Research · Training · 30 October 2026

Zone 2 for ultra runners: useful, not magic

Zone 2 is everywhere: podcasts, YouTube, your watch. It's often sold as the single best intensity for building your aerobic engine. So what does the research actually say, and what does it mean for ultra runners?

The 2025 review that questioned the hype

In 2025, researchers including Martin Gibala, well known for his interval training research, published a review in Sports Medicine titled "Much Ado About Zone 2".

They point out that the popular advice largely comes from observational data on elite endurance athletes, who do huge amounts of zone 2 training and have very high mitochondrial and fat-burning capacity. But that doesn't show zone 2 is why they're so fit, or that it's the best approach for everyone else.

Their conclusion:

• Current evidence does not support zone 2 as the optimal intensity for improving mitochondrial or fat-burning capacity.
• Higher intensities are important for maximising health benefits, especially for people who don't train many hours.

One important detail: this review focused on the general population, people training a few hours a week for health. It isn't a verdict on how high-volume endurance athletes should train.

Nobody agrees what "zone 2" means

Part of the confusion is that "zone 2" means different things depending on the system:

• In three-zone research models, zones are set by two thresholds. Zone 1 is below the first threshold, zone 2 is between the two, and zone 3 is above the second. Here, zone 2 is a moderate, "comfortably hard" effort.
• In popular five-zone systems, often based on a percentage of your maximum heart rate, "zone 2" is an easy effort.

So "do more zone 2" can mean two quite different things.

In 2025, a panel of 14 sport scientists and coaches published an expert viewpoint to try to settle it. They agreed zone 2 training should preferably be done at an intensity just below your first lactate or ventilatory threshold: the top end of easy. They also noted that the adaptations people expect from zone 2 might not be unique to zone 2, and could come from sessions slightly easier or harder too.

What elite runners actually do

Here the evidence is solid, though it's often oversimplified:

• In 2010, Stephen Seiler reviewed how elite endurance athletes train and found that about 80% of training sessions are done at low intensity. That's where the famous "80/20" comes from. It's a description of what top athletes do, not a proven formula.
• A 2022 systematic review of highly trained and elite distance runners found most use a pyramidal approach: lots of easy running, less moderate running, and the least hard running. Marathon runners leaned pyramidal; 1500m runners leaned more polarised.
• In a 2007 study of 12 sub-elite runners over about 5 months, the group doing more low-intensity training (about 80% of their time) improved more in a race simulation than the group doing more moderate training, with the amount of hard training kept the same.

The common thread: a lot of easy running, plus some harder work. Not easy running alone.

So what should ultra runners do?

Ultras are long, mostly low-intensity events, so the case for plenty of easy running is strong. It builds the volume and durability you need, with less fatigue and injury risk than lots of hard running. Just don't treat it as magic.

1. Keep most of your running genuinely easy. You should be able to hold a conversation.
2. Don't drop the hard sessions. Hills and intervals add things easy running doesn't. See our post on polarised training.
3. Know which zones you're using. Check whether your watch uses a percentage of max heart rate or your own thresholds. Our heart rate zones calculator can work out zones from a 30-minute threshold test.
4. On long days, trust effort as well as heart rate. Heart rate drifts upward on long runs at the same pace, especially in heat, so a fixed number can mislead you late in a long run.
5. Short on time? If you only run a few hours a week, the 2025 review suggests making some of it harder will give you more benefit.

Limitations

• The 2025 review is a narrative review focused on the general population, not trained ultra runners.
• The expert definition is opinion from a panel, not new experimental data.
• The 80/20 figure and the 2022 review describe what elite athletes do. They don't prove it's the best approach for every runner.
• The 2007 study was small (12 runners).

References

• Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ (2025). Much Ado About Zone 2. Sports Medicine, 55(7), 1611–1624. doi:10.1007/s40279-025-02261-y
• Sitko S et al. (2025). What Is "Zone 2 Training"?: Experts' Viewpoint on Definition, Training Methods, and Expected Adaptations. International Journal of Sports Physiology and Performance, 20(11), 1614–1617. doi:10.1123/ijspp.2024-0303
• 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), 276–291. doi:10.1123/ijspp.5.3.276
• Casado A et al. (2022). Training Periodization, Methods, Intensity Distribution, and Volume in Highly Trained and Elite Distance Runners: A Systematic Review. International Journal of Sports Physiology and Performance, 17(6), 820–833. doi:10.1123/ijspp.2021-0435
• Esteve-Lanao J, Foster C, Seiler S, Lucía A (2007). Impact of training intensity distribution on performance in endurance athletes. Journal of Strength and Conditioning Research, 21(3), 943–949. doi:10.1519/R-19725.1

This is a summary for information only, not medical or dietary advice. Read the original study for full details.