Intensity Zones

Definition

Intensity zones partition the space of possible running efforts into discrete bands, each corresponding to a different physiological state. The simplest model — the one Your Pacer uses in most letters — is the three-zone polarized scheme anchored at the two thresholds:

• Z1 — below AeT. Fat-dominant metabolism, conversational. The "easy" zone.
• Z2 — the narrow band above AeT and below AnT. The zone many runners drift into without intending to. Often called the "grey zone" or "moderate" in polarized discussion.
• Z3 — at or above AnT. Threshold and faster work. The sharp, deliberately hard zone.

Cycling and running platforms sometimes use a five-zone or seven-zone model. Those are refinements of the same physiology — they split Z1 into recovery and easy, and split Z3 into threshold, VO2max, and anaerobic — but the two boundary lines (AeT and AnT) are what truly carry physiological meaning.

Why it matters to runners

Zones exist so that prescription can be physiological rather than nominal. "Go easy" and "run hard" mean different things on different days for the same athlete, and radically different things for two athletes of different fitness. Heart rate or pace ranges that map to your AeT and AnT make "easy" and "hard" concrete.

The central insight of polarized training is that Z2 — the middle band — is where recreational runners spend the most time, and where adaptation per unit fatigue is lowest. Time below AeT (Z1) builds the engine cheaply; time at or above AnT (Z3) stimulates the top end precisely. Time in Z2 produces intermediate fatigue without the peak-end adaptation. See Polarized Training for the distribution argument.

Your Pacer treats your AeT and AnT as living numbers, re-estimated from observed data every week. As those numbers move, so do the zones. A Z1 session today may have a different pace range than a Z1 session six weeks ago.

How zones are bounded

Heart rate is the most common anchor because it integrates effort, environment, and fatigue. Pace-based zones work when terrain and conditions are stable. Power-based zones (running power meters, or cycling) offer the most direct mechanical reading but add hardware.

• Z1 upper bound — AeT. In an HR drift test, this is the heart rate at which 30 minutes of steady running produces less than ~3.5% heart-rate drift.
• Z2 band — the heart-rate range between AeT and AnT. Often about 10 to 20 bpm wide, depending on the athlete.
• Z3 lower bound — AnT, approximately the heart rate sustainable for about one hour of all-out effort.

First-week prescriptions may use formula-based estimates (age-predicted max heart rate, percentages) when thresholds have not yet been measured. These are overwritten as soon as measured data accumulates.

Related terms

• AeT (Aerobic Threshold) — the Z1 / Z2 boundary.
• AnT (Anaerobic Threshold) — the Z2 / Z3 boundary.
• Polarized Training — why 80% of volume sits in Z1 and 20% sits in Z3.
• HR Drift Test — the protocol that tells you where the Z1 ceiling actually sits.

Further reading

• Seiler, What is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes?, International Journal of Sports Physiology and Performance (2010).
• Skinner & McLellan, The Transition from Aerobic to Anaerobic Metabolism, Research Quarterly for Exercise and Sport (1980).
• Daniels, Daniels' Running Formula. VDOT-based pace zones.