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

If you've ever exported your zones from one app and found they don't match another, Joe Friel's heart-rate model is usually why. It's the most widely used heart-rate zone system in endurance sport, and also the most relabelled. Here's what it actually is, why it's sometimes 5 zones and sometimes 7, and how platforms like intervals.icu use Friel's exact numbers under their own names.

What Friel's zones actually 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, not to an age-estimated maximum. That single choice is what sets the model apart: instead of 220 − age (a guess that can be off by ±10–12 bpm), every band is pinned to a marker you measure on yourself.

Because threshold heart rate differs by sport (running LTHR usually sits a few percent above cycling LTHR for the same athlete), Friel publishes separate cycling and running boundaries.

Five zones, or seven?

Both. Friel's model is fundamentally five zones, but he splits the hardest one, Zone 5, into three sub-zones: 5a, 5b and 5c. So it's most often shown as seven. Zones 1–4 are identical in either view; only the top end changes:

• 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 right here:

Why every platform renumbers them

This is where the confusion comes from. The boundaries are Friel's, but the labels aren't standardised.

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

Same maths, different counting. If your zones "don't match," check whether one tool split Zone 5 into a/b/c and the other counted straight up to 7. Nine times out of ten that's the whole discrepancy. The advantage of Friel's a/b/c labels is that they keep the shared threshold of the top three zones obvious; the advantage of straight numbering is that it's simpler to read at a glance.

What the model gets right

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

Where it falls short

Heart rate is a brilliant anchor for the aerobic end and a blunt one for the top end:

• HR lags and drifts. Your heart rate rises slowly at the start of an interval and keeps climbing over a long effort (cardiac drift), and it spikes in heat or when you're dehydrated. So the same effort can land in different "zones" on different days.
• It saturates above threshold. In Z5a–Z5c the heart rate differences are tiny and arrive late, so HR is a poor governor for short, hard reps: 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's built around the upper threshold and doesn't explicitly mark the lower aerobic threshold the way dual-threshold and polarized models do, which matters if most of your training is meant to be genuinely easy.
• It's only as good as your test. A stale or badly-paced LTHR test mis-sets every zone at once.

For the why-not-just-use-a-lab question, see lab vs. field testing.

How to find your LTHR

The classic Friel field test:

1. Warm up 10–15 minutes easy with a couple of short pickups.
2. Time-trial for 30 minutes, solo, flat or on a trainer, hard and evenly paced (don't sprint off the line).
3. Press lap at 10 minutes, so your monitor averages the final 20 minutes separately.
4. Your LTHR is the average heart rate of those last 20 minutes.

Test rested, and do it separately for cycling and running. Re-test every 4–8 weeks of focused training. Then drop the number into the LTHR zone calculator above, or compare it against your max-heart-rate and %HRmax zones to see how different the two approaches really are.

Bottom line

Friel's model earned its ubiquity by being individual, measurable, and threshold-anchored. Treat it as the aerobic backbone of your heart-rate training, and when you're doing short, sharp work above threshold, trust power or pace over the number on your wrist. And whichever platform you use, remember: if the zones look off, it's almost always just Z5a/b/c versus Z5/6/7.

Further reading: Joe Friel's blog, intervals.icu.