Friel's Heart-Rate Zones: 5, 7, and Why Every Platform Numbers Them Differently

Joe Friel's LTHR zones explained: the 5-zone model, the 5a/5b/5c split that makes it 7, how intervals.icu renumbers them, and how to test your LTHR.

Sven Ahrens12. juni 2026Senest opdateret 6. september 2026

heart rateLTHRFrieltraining zonesintervals.icu

If your zones from one app don't match another, Joe Friel's heart-rate model is usually why. It is the most used heart-rate zone system in endurance sport, and the most relabelled. Here is why it is 5 zones or 7, and how intervals.icu relabels them.

What Friel's zones are

Friel's zones are anchored to your lactate-threshold heart rate (LTHR), the heart rate you can hold for roughly an hour of hard, steady effort. Every band is pinned to a marker you measure, not to 220 − age (off by ±10–12 bpm).

Friel publishes separate cycling and running boundaries, since running LTHR sits a few percent above cycling LTHR.

Five zones, or seven?

Both. The model is five zones, but Friel splits Zone 5 into 5a, 5b and 5c, so it is most often shown as seven. Zones 1–4 are identical in either view:

Z1 Recovery
Z2 Aerobic / endurance
Z3 Tempo
Z4 Threshold (94–99% LTHR cycling, 95–99% running)
Z5a just above threshold
Z5b VO₂max / aerobic capacity
Z5c anaerobic, short and sharp

Plug in your LTHR and toggle between the 5- and 7-zone views:

Laktattærskel-puls

165bpm

cykel-LTHR

Zone	Interval	Hvad det træner
Z1	Aktiv restitution ≤ 132 bpm	Restitutionsruller; renser benene. RPE 1–2, meget let.
Z2	Udholdenhed 133–147 bpm	Aerob base og fedtstofskifte. RPE 3–4, tempo til hele dagen.
Z3	Tempo 148–153 bpm	Aerob udholdenhed, 'sweet' jævnt arbejde. RPE 5–6, behageligt hårdt.
Z4	Tærskel 154–163 bpm	Laktattærskel; holdbart hårdt. RPE 7–8, grænse 20–60 min.
Z5a	VO₂max 164–168 bpm	Maksimal aerob effekt. RPE 9, 3–8 min forsøg.
Z5b	Anaerob kapacitet 169–175 bpm	Anaerob kapacitet, kort og skarpt. RPE 9–10, 30 s–3 min.
Z5c	Neuromuskulær ≥ 176 bpm	Sprintkraft og neuromuskulært drev. RPE 10, alt-ud.

Aktiv restitution

≤ 132 bpm

Udholdenhed

133–147 bpm

Tempo

148–153 bpm

Tærskel

154–163 bpm

Z5a

VO₂max

164–168 bpm

Z5b

Anaerob kapacitet

169–175 bpm

Z5c

Neuromuskulær

≥ 176 bpm

Z1 · Aktiv restitution. Restitutionsruller; renser benene. RPE 1–2, meget let.

Z2 · Udholdenhed. Aerob base og fedtstofskifte. RPE 3–4, tempo til hele dagen.

Z3 · Tempo. Aerob udholdenhed, 'sweet' jævnt arbejde. RPE 5–6, behageligt hårdt.

Z4 · Tærskel. Laktattærskel; holdbart hårdt. RPE 7–8, grænse 20–60 min.

Z5a · VO₂max. Maksimal aerob effekt. RPE 9, 3–8 min forsøg.

Z5b · Anaerob kapacitet. Anaerob kapacitet, kort og skarpt. RPE 9–10, 30 s–3 min.

Z5c · Neuromuskulær. Sprintkraft og neuromuskulært drev. RPE 10, alt-ud.

LTHR-baserede zoner er mere individuelle end %HRmax, fordi de forankres til din faktiske tærskel, ikke et aldersestimat. Cykel- og løbe-LTHR er forskellige, så test hver sportsgren for sig.
Feltprøve: varm op, og kør eller løb derefter en hård 30-minutters solo-enkeltstart. Din LTHR er den gennemsnitlige puls i de sidste 20 minutter (Friels protokol).

En PDF med dine personlige resultater, plus en QR-kode til at åbne dem igen når som helst.

Why every platform renumbers them

The boundaries are Friel's. The labels are not standardized.

intervals.icu uses the same Friel LTHR percentages but numbers the zones sequentially, Z1 to Z7. Friel's Z5a, Z5b and Z5c appear as Z5, Z6 and Z7.
TrainingPeaks, which Friel co-founded, keeps the original Z5a / Z5b / Z5c notation.

Same maths, different counting. If your zones "don't match", one tool split Zone 5 into a/b/c and the other counted to 7.

What the model gets right

It's individual. LTHR is your measured threshold, not a population average scaled by age.
It's measurable without a lab. A 30-minute time trial gives a usable number for free.
It's threshold-pinned. The zones stay sensible even as an age-based HRmax estimate drifts.
It's sport-specific. Separate cycling and running anchors stop you training one sport with the other's zones.

Where it falls short

Heart rate is blunt at the top end:

HR lags and drifts. It rises slowly at the start of an interval, climbs over a long effort (cardiac drift), and spikes in heat or dehydration.
It saturates above threshold. In Z5a–Z5c the heart-rate differences are tiny and arrive late. Power (cycling) or pace (running) controls those efforts far better. Friel himself moved toward power for the high zones.
It's a single-threshold model. It marks the upper threshold but not the lower aerobic threshold the way dual-threshold and polarized models do.
It's only as good as your test. A stale or badly paced LTHR test mis-sets every zone.

How to find your LTHR

The classic Friel field test:

Warm up 10–15 minutes easy with a couple of short pickups.
Time-trial for 30 minutes, solo, flat or on a trainer, hard and evenly paced.
Press lap at 10 minutes, so your monitor averages the final 20 minutes separately.
Your LTHR is the average heart rate of those last 20 minutes.

Test rested, separately for cycling and running. Re-test every 4–8 weeks of focused training. Then use the LTHR zone calculator, or compare against your max-heart-rate and %HRmax zones.