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

Sven Ahrens2026년 6월 12일

heart rateLTHRFrieltraining zonesintervals.icu

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:

젖산 역치 심박수

165bpm

사이클링 LTHR

존	범위	무엇을 단련하는가
Z1	능동적 회복 ≤ 132 bpm	회복용 스핀; 다리를 풀어줍니다. RPE 1–2, 매우 쉬움.
Z2	지구력 134–147 bpm	유산소 기초와 지방 대사. RPE 3–4, 하루 종일 갈 수 있는 페이스.
Z3	템포 149–153 bpm	유산소 지구력, '스위트'한 꾸준한 운동. RPE 5–6, 편안하게 힘듦.
Z4	역치 155–163 bpm	젖산 역치; 지속 가능한 강도. RPE 7–8, 20–60분 한계.
Z5a	VO₂max 165–168 bpm	최대 유산소 파워. RPE 9, 3–8분 노력.
Z5b	무산소 능력 170–175 bpm	무산소 능력, 짧고 날카로운 노력. RPE 9–10, 30초–3분.
Z5c	신경근 ≥ 177 bpm	스프린트 파워와 신경근 구동력. RPE 10, 전력 질주.

능동적 회복

≤ 132 bpm

지구력

134–147 bpm

템포

149–153 bpm

역치

155–163 bpm

Z5a

VO₂max

165–168 bpm

Z5b

무산소 능력

170–175 bpm

Z5c

신경근

≥ 177 bpm

Z1 · 능동적 회복. 회복용 스핀; 다리를 풀어줍니다. RPE 1–2, 매우 쉬움.

Z2 · 지구력. 유산소 기초와 지방 대사. RPE 3–4, 하루 종일 갈 수 있는 페이스.

Z3 · 템포. 유산소 지구력, '스위트'한 꾸준한 운동. RPE 5–6, 편안하게 힘듦.

Z4 · 역치. 젖산 역치; 지속 가능한 강도. RPE 7–8, 20–60분 한계.

Z5a · VO₂max. 최대 유산소 파워. RPE 9, 3–8분 노력.

Z5b · 무산소 능력. 무산소 능력, 짧고 날카로운 노력. RPE 9–10, 30초–3분.

Z5c · 신경근. 스프린트 파워와 신경근 구동력. RPE 10, 전력 질주.

LTHR 기반 존은 나이 추정치가 아닌 실제 역치에 기준을 두므로 %HRmax보다 더 개인화되어 있습니다. 사이클링과 러닝의 LTHR은 다르므로 종목별로 따로 테스트하세요.
필드 테스트: 워밍업 후 강도 높은 30분 단독 타임 트라이얼을 라이딩하거나 달립니다. LTHR은 마지막 20분의 평균 심박수입니다(Friel 프로토콜).

당신의 개인 맞춤 결과가 담긴 PDF와, 언제든 다시 열 수 있는 QR 코드.

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:

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 (don't sprint off the line).
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, 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.