Training intensity zones
Evidence: strong
Running intensity is a continuum, but it has two non-arbitrary landmarks: the two physiological thresholds, which divide effort into three domains (easy, threshold, hard). Every zone model, three-zone, five-zone, seven-zone, is a way of slicing that continuum, and the slices are conventions, not physiology. Because the models anchor to different references, a “Zone 2” in one is not the same intensity as in another.
A training zone is a band of intensity treated as a unit, so that “run in Zone 2” is a usable instruction. The trouble is that there is no single zone system: watches, coaches and research papers use models with three, five or seven zones, anchored to different things and sliced at different points. Underneath all of them, though, is one physiological structure that does not depend on the model, and it is the thing worth understanding first.
The backbone: two thresholds, three domains
Intensity is best understood through two physiological landmarks. As pace rises, blood lactate first lifts above its resting level at the first threshold (LT1, the aerobic threshold, roughly coinciding with the first ventilatory threshold, VT1). It then climbs steeply and stops being sustainable at the second threshold (LT2, approximating the maximal lactate steady state, roughly coinciding with the second ventilatory threshold, VT2) (Faude, Kindermann & Meyer 2009; see lactate threshold).
These two lines carve effort into three intensity domains, the same structure the critical-power literature describes as moderate, heavy and severe (Poole et al. 2016; Jones et al. 2010):
- Below the first threshold (easy / moderate domain). Comfortable, fully aerobic, sustainable for hours. Lactate stays near baseline. This is easy-run and long-run territory.
- Between the thresholds (threshold / heavy domain). Lactate is elevated but can reach a steady state for a while. This spans steady, marathon-effort and tempo running up to the second threshold.
- Above the second threshold (hard / severe domain). Lactate rises relentlessly and the effort is time-limited. This is the domain of VO₂max intervals and racing short distances.
The two thresholds are the only boundaries that mean the same thing across runners and models. Everything else is a labelling choice on top of them.
The popular models
The systems differ mainly in how finely they cut the continuum, not in the underlying physiology (Stöggl & Sperlich 2015):
- Three-zone model. The model used in most training-distribution research: Zone 1 below the first threshold, Zone 2 between the thresholds, Zone 3 above the second. It maps directly onto the three domains and is the cleanest way to describe how a week’s training is distributed (the basis of polarised and pyramidal training).
- Five-zone model. The common watch and coaching scheme, which subdivides the three domains: recovery and easy (both below the first threshold), a tempo or marathon zone (between the thresholds), a threshold zone (at the second), and a VO₂max zone (above it).
- Seven-zone model. A finer scheme, often power- or pace-based, that splits the hard domain further into VO₂max, anaerobic and neuromuscular bands.
A five-zone “Zone 2” sits below the first threshold (easy aerobic running), whereas the three-zone “Zone 2” is the whole band between the thresholds. They are different intensities with the same name. Comparing zone numbers between a friend, a watch and a study is meaningless unless they share a model.
How zones are anchored
Within a model, the boundaries have to be pegged to something measurable. The common anchors are percentage of maximum heart rate, percentage of heart-rate reserve (Karvonen), percentage of lactate-threshold heart rate (Friel), percentage of VO₂max or maximal aerobic speed, rating of perceived exertion (Chen et al. 2002), and the talk test, where the point at which full sentences become uncomfortable falls in the threshold region (Quinn & Coons 2011). These do not line up exactly, because they are anchored differently: a percentage of maximum heart rate, a percentage of reserve and a percentage of threshold heart rate give three different target intensities. How to choose and apply them in practice, and why an estimated maximum heart rate is a weak anchor, is covered in heart-rate and effort-based training.
This table maps the systems onto the physiological backbone. Treat the heart-rate and effort figures as broad guides, not precise boundaries.
| Domain | 3-zone | 5-zone | ~% max HR | RPE (1–10) | Talk test |
|---|---|---|---|---|---|
| Easy (below LT1) | Zone 1 | Zones 1–2 | < ~80% | 2–4 | Full sentences |
| Threshold (LT1–LT2) | Zone 2 | Zones 3–4 | ~80–90% | 5–7 | Short phrases |
| Hard (above LT2) | Zone 3 | Zone 5 | > ~90% | 8–10 | A few words at most |
The numbers are conventions
The single most useful caveat is that the zone boundaries are operationalisations, not biological constants. The thresholds themselves are real and individual, but the percentages used to estimate them are population averages with wide scatter, and the fixed values (a 4 mmol/L lactate line, a “Zone 2 is 60 to 70% of max” rule) are conventions that misplace the boundary for many runners (Faude et al. 2009). Anchoring zones to a lactate or threshold field test places them far better than any age-based formula. And the popular idea that one narrow zone is uniquely magical, the Zone 2 claim in particular, outruns the evidence (Storoschuk et al. 2025): the zones are a navigation aid for distributing training, not a set of switches that each unlock a separate adaptation.