Finding Zone 2: Heart Rate, Lactate, Talk Test or Breathing Threshold?

Few training ideas have spread as fast as Zone 2. Podcasts, apps and smartwatches all push it, and many triathletes now try to do most of their winter training "in Zone 2". The trouble is that people often mean different things by it, and the most common way of setting it, a percentage of maximum heart rate, is one of the least accurate.

This article explains what Zone 2 is meant to capture, why fixed percentages misclassify many athletes, and how lab tests, lactate, the talk test and heart-rate variability methods compare. It's about finding the intensity, not about whether Zone 2 should dominate your week. For that, see TriForward's "Polarised, Pyramidal or Threshold: How to Spread Your Winter Intensity".

What Zone 2 is supposed to mean

There are two common zone systems, and they use the number 2 differently:

• Three-zone research models. Zone 1 is below the first threshold, Zone 2 is between the first and second thresholds, and Zone 3 is above the second.
• Five-zone coaching and app models. Zone 2 usually means the upper part of easy training, up to roughly the first threshold.

When most people say "Zone 2" today, they mean the second: the highest intensity that still counts as easy. That makes the first threshold the key marker. It's known by several names depending on how it's measured:

• First lactate threshold (LT1): the point where blood lactate starts to rise above resting levels.
• First ventilatory threshold (VT1): the point where breathing starts to rise faster than oxygen use.
• Gas-exchange threshold: a closely related marker from breath analysis.

Below it, you can ride or run for a long time with modest fatigue. Above it, you move into the heavy domain, where fatigue builds faster. A 2020 review by Jamnick, Pettitt, Granata, Pyne and Bishop in Sports Medicine concluded that LT1, the gas-exchange threshold and VT1 all have some support for marking that boundary.

The problem with percentages

Thresholds vary widely between people

A 2020 study by Iannetta and colleagues in Medicine and Science in Sports and Exercise tested 100 adults, 46 women and 54 men. Each did a ramp test to measure maximum oxygen uptake (VO2max), peak power and maximum heart rate, plus the lactate threshold, and 30-minute constant rides to find the maximal lactate steady state, the upper boundary of the heavy domain.

Where those thresholds fell varied enormously:

• The lactate threshold sat between 60% and 90% of maximum heart rate.
• It sat between 45% and 74% of VO2max, and between 23% and 57% of peak power.
• The maximal lactate steady state sat between 75% and 97% of maximum heart rate.
• Women's lactate threshold occurred at a higher percentage of maximum heart rate and VO2max than men's.

The authors concluded that fixed-percentage methods conform poorly to the intensity domains and don't adequately control the training stimulus.

Inference: if you set Zone 2 at a fixed percentage of maximum heart rate, it could be comfortably easy for one athlete and in the heavy domain for another. The same applies to a percentage of FTP or of maximum power.

What a review concluded

The Jamnick review compared methods by how well they produce distinct physiological responses in each domain. Percentages of VO2max, maximum heart rate and maximum work rate were judged to have little merit for producing domain-specific responses. Submaximal markers such as LT1 and VT1 did better for the easy boundary, and critical power or critical speed was best supported for the boundary between heavy and severe exercise. TriForward's "Critical Speed for Runners" and "Is FTP Dead?" cover those upper markers.

Your maximum heart rate is also an estimate

Most percentage methods start from maximum heart rate, which itself is often guessed from an age formula or taken from a single hard effort. Any error in the maximum carries straight into every zone. Inference: a percentage of an estimated maximum adds a second layer of uncertainty to the first.

Lab tests: lactate and breathing

How well do they agree?

If you can access a lab test, the two main methods agree reasonably well. A 2016 study by Pallarés and colleagues in PLOS ONE tested 14 trained male cyclists twice on a graded test with both blood lactate and breath analysis.

• A lactate threshold method agreed with VT1: no significant difference, with a correlation of 0.72.
• Reproducibility was good: most lactate threshold methods had a coefficient of variation of 3.4 to 3.7% between tests, and ventilatory thresholds 1.6 to 3.5%.
• Not every method was equal. One popular lactate method (DMAX) was much less reproducible, with a coefficient of variation of 10.3%.

The authors concluded that lactate threshold tests can be a valid and reliable alternative to ventilatory thresholds.

