← All guidesHeart Rate Zones: What Is Zone 2, and How Should You Read the Zones?
Your watch shows five heart-rate zones. The research uses three. And of all those dividing lines, only one is well validated — this page is about which one, why it is that one, and how to find yours without spending anything.
Updated August 13, 2026
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AI-researched
The answer, with evidence
How many: the research uses three, split by two ventilatory thresholds — not the five on your watch.
A ventilatory threshold is a point where your breathing pattern visibly changes; there are two, usually written VT1 and VT2. The study that quantified 384 sessions over 32 days in 11 junior cross-country skiers used exactly VT1 and VT2 to cut intensity into three zones. [2] Worth stating plainly: that is descriptive work on 11 athletes. It establishes that three zones is the model the field works in, not that three zones trains you better than five.
The anchor: slicing zones off a percentage of maximum heart rate does not separate genuinely different physiological states.
A review that assessed the construct validity of the various methods put it this way: prescribing intensity from a fixed percentage of these maximal anchors "has little merit for eliciting distinct or domain-specific homeostatic perturbations." [1] Zone 1–5 on a watch is precisely a set of %HRmax bands. This does not make the watch useless — it makes those particular lines a convention rather than a measurement.
The boundary: the Easy/Moderate line holds up; the Moderate/Hard line is much softer.
The same review splits its verdict: there is "some evidence" that the first lactate threshold, the gas exchange threshold and the ventilatory threshold delineate the moderate from the heavy domain, but "little evidence" for any commonly used method except critical power and critical speed to delineate heavy from severe. [1] That is why this page speaks with different confidence about the two lines — the asymmetry is in the evidence, not in the tone.
The instrument: the most reliable free way to know which side you are on is whether you can finish a sentence.
The strongest thing here is a design, not a p-value: researchers pushed the ventilatory threshold down with blood donation and up with training, and both times the point where speech stopped being comfortable moved with it. [3] A later review summarised the talk test as "valid, reliable, practical and inexpensive". [4] One honest limit: the errors ran toward passing the test while already above the threshold [3], so it is best used to defend the ceiling of easy.
The mapping: this app's Easy / Moderate / Hard are those three zones, one for one.
Definitional, not a finding — it follows from using the same split, and no trial needs to establish it. It is stated because it means what you learn on this page transfers straight to every cardio session in your plan, with no second vocabulary to keep in your head.
Why does my watch say five and the research say three?
Because the two are measuring different things. The three-zone lines are found on you: as effort rises, your breathing visibly changes rhythm at two points, and those two turns are the boundaries. A watch cannot do that — all it has at your wrist is heart rate, so it does the next best thing and cuts your maximum into percentage bands. Whether that is five bands or three, and where each one starts, differs between brands. That disagreement is itself the tell: these are conventions, not measurements.
Zone on your watch
Where it sits in the three
What this app calls it
Zone 1–2 (about 50–70% of max HR)
Below the first threshold
Easy — a full sentence, comfortably
Zone 3 (about 70–80%)
Between the two thresholds
Moderate — you can talk but not sing
Zone 4–5 (about 80–100%)
Above the second threshold
Hard — a few words at a time
Read the table as a translation, not a conversion. Where your own first threshold falls as a percentage of your maximum is personal, and it can sit ten or more percentage points away from someone else’s. So do not run it backwards: seeing 68% on the watch is not proof you are still in the easy zone.
The line you can trust, and the line you cannot
Treating both boundaries as equally solid is the most common mistake on this topic, and the evidence does not support it. The line between easy and moderate — the first threshold — has research behind it as a real divide between two physiological states. The line between moderate and hard does not: of the methods people commonly use, almost none are supported, with the exception of critical power and critical speed, both of which need a dedicated test. [1]
Spend your attention on the lower line — on making sure your easy days are genuinely easy. Do not chase a heart-rate number for the upper one: hard training is defined by its structure and by working near your limit, and the
HIIT guide is where that is laid out.
You already own the instrument
The most convincing thing about the talk test is an experiment, not a statistic. Researchers wanted to know whether "can you still finish a sentence" really tracks the physiological threshold, so they went and moved the threshold: first down, by having subjects donate blood, then up, with a block of training. Both times, the point where speaking became difficult moved with it. [3]
How to use it: while moving, say a full sentence of a dozen or so words out loud. The content does not matter — recite something, or count.
Still comfortable → you are in the easy zone, below the first threshold.
You get it out but have to breathe mid-sentence → you are into the middle band.
A few words at a time → you are on the hard side.
First, it does not work during intervals — the review that recommends it says outright that it "may not be practical for high-intensity interval training" [4], which is the same boundary that was already blurry. Second, its errors have a direction: it more often passes you when you are in fact already past the threshold [3]. So its best use is defending the ceiling of easy, not confirming that you went hard enough.
Then why is a percentage of max heart rate on everything?
Because it is the only thing a wrist strap can compute. And there is a second estimate stacked underneath: most devices have never measured your maximum at all, they predicted it from your age. The analysis that pooled 351 studies, 492 groups and 18,712 subjects landed on 208 − 0.7 × age, cross-validated it in a further 514 people, and noted that the more familiar 220 − age systematically underestimates older adults. [5] So a watch zone is a percentage of an estimate. That is fine for trends — the same route at a lower heart rate this month than last is real information — and poor for deciding whether today counts as easy. For how to estimate your maximum and measure your fitness, the
cardio guide goes into it properly.
