Why LTHR beats %HRmax
Lactate-threshold heart rate (LTHR) is your heart rate at lactate threshold, roughly the hardest pace you can hold for an hour. Zones anchored to it use a marker you can measure, not an age estimate. Joe Friel popularized LTHR zones in his Training Bible books. Running LTHR sits a few percent higher than cycling, so the bands differ by sport.
Friel's model: 5 zones, or 7?
Friel's model is five zones, but Zone 5 splits into 5a, 5b and 5c, so it is usually shown as seven. Zones 1 to 4 are identical in both views: active recovery, aerobic endurance, tempo, and threshold at 94–99% LTHR (cycling) or 95–99% (running). Above threshold, 5a, 5b and 5c separate VO₂max, anaerobic capacity and neuromuscular work. Toggle between views above.
How intervals.icu numbers the same zones
intervals.icu uses Friel's exact percentages but numbers the zones Z1 through Z7. Friel's Z5a, Z5b and Z5c are intervals.icu's Z5, Z6 and Z7. Same boundaries, different labels.
The 30-minute LTHR field test (Friel protocol)
Solo, rested, flat terrain or a trainer. Pace it like a hard, even time trial.
- 1
Warm up
10–15 minutes easy with a couple of short pickups.
- 2
Start the 30-minute time trial
Hard, evenly paced, solo. Do not start too fast.
- 3
Press lap at 10 minutes
So your monitor averages the final 20 minutes separately.
- 4
Read your LTHR
The average heart rate of those final 20 minutes.
Worked example
A cyclist with a tested LTHR of 165 bpm, in the 7-zone Friel view:
| Zone 2, Endurance (81–89%) | 134–147 bpm |
| Zone 3, Tempo (90–93%) | 149–153 bpm |
| Zone 4, Threshold (94–99%) | 155–163 bpm |
| Zone 5a (100–102%), intervals.icu Z5 | 165–168 bpm |
In the 5-zone view, 5a, 5b and 5c merge into one Zone 5 from 165 bpm.
Frequently asked questions
Is Friel's model 5 zones or 7?
Friel's model is five zones in which Zone 5 splits into 5a, 5b and 5c, so it is usually shown as seven. Zones 1 to 4 are the same either way. This calculator toggles between both views.
How does this compare to intervals.icu's zones?
intervals.icu uses the same Friel LTHR percentages but numbers the zones Z1 to Z7. Friel's Z5a, Z5b and Z5c are intervals.icu's Z5, Z6 and Z7. The boundaries are identical.
How do I find my LTHR?
Find LTHR with a 30-minute solo time trial at a hard, even effort. Press lap at 10 minutes. Your lactate-threshold heart rate is the average over the final 20 minutes. Test rested, and separately per sport.
Is LTHR the same as maximum heart rate?
No. Lactate-threshold heart rate is the rate you can hold for about an hour, typically 85–92% of maximum for trained athletes. It is the better anchor because a field test measures it directly.
Why are cycling and running LTHR different?
Running produces a higher threshold heart rate than cycling for the same athlete, because it recruits more muscle and you support your body weight. Test each sport separately or one sport's zones will be wrong.
Sources
- Joe Friel, The Cyclist's / Triathlete's Training Bible. The LTHR field-test protocol and the 7-zone (Z1–Z4, Z5a/Z5b/Z5c) %LTHR zone definitions.
- intervals.icu: HR zone models. Free training platform offering LTHR-based Friel zones, numbered Z1–Z7 (Friel's Z5a/Z5b/Z5c shown as Z5/Z6/Z7). Link
- Allen & Coggan, Training and Racing with a Power Meter. LTHR and threshold-based zone framework.