Limits of lab tests

A lab threshold is measured fresh, usually in a short ramp test. Thresholds aren't fixed: TriForward's "Can the Turbo Replace the Long Ride? Indoor Volume and Winter Durability" describes a study in which power at VT1 fell after two hours of easy riding. Your Zone 2 at hour three isn't the same as at minute ten. Heart rate also drifts upwards on long sessions, as TriForward's "Why the Ironman Marathon Falls Apart After 25 km" explains.

The talk test

The talk test is the oldest and cheapest method: if you can talk comfortably, you're probably below your first threshold.

A 2013 study by Rodríguez-Marroyo, Villa, García-López and Foster in the Journal of Strength and Conditioning Research tested whether it works in 18 highly trained cyclists (sex not stated in the abstract). Each did one incremental test with breath analysis and, on another day, the same test while reading a standard paragraph aloud at each stage.

• The first stage where talking was no longer comfortable came at 3.6 W/kg and an average heart rate of 148 bpm. VT1 came at 3.7 W/kg and 150 bpm. They weren't significantly different.
• The first stage where talking wasn't possible came at 5.3 W/kg and 176 bpm, against 5.3 W/kg and 177 bpm at the second ventilatory threshold.
• The correlations were strong: 0.86 for workload at VT1 and the first uncomfortable stage.

The authors concluded the talk test is a simple, practical surrogate for the ventilatory thresholds in highly trained cyclists. Inference: the point where you'd rather not hold a conversation is a reasonable field marker for the top of Zone 2.

Heart-rate variability methods

A newer approach uses the pattern of beat-to-beat heart-rate variation during exercise, a measure called DFA alpha1. Some apps and watches now use it to estimate the first threshold.

A 2021 study by Rogers, Giles, Draper, Hoos and Gronwald in Frontiers in Physiology compared it with VT1 in 15 men aged 19 to 52 during a treadmill test. Two more volunteers were excluded because of frequent irregular heartbeats.

• Average heart rate was close: 154 bpm at the DFA alpha1 marker against 152 bpm at VT1.
• The correlation was high: 0.97 for heart rate.
• Individual error was wider: for any one person, the DFA estimate could sit roughly 12 bpm below to 8 bpm above their VT1.

The authors called for more research in women, during steady exercise and with different devices. Inference: it's a promising free method, but an individual reading can be several beats out, and it needs a chest strap that records every beat accurately.

Comparing the options

• Percentage of maximum heart rate or FTP: easy, free and least accurate. In Iannetta's study, the threshold could fall anywhere across 30 percentage points of maximum heart rate.
• Lab lactate or breathing test: most precise and reproducible, but costs money and is measured fresh.
• Talk test: free, surprisingly accurate on average, and works in any session. It's subjective, and harder on the bike in traffic or in the pool.
• HRV-based estimates: free with the right strap, close on average, but individual readings can be several beats out.
• Perceived effort: in the talk-test study, average perceived exertion at the first threshold was about 4 out of 10. Feeling easy is a reasonable check.

Running and swimming

All of the threshold studies here used cycling or treadmill running, and none tested swimming. A few practical points follow, all inference:

• Running: the talk test works the same way, and a treadmill ramp is easy to control. Hills and wind make heart rate jump outdoors, so judge easy runs over the whole session rather than chasing a number on every climb.
• Swimming: heart rate is hard to measure accurately in the water, and talking isn't an option. Pace-based methods are more practical; TriForward's "Critical Swim Speed" explains how to set swim intensity from a simple test.
• Brick sessions: heart rate often runs higher at the start of a run off the bike, so a cycling-based ceiling won't transfer directly.

Is Zone 2 the magic intensity?

Part of the Zone 2 boom comes from claims that it uniquely builds mitochondria and fat-burning capacity. A 2025 narrative review by Storoschuk, Moran-MacDonald, Gibala and Gurd in Sports Medicine examined those claims for the general public. They found the recommendations stem largely from observational data on elite endurance athletes who do large volumes of easy training, and concluded that current evidence doesn't support Zone 2 as the optimal intensity for improving mitochondrial capacity or cardiorespiratory fitness. Higher intensities matter, especially when training time is limited.

For triathletes, that doesn't make easy training pointless. It means easy training is valuable mainly because it lets you train more often and longer without wrecking recovery, not because a precise heart-rate band has special properties.

