June 12, 2026 · Coach Kalle
5 zones or 7? Making sense of Joe Friel's heart-rate model — and how intervals.icu uses it
If you have ever wired up a new training platform and wondered why your heart-rate zones suddenly went from five to seven, or why "Zone 3" means something different than it did in your last app, you are not alone. The culprit is almost always Joe Friel's heart-rate model, and the "5 versus 7" question has a simple answer once you see how it is built.
Friel zones are anchored to LTHR, not max HR
Most casual HR-zone calculators hang everything off your maximum heart rate (the old "220 minus age" trap) or your heart-rate reserve (Karvonen). Friel does neither. His zones are anchored to your Lactate Threshold Heart Rate (LTHR) — roughly the highest heart rate you can hold in a steady, hour-long effort.
That choice matters: LTHR is a trainable, individual number that tracks your fitness far better than an age estimate. Friel's field test for it is refreshingly low-tech: do a hard 30-minute solo time trial, hit the lap button 10 minutes in, and take your average heart rate over the final 20 minutes. That average is a good proxy for LTHR. (Full method on Joe Friel's own guide and the TrainingPeaks write-up.)
So is it 5 zones or 7?
Both — and that is the whole confusion. Friel describes five effort levels: recovery, aerobic, tempo, sub-threshold, and supra-threshold. But the top level (Zone 5) is split into three slices — 5a, 5b, 5c — to separate "just over threshold" from "VO2max" from "anaerobic". Count the rows and you get seven.
So "Friel's 5-zone model" and "Friel's 7-zone model" are the same model. One view collapses Zone 5; the other expands it. Nothing physiological changes — only how finely the top end is sliced.
Here are the boundaries, as percentages of LTHR (note that running and cycling differ slightly):
| Zone | Running (% LTHR) | Cycling (% LTHR) |
|---|---|---|
| Zone 1 | < 85% | < 81% |
| Zone 2 | 85–89% | 81–89% |
| Zone 3 | 90–94% | 90–93% |
| Zone 4 | 95–99% | 94–99% |
| Zone 5a | 100–102% | 100–102% |
| Zone 5b | 103–106% | 103–106% |
| Zone 5c | 106%+ | 106%+ |
Because the percentages are sport-specific, your "Zone 2" ceiling on the bike is not the same heart rate as on the run, even at the same LTHR. That is a feature, not a bug.
The other "7-zone" — and why people conflate them
There is a second famous seven-zone system in endurance sport: Andrew Coggan's power zones, anchored to FTP (functional threshold power). When someone says "I train in seven zones," they might mean Friel's HR zones or Coggan's power zones — and the two are not interchangeable:
- Friel = heart rate, anchored to LTHR, 5 levels (7 rows with 5a/b/c).
- Coggan = power, anchored to FTP, 7 zones (Z1 active recovery → Z7 neuromuscular).
They answer different questions. Power tells you what you are producing right now; heart rate tells you what it is costing you. Most cyclists with a power meter end up watching Coggan power zones for intervals and Friel HR zones for drift and aerobic work. Comparing your "Zone 3" across the two is meaningless — they are different metrics with different anchors.
How intervals.icu uses it
intervals.icu leans on Friel for heart rate. You enter your LTHR and max HR per sport, and it lays out seven HR zones (Z1–Z4 plus Z5a/5b/5c) — Friel's model in its expanded form — separately for the Ride, Run, and Swim profiles. Power zones, if you ride with a meter, default to Coggan's seven. Everything is fully editable: the percentages are sensible defaults, not handcuffs, and there is a presets/defaults option to reload Friel's numbers if you have tweaked them.
Two practical notes from the intervals.icu community guides (setting up HR zones, popular zone setups):
- Set your zones per sport. Your running LTHR is typically a few beats higher than your cycling LTHR, and the zone percentages differ too.
- Re-test LTHR every couple of months in a build block. It rises as you get fit, and stale zones quietly make all your "easy" days too hard.
The takeaway
Stop worrying about whether your app says 5 or 7. What matters is the anchor (LTHR for Friel HR zones, FTP for Coggan power) and that you actually re-test it. The zone count is just how the top end is drawn on the page.
That is also exactly how Coach Kalle reads your training: it pulls your stored, sport-specific zones — your LTHR-based HR zones, your power zones, your pace zones — and prescribes and analyses in the metric you have chosen for each sport, rather than forcing one model onto everything. Set the anchors once, train to them, and let the zones take care of themselves.