Why LTHR beats %HRmax
Lactate-threshold heart rate (LTHR) is the heart rate at your lactate threshold, roughly the hardest pace you could hold for an hour. Anchoring zones to it, rather than to an age-estimated maximum, ties your training to a physiological marker you can actually measure.
Joe Friel popularized LTHR heart-rate zones for endurance athletes in his Training Bible books. The bands differ slightly between cycling and running because threshold heart rate is sport-specific: running LTHR usually sits a few percent higher.
Friel's model: 5 zones, or 7?
Friel's model is fundamentally five zones, but he splits the hardest one, Zone 5, into three sub-zones: 5a, 5b and 5c. That is why it is most often presented as seven zones. Zones 1 to 4 are identical in both views; only the top end changes. Use the toggle above to switch between the 7-zone breakout and the simpler 5-zone view.
Zone 1 is active recovery; Zone 2 builds aerobic endurance; Zone 3 is tempo; Zone 4 sits right at threshold (94–99% LTHR cycling, 95–99% running). Above threshold, 5a, 5b and 5c separate VO₂max work, anaerobic capacity and short neuromuscular efforts. Because every band is pinned to threshold, the zones stay valid even as an age-based HRmax estimate drifts.
How intervals.icu numbers the same zones
intervals.icu, the popular free training platform, offers an LTHR-based heart-rate model built on Friel's exact percentages, but labels the zones differently. Where Friel writes Z1, Z2, Z3, Z4, Z5a, Z5b, Z5c, intervals.icu numbers them sequentially Z1 through Z7. So Friel's Z5a, Z5b and Z5c become intervals.icu's Z5, Z6 and Z7.
The math underneath is the same Friel split off LTHR; only the naming differs (sub-letters versus running numbers). If you set your zones from this calculator and your intervals.icu zones from %LTHR, the boundaries line up, just remember that this page's Z5a/Z5b/Z5c are intervals.icu's Z5/Z6/Z7. Many coaches keep the Friel a/b/c labels because they make the shared threshold of the top three zones obvious at a glance.
The 30-minute LTHR field test (Friel protocol)
Test solo, on flat terrain or a trainer, when rested. Aim for a steady, maximal 30-minute effort, pace it like a hard time trial, not a sprint start.
- 1
Warm up
Ride or run easy for 10–15 minutes with a couple of short pickups.
- 2
Start the 30-minute time trial
Begin a hard, evenly-paced 30-minute solo effort as if racing. Don't start too fast.
- 3
Press lap at 10 minutes
Ten minutes in, hit the lap button so your monitor averages the final 20 minutes separately.
- 4
Read your LTHR
Your lactate-threshold heart rate is 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 |
Switch to the 5-zone view and 5a, 5b and 5c merge into a single Zone 5 from 165 bpm up.
Frequently asked questions
Is Friel's model 5 zones or 7?
Both: it is a five-zone model in which Zone 5 splits into 5a, 5b and 5c, so it is usually shown as seven zones. Zones 1 to 4 are the same either way; only the top end differs. This calculator lets you toggle between the 7-zone breakout and the combined 5-zone view.
How does this compare to intervals.icu's zones?
intervals.icu uses the same Friel LTHR percentages but numbers the zones Z1 to Z7 instead of using 5a/5b/5c. So Friel's Z5a, Z5b and Z5c are intervals.icu's Z5, Z6 and Z7. The boundaries are identical; only the labels differ.
How do I find my LTHR?
Do a 30-minute solo time trial at a hard, even effort. Press lap 10 minutes in; your lactate-threshold heart rate is the average over the final 20 minutes. Test when rested, and repeat separately for cycling and running because threshold heart rate differs between the two.
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 a far more useful training anchor than maximum heart rate because it reflects your actual threshold and can be measured directly with a field test.
Why are cycling and running LTHR different?
Running generally produces a higher threshold heart rate than cycling for the same athlete, because it recruits more muscle mass and you support your body weight. Test each sport separately, using your cycling LTHR for running (or vice versa) sets one sport's zones noticeably 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.