How Runsei estimates your training zones

Runsei estimates your zones from your own runs, and is honest about how sure it is — every value carries a source and a confidence level (measured > derived > estimated > age-based).

The key idea: heart-rate zones anchor on your lactate threshold (LTHR / LT2), not a percentage of max heart rate. Anchoring on %max reads low, because normal training rarely reveals your true max. Runsei finds your threshold from the *signature* of your efforts — a sustained, stable, high heart rate you held without drifting up — not from a run's title (a run called "threshold" is often run above threshold).

Your aerobic threshold (LT1) is the ceiling of genuinely *easy* running — the effort above which easy running stops being easy. It's a physiological point, so Runsei estimates it as a tight range, and it's the honest cap on your easy days. It's found three ways that have to agree: a band below your LTHR, a band as a fraction of max HR, and — the strongest — heart-rate drift (aerobic decoupling): when you hold a steady effort for 30+ minutes and your heart rate barely rises relative to pace (under ~5% drift), you've *proven* that effort was aerobic, so your LT1 is at least that high. This is Joe Friel's aerobic-decoupling test (used in TrainingPeaks), and it means your easy ceiling is grounded in what you've actually shown, not a generic formula.

Easy pace is the pace you actually hold on runs that were genuinely easy — Runsei deliberately ignores "easy" runs where your heart rate sat above your aerobic threshold (run too hard), so running easy days too fast can't quietly redefine your easy pace upward.

Your zones are estimates that shift day to day with heat, sleep, fatigue and altitude — treat them as a guide, not an exact measurement. A short field test or a lab test sharpens them.