Zone 2: What It Does for Running, Cycling and Swimming

Your watch shows "zone 2" during your Sunday easy run. A podcast tells you it's the key to endurance. Somewhere else, you hear it's just a trend. The answer is somewhere in between, and it changes with your sport.

Here's what the published studies show, then how to use this zone for running, cycling and swimming with your watch data.

Zone 2: One Name, Two Scales

The first trap is the vocabulary. Researchers split effort in two ways.

• The 5-zone model, used by most watches. In Meixner et al. (2025), zone 2 usually sits at about 72 to 82% of max heart rate (your highest possible heart rate), 1.5 to 2.5 mmol/L of lactate, or 60 to 75% of FTP on the bike.
• The 3-zone model, used by the Scandinavian researchers, including Seiler. Zone 1 covers everything below the first threshold, so it combines zones 1 and 2 from the 5-zone scale. Their "zone 2" is the middle zone between the two thresholds, the one polarized training tries to limit.

When someone tells you to do zone 2, ask which scale they mean.

In the same study, the first ventilatory threshold and the point where the body burns the most fat lined up well. A zone 2 measured on you is more reliable than a percentage from a chart.

What Zone 2 Is For: Easy Volume

The interest comes from one observation. In young national-level cross-country skiers, about 75% of workouts were clearly below the threshold zone, and 15 to 20% were well above it (Seiler and Kjerland, 2006). Elite athletes train surprisingly little at threshold intensity. A large part of their week is easy.

These observations show that elites do a lot of easy workouts. The idea then came from coaching practice: make easy work the base of a recreational athlete's week too, so you stay fresh for the hard sessions.

The review by Storoschuk et al. (2025), published in Sports Medicine, puts things in perspective. Recommendations in favor of zone 2 mostly come from elite athletes who do high volumes. The authors conclude that current evidence does not support zone 2 as the optimal intensity for mitochondrial capacity or fatty acid oxidation. Higher intensities matter a lot for cardiometabolic health, especially when training volume is low.

So if your training volume is low, putting everything into easy work is not the right conclusion. If you do more, an easy portion has its place. For running, this also depends on how you choose your workouts.

What Changes by Sport

Running

Three markers work together: heart rate, pace and how it feels. Coaches use a simple test: you should be able to talk in full sentences. For a first estimate of heart rate, use the heart rate zones calculator. Treat it as a starting point, not a measurement.

Comparing over time tells you the most. Take an easy run at 6:30 per km (10:28 per mile) with a heart rate of 140 bpm. Three weeks later, the same heart rate at 6:15 per km (10:04 per mile) means your base is improving. If the heart rate on your "easy" run creeps up toward your fast workouts, you're probably going too fast.

Cycling

You can use power, not just heart rate, to find zone 2. The marker from Meixner et al. (2025) is 60 to 75% of FTP, so estimate your FTP first.

Keep heart rate as a second check, to see whether today's "easy" power really is easy.

Swimming

This is the hardest sport to measure. In 26 competitive swimmers, an optical sensor on the temple (Polar OH1) was valid for heart rate. A Polar M600 on the wrist was less accurate: its differences from a chest strap ranged from -53 to +33.9 bpm (Olstad and Zinner, 2020). Distance is also uneven. In a 2024 study in a 50 m pool (164 ft), watch error ranged from 0% on a 400 m freestyle to 91.7% on a 200 m individual medley.

So go by pace per 100 m and how it feels to pace your zone 2. The critical swim speed (CSS) test gives you a reference pace to compare your workouts.

What Your Watch Really Measures

In a treadmill test with 10 watches and 30 participants, the average error of optical heart rate, compared with a chest strap, ranged from 3.1 to 8.3% depending on the watch (2024 study). At 140 bpm, that means about 4 to 12 bpm of difference, while zone 2 fits in a narrow window.

A 2025 study of a Polar Vantage V2 found good accuracy at moderate to hard intensities, and less accuracy when heart rate is low and the arm moves a lot. It did not test zone 2 directly. These tests use specific models (Polar M600 in 2020, Polar Vantage V2), so don't generalize them to your watch.

Compare your watch with a chest strap for one or two workouts, and look at trends rather than a single value.

What You Can Do at Your Next Workout

• ✓Pick one easy workout this week and set a feeling for it: you can talk in full sentences.
• ✓Note your pace (or power) and heart rate for the first half and the second half, the way you check your fueling on a long run.
• ✓Wear a chest strap once to check your watch.
• ✓If your training volume is low, keep some intensity in your week.

That's what ELYO does with your workouts. It reads your runs from your watch, compares pace, heart rate and volume week to week, checks whether your "easy" workouts really are easy, and adjusts your training plan.

Written with the help of AI.