session, because the distribution target is weekly.
Caveats:
- Zone boundaries are only as good as their anchors (HRmax, LT, FTP). Mis-set zones make TIZ misleading (see Logging Heart Rate (Avg / Max)).
- HR lag smears TIZ on interval sessions — time "in zone 5" is undercounted because HR arrives late and lingers. Power-based TIZ avoids this for cycling.
For programming, drifting TIZ (e.g. easy runs creeping into zone 3) is a flag to correct intensity discipline; rising hard-zone tolerance can be a progression cue (see Applying Progressive Overload).
Key facts
- TIZ = minutes per HR or power zone, per session and per week.
- Operationalizes intensity distribution (polarized / pyramidal targets).
- Weekly aggregation is the meaningful unit.
- Accuracy depends on correctly set zone anchors.
- HR lag distorts interval TIZ; power-based TIZ is cleaner for intervals.
Connections
- Heart-Rate Zones (5-Zone Model) — the HR zone definitions.
- Training-Intensity Distribution (Polarized / Threshold / Pyramidal) — the target TIZ tracks against.
- Power and FTP — power zones avoid HR-lag distortion.
- Zone 2 Training — the zone most endurance volume should sit in.
- KPIs for Endurance — weekly TIZ is a core endurance KPI.
Sources
Keep this lesson, or ask how it applies to you.
Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.