Zone 2: where the trend for slow cardio came from and what comparisons have shown

Zone 2 is an intensity at which you can speak in full sentences and blood lactate levels barely rise. Over the past two or three years, it has evolved from a technical term in sports physiology into a standalone "longevity protocol" with promises regarding mitochondria. The term itself remained technical, while the promises were attached to it later.

What the term "zone" actually means

Heart rate zones are a way to map the intensity range between rest and maximum. There are several mapping systems, and they do not overlap: a five-zone model based on a percentage of maximum heart rate, a three-zone model based on two lactate thresholds, and a seven-zone model based on power for cyclists.

In the three-zone model, zone 2 is the segment between the first and second thresholds. In the five-zone model, zone 2 is approximately 60–70% of the maximum heart rate. These are different things with the same name, and part of the confusion in conversations about "zone 2" stems from this.

A separate issue is the maximum heart rate itself. The "220 minus age" formula has a standard deviation of about ten beats, meaning that for two people of the same age, the actual maximum can differ by twenty beats. A zone calculated from such an estimate inherits this error entirely.

Where the 80/20 rule came from

Observation of intensity distribution in endurance athletes has shown that most of the volume falls on low intensity, a smaller portion on high intensity, and the middle zone is used little. This pattern is called polarized.

It was then tested as an intervention. A meta-analysis of 17 studies with 437 participants compared the polarized distribution with other schemes: in terms of VO2peak gain, the polarized model proved better with a standardized effect of 0.24, but the advantage appeared only in short programs of up to 12 weeks and only in well-trained athletes (0.46). For other endurance indicators—time trials, thresholds—there was no difference between the schemes. Review in Sports Medicine.

An earlier meta-analysis comparing polarized and threshold schemes yielded a similarly cautious conclusion — Journal of Strength and Conditioning Research. A network meta-analysis of 13 studies with 348 athletes found no differences between polarized and pyramidal schemes in either VO2max or time trials, and formulated the conclusion as follows: the response to intensity distribution depends on the level of training—a polarized scheme may suit competitive athletes, while a pyramidal one may suit amateurs. Study in Sports Medicine.

Three meta-analyses on the same data provide three slightly different answers. This in itself is information: if the difference between the schemes were large, it would not fluctuate like this.

What has migrated into biohacking

In sports science, zone 2 is a way to accumulate volume without driving oneself into a pit of fatigue. In biohacking, a different promise has been attached to it: that this specific intensity "builds mitochondria" and therefore affects metabolic health.

The first part is true in the sense that aerobic training increases the oxidative capacity of the muscle. The second part—about the uniqueness of zone 2 in this regard—does not follow from the studies: high-intensity intervals provide comparable or greater shifts in mitochondrial markers in less time. What exactly stands behind the word "mitochondria" in advertising is covered in the article on mitochondria and cellular energy.

How the zone is determined without a lab

The accurate method is a lactate test: a series of stages with a finger prick at each. Approximate methods include:

• The talk test. You can speak in full sentences, but you can no longer sing.
• Nasal breathing. Maintaining a steady rhythm without switching to mouth breathing.
• Percentage of actual maximum heart rate. Actual—meaning measured in a test to exhaustion, not derived from a formula.

All three provide a range, not a number. Only a lab provides a number, and it is important to understand that heart rate drift in the heat, after poor sleep, or during the second hour of exercise shifts the picture.

What remains if you remove the promises

Reliable: aerobic exercise increases endurance, and endurance is strongly associated with mortality in cohorts—the figures are in the article on VO2max. Unreliable: that a specific intensity distribution has independent significance for the health of a person who does not compete. In meta-analyses, the difference between schemes is small and manifests in well-trained individuals, not beginners.

Count it from a photo

Frequently asked questions

What is zone 2?

The intensity between the first and second lactate thresholds in the three-zone model: lactate is still barely accumulating, and speaking in full sentences is possible. In the five-zone model, zone 2 refers to a different range—approximately 60–70% of the maximum heart rate.

Is it true that 80% of training should be at low intensity?

This describes how endurance athletes distribute their training volume. In a meta-analysis of 17 studies, a polarized scheme showed an advantage in VO2peak with a standardized effect of 0.24, but only in programs up to 12 weeks and among well-trained participants.

Does zone 2 build mitochondria better than other modes?

This is not supported by research. Aerobic exercise increases muscle oxidative capacity, but high-intensity intervals produce comparable shifts in mitochondrial markers in less time.

How to determine your zone without a laboratory?

Approximately: talk test, steady nasal breathing, percentage of actually measured maximum heart rate. All three provide a range. The '220 minus age' formula gives a standard deviation of about ten beats, and the zone inherits this error.

Is there a difference between schemes for an untrained person?

In meta-analyses, differences between intensity distributions are small and depend on the fitness level: a network meta-analysis of 13 studies with 348 athletes found no difference between polarized and pyramidal schemes and concluded that the former might be suitable for competitors, and the latter for amateurs.

Read next

• VO2max: the indicator with the strongest link to mortality — VO2max is the maximum volume of oxygen the body can consume per minute during maximal exertion.
• How many calories are burned during a workout: running, cycling, swimming, gym — An hour of running at a moderate pace for a 70 kg person is about 550–700 kcal, cycling at a leisurely pace is 350–450, swimming is 400–600, and strength training is 200–350.
• Carbohydrates during long workouts: when food is needed during exercise — Gels, isotonic drinks, and bars on the course solve one problem — preventing fuel from running out where it is truly needed.

This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.

Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.