Thresholds and Endurance Zones: What Each One Is For
Introduction
"Zone 2" became one of the most repeated phrases in recent training talk, and with it came considerable confusion: there are at least two different zone systems, zone 2 in one isn't zone 2 in the other, and almost nobody says which they mean.
Underneath the labels there's something simple: two breakpoints in your physiology, and three territories between them.
You don't need a lab to find your thresholds. You need to know how much you can talk.
The two thresholds
First threshold (aerobic). The point where effort stops being entirely comfortable. Below it you can sustain hours; lactate stays low and stable. It's the ceiling of the genuinely easy.
Second threshold (anaerobic, or critical power). The point where lactate starts accumulating faster than you can clear it. Above it the clock is running: fatigue becomes inevitable within a measurable window. It corresponds roughly to the pace you could hold for 30 to 60 minutes.
Everything else is a way of naming the territories those two points delimit.
The two zone systems
Here's the source of the confusion, and it's worth settling once.
Three-zone model (used by exercise physiologists):
- Zone 1: below the first threshold. Easy.
- Zone 2: between the thresholds. Moderate.
- Zone 3: above the second threshold. Hard.
Five-zone model (used by watches and apps):
- Zone 1: very easy.
- Zone 2: easy, aerobic.
- Zone 3: moderate.
- Zone 4: threshold.
- Zone 5: maximal.
The famous zone 2 — the long, comfortable aerobic base work — is zone 2 of the five-zone model, which corresponds to zone 1 of the three-zone model. Zone 2 in the three-zone model is a considerably more demanding territory, the one coaches usually call "no man's land".
When someone tells you to "train in zone 2", the right question is: which of the two models?
Finding your thresholds without a lab
The talk test. Surprisingly good and it costs nothing.
- You can converse in full sentences and breathe through your nose → you're below the first threshold.
- You can manage short, broken sentences → you're between thresholds.
- You can only manage two or three words → you're above the second threshold.
Field test for the second threshold. After warming up, run or cycle 30 minutes at your maximum sustainable effort. The average pace or power of the final 20 minutes approximates your second threshold, and the average heart rate of that stretch approximates your threshold heart rate.
With a heart rate monitor. Zones calculated from "220 minus your age" are unreliable: that equation has large individual error. If you're going to use heart rate, calculate your zones from your threshold heart rate measured in the test above, not from an estimated maximum.
What each territory is for
Below the first threshold (the easy stuff). It builds the base: more capillaries, more mitochondria, better fat utilization, greater cardiac volume. Its great virtue is that it accumulates a lot of time with little fatigue, which allows high volume without breaking. It's boring and it's most of any endurance athlete's training.
Between thresholds (the moderate stuff). It improves your ability to sustain high paces. It's useful and it has a problem: it's quite tiring for what it delivers, so overusing it leaves fatigue without the advantages of either extreme. Hence the "no man's land" nickname.
Above the second threshold (the hard stuff). It raises the roof: it improves VO₂ max and tolerance for intense effort. Low volume, high stimulus, high fatigue.
What to do with this
For someone training for health who does some endurance work:
1. Measure your second threshold once with the 30-minute test. Without that figure, your zones are invented. 2. Make most of your volume comfortable. Being able to talk is the criterion. 3. Include some hard work each week, short and well separated. 4. Don't live in the middle. It's the most common error of people who always run at the same pace: too hard to be easy, too easy to be hard.
Common mistakes
Using your watch's zones without calibrating them. If they come from an age-estimated maximum, they're wrong.
Doing all your volume at moderate intensity. Very tiring and it improves little.
Believing the easy zone is useless because "you don't feel it". It's the one that lets you accumulate the most volume, and volume builds the base.
Chasing the "fat-burning zone". You do burn proportionally more fat at low intensity, and that isn't the same as losing more fat: total energy balance governs.
Confusing the two zone models. The origin of half the misunderstandings about this topic.
Conclusion
Beneath all the jargon there are two points — the ceiling of comfortable and the ceiling of sustainable — and three territories. Locating them with the talk test and a 30-minute test is enough to train with judgment.
Do this in your next endurance session: pay attention to how much you can talk. If you've spent months heading out at a pace where you only manage broken sentences, you're living in no man's land — and slowing down on most days will probably improve you more than continuing to push.
References
- Seiler, S. (2010). What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance, 5(3).
- Jamnick, N. A., et al. (2020). An examination and critique of current methods to determine exercise intensity. Sports Medicine, 50(10).
- Poole, D. C., et al. (2016). Critical power: An important fatigue threshold in exercise physiology. Medicine & Science in Sports & Exercise, 48(11).
- Tanaka, H., Monahan, K. D., & Seals, D. R. (2001). Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology, 37(1).
- Foster, C., et al. (2008). The talk test as a marker of exercise training intensity. Journal of Cardiopulmonary Rehabilitation and Prevention, 28(1).