So what about Zone 2 training?
What people mean by "zone 2" is essentially that whole band below the first threshold — which in this app is Easy. That it is worth doing is not in dispute. What it gets turned into is.
Its case is mostly descriptive: watch how high-level endurance athletes actually train, and about 75% of sessions land below the first threshold [2], with multiple studies converging on roughly 80% low intensity and 20% high [6]. That is "this is what they do", not "this won when people were randomised to it" — and the difference matters. The same review notes that studies intensifying training in already well-trained athletes have not produced convincing evidence of long-term gains. [6]
There is a complication worth knowing. A network meta-analysis of individual participant data from 13 studies and 348 athletes found no difference between polarized and pyramidal distributions on VO₂max (SMD −0.06, p = 0.68) or time-trial performance (SMD −0.05, p = 0.34) — and, in subgroup analysis, that recreational and competitive athletes responded differently (SMD −0.63, p < 0.05), with recreational athletes if anything favouring the pyramidal distribution, the one containing more moderate work. [7] Put plainly: "keep almost everything easy" comes out of elite training logs, and most people are not elite athletes. For how to lay out an actual week, the
weekly cardio plan owns that.
Find your own line in one session
Warm up easy for ten minutes
Let your heart rate and breathing settle. The first few minutes of any session are unsettled, and nothing measured there means much.
Raise the effort gradually, talking as you go
Every minute or two, say a full sentence of a dozen words out loud. Stay where you can finish it comfortably, then nudge the effort up.
Mark the moment speaking gets hard
The point where you have to breathe mid-sentence is your working estimate of the first threshold, and the ceiling of your easy zone. Note the pace, the incline, or the heart rate there.
Repeat it in a few weeks
If training is working, the same pace becomes easier to talk at, or you hold the same speech at a faster pace. That is far more useful than chasing a fixed heart-rate number.
On the status of that procedure: it is a practical routine derived from the talk-test research, not itself a protocol that has been validated as a package. Its value is giving you a repeatable reference point, not a precise physiological measurement.
Do not memorise five percentage bands — find the one line you can feel for yourself: whether you can still finish a sentence. Defend it, and your easy days will actually be easy.
Once you know which side to be on, the rest is laying the time out. Omini handles the clock and the rhythm of the intervals — the half of intensity a talk test cannot give you.
Start a cardio session
Frequently asked questions
What are heart rate zones, and how many are there?
It depends on who is counting. Watches typically show five zones cut from percentages of your maximum heart rate. Exercise research generally uses three, separated by the two ventilatory thresholds — the points where your breathing pattern changes as effort rises. The three-zone version is the one with physiological boundaries behind it; the five-zone version is a convention that a wrist sensor can compute, and brands do not agree on where its lines fall.
What is Zone 2 training?
What people call zone 2 is essentially everything below the first ventilatory threshold — easy work where you can still hold a conversation. It is worth doing. But its case is largely descriptive: studies of high-level endurance athletes find about 75% of their sessions land there, converging on a rough 80/20 split. That is what elite athletes do, not the result of randomising people to it, and a 2025 network meta-analysis found recreational athletes may actually respond better to a distribution containing more moderate work.
What does Zone 4 on my watch mean?
Roughly 80–90% of your estimated maximum heart rate, which usually places you above the second ventilatory threshold — hard work where you can only manage a few words at a time. Treat it as approximate. Your maximum was probably predicted from your age rather than measured, and the boundary between moderate and hard is the one that exercise science supports least well, so the number on the screen is a rough guide rather than a verdict.
Do I need a chest strap or a lab test to train by zones?
No. The single most useful boundary — the ceiling of easy — is findable for free with the talk test. Researchers verified this by moving the ventilatory threshold down with blood donation and up with training; both times the point where speech became difficult moved with it. A lab test gives you a precise number, and a chest strap gives you a more accurate heart rate than a wrist sensor, but neither is required to train at the right intensity.
Is the talk test actually accurate?
For the easy/moderate boundary, yes — a review describes it as valid, reliable, practical and inexpensive, and it tracks the ventilatory threshold across changes in fitness. It has two known limits. Its errors lean toward passing you when you are already slightly above the threshold, and the same review notes it may not be practical during high-intensity intervals. So it is best used to keep your easy sessions honest rather than to confirm a hard one.
Why does my maximum heart rate estimate matter?
Because every zone on your watch is a percentage of it. Most devices never measured your maximum — they predicted it from your age. The best-supported formula, from a pooled analysis of 351 studies and 18,712 subjects, is 208 − 0.7 × age; the familiar 220 − age systematically underestimates older adults. So a watch zone is a percentage of an estimate, which makes it good for tracking your own trend and poor for deciding whether today was easy.
Which zone should most of my cardio be in?
Most of it easy, with a smaller amount genuinely hard — but do not treat the exact ratio as settled. The 80/20 pattern comes from observing elite endurance athletes, and a network meta-analysis of 13 studies and 348 athletes found no overall difference between polarized and pyramidal distributions, with recreational athletes if anything favouring the one containing more moderate work. The more useful discipline is making sure your easy days are actually easy.
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