Time-in-Zone

Time-in-zone (TIZ) is the breakdown of a session (or week) into minutes spent in each HR zone (see heart-rate-zones) or power zone (see power-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).

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 heart-rate-logging).
• 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 progressive-overload-application).

• 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.

• heart-rate-zones — the HR zone definitions.
• training-intensity-distribution — the target TIZ tracks against.
• power-ftp — power zones avoid HR-lag distortion.
• zone-2-training — the zone most endurance volume should sit in.
• kpis-endurance — weekly TIZ is a core endurance KPI.