How to set your easy zone

The following combines the studies above. It's inference for triathletes, not a tested protocol:

1. Use the best measure you have. A lab lactate or breathing test is most precise. Without one, the talk test is a better starting point than a percentage of maximum heart rate.
2. Cross-check one method against another. If your watch's Zone 2 ceiling is a heart rate at which you can't speak comfortably, trust the talk test.
3. Set zones separately for each sport. Inference: heart rate at the first threshold is often different on the bike and the run, so one set of zones rarely fits both.
4. Use the zone as a ceiling, not a target. Easy days below the top of Zone 2 still count.
5. Expect drift on long sessions. Heart rate rises over time at the same power or pace, so on long rides use a mix of power, heart rate and talk test.
6. Recheck after a training block or a long break. Thresholds change with fitness.

A triathlon scenario

This example is illustrative, not drawn from a study. A 50-year-old triathlete sets his Zone 2 from his watch's age-predicted maximum of 170 bpm, giving a ceiling of about 119 bpm at 70%. On the bike he finds he can barely turn the pedals, and on the run he has to walk up every hill. He suspects he's doing it wrong.

He tries a talk-test ride: a gradual ramp on the trainer, reading a paragraph aloud every three minutes. Talking becomes uncomfortable at 138 bpm. A treadmill version gives 145 bpm. His winter easy rides now top out at around 135 bpm and his runs at around 142 bpm, with a check that he can still speak in full sentences. Next spring, a lab test confirms his first threshold is close to those figures.

His case isn't unusual. In Iannetta's data, the first threshold for some people was as high as 90% of maximum heart rate.

Common mistakes

• Using an age formula for maximum heart rate and building every zone on it.
• Using the same heart-rate zones for cycling and running.
• Treating Zone 2 as a target to hit exactly, rather than a ceiling for easy days.
• Ignoring heart-rate drift on long sessions and slowing to a crawl late in a ride to stay "in zone".
• Assuming the watch knows best. Device estimates are a starting point.
• Forgetting the point. Easy training is about recovery and volume, not a magic heart rate.

What we don't know

• Women. The talk-test and HRV studies didn't include women, and Iannetta found women's thresholds sit at different percentages.
• Triathletes across three sports. No study has tested these methods in the swim, bike and run in the same athletes.
• Long sessions. Almost all validation is from short incremental tests, not hours of steady riding.
• Consumer devices. Watch and app threshold estimates vary in how they're calculated and have rarely been independently tested.
• The training benefit of precision. No trial shows that a precisely set Zone 2 ceiling produces better results than a sensible talk-test ceiling.

References

• Iannetta D, Inglis EC, Mattu AT, Fontana FY, Pogliaghi S, Keir DA, Murias JM. A critical evaluation of current methods for exercise prescription in women and men. Medicine and Science in Sports and Exercise, 2020;52(2):466–473. doi:10.1249/MSS.0000000000002147
• Jamnick NA, Pettitt RW, Granata C, Pyne DB, Bishop DJ. An examination and critique of current methods to determine exercise intensity. Sports Medicine, 2020;50(10):1729–1756. doi:10.1007/s40279-020-01322-8
• Pallarés JG, Morán-Navarro R, Ortega JF, Fernández-Elías VE, Mora-Rodriguez R. Validity and reliability of ventilatory and blood lactate thresholds in well-trained cyclists. PLOS ONE, 2016;11(9):e0163389. doi:10.1371/journal.pone.0163389
• Rodríguez-Marroyo JA, Villa JG, García-López J, Foster C. Relationship between the talk test and ventilatory thresholds in well-trained cyclists. Journal of Strength and Conditioning Research, 2013;27(7):1942–1949. doi:10.1519/JSC.0b013e3182736af3
• Rogers B, Giles D, Draper N, Hoos O, Gronwald T. A new detection method defining the aerobic threshold for endurance exercise and training prescription based on fractal correlation properties of heart rate variability. Frontiers in Physiology, 2021;11:596567. doi:10.3389/fphys.2020.596567
• Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ. Much ado about Zone 2: a narrative review assessing the efficacy of Zone 2 training for improving mitochondrial capacity and cardiorespiratory fitness in the general population. Sports Medicine, 2025;55(7):1611–1624. doi:10.1007/s40279-025-02261-y