Zone 2 training: what it is, what studies of easy endurance exercise show and what is still unproven

A few years ago “zone 2” was a term used by cycling coaches. Now it turns up on podcasts about longevity, on the screens of fitness watches and in gym conversation, usually attached to a promise: long, easy sessions at a carefully held low heart rate are said to build the cell’s energy machinery and protect against chronic disease in a way that harder exercise does not.

Easy endurance exercise is good for people, and nothing in the research disputes that. The narrower claim, that this one intensity is the best one, has been examined by exercise scientists and has not held up well. A group of Canadian researchers wrote in the journal Sports Medicine last year that current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity. The largest pooled analysis of training studies points the same way. Easy and hard training both work, and for a given amount of time, hard training does more.

What is zone 2 training?

Coaches divide exercise effort into zones, and the numbering depends on the system. In one common five-zone version, zone 1 is very light and zone 5 is all-out. Zone 2 is the second-easiest: a pace that can be held for a long time, with breathing still under control.

Physiologists pin it to something measurable. As effort rises from a walk toward a sprint, there is a point at which lactate, a by-product of burning fuel fast, first begins to rise in the blood and breathing first begins to quicken. This is called the first lactate or ventilatory threshold. Zone 2 sits just under it.

For all the confidence with which the term is used, it had no agreed definition until recently. A 2025 commentary led by Sebastian Sitko of the University of Zaragoza began from the observation that there is no clear consensus among coaches, athletes, and scientists regarding the definition of zone 2 training. The authors convened an expert panel of 14 applied sport scientists and professional coaches, who settled on intensities immediately below the first lactate or ventilatory threshold. Finding that threshold precisely takes a laboratory test. Heart-rate formulas on a watch give an estimate.

Where the case for zone 2 training comes from

The idea began with observation of the best endurance athletes in the world. Stephen Seiler, a sport scientist at the University of Agder in Norway, reviewed studies of how rowers, runners, cyclists and skiers at national and international level actually spend their training time. These were nationally or internationally competitive endurance athletes training 10 to 13 times per week, and the studies converged on a pattern in which about 80% of training sessions are performed at low intensity, with most of the remainder built around hard intervals.

Seiler’s explanation was practical. Endurance athletes appear to self-organize toward a high-volume training approach, and nobody can train twice a day at high intensity without breaking down. Easy sessions are what make the volume survivable.

The Canadian reviewers, led by Kristi Storoschuk and Brendon Gurd at Queen’s University with Martin Gibala of McMaster University, argue that the popular version of zone 2 took a wrong turn at this point. Popular media has recently positioned Zone 2 training as the best intensity for everyone, they write, and these recommendations largely stem from observational data of elite endurance athletes who engage in large volumes of Zone 2 training. What suits a person training 20 hours a week may not be the best use of time for one who has three.

What training studies show about easy and hard exercise

The fullest test comes from Knut Sindre Mølmen and colleagues at Inland Norway University of Applied Sciences. They gathered every training study they could find that had measured muscle before and after, pooling data from 5973 participants in 353 and 131 research articles on two things zone 2 is promoted for. One is mitochondria, the structures inside cells that turn fuel and oxygen into usable energy. The other is capillaries, the smallest blood vessels, which deliver that oxygen to muscle.

They sorted the training programs into three kinds: continuous exercise at low or moderate intensity, high-intensity training, and sprint intervals, meaning repeated all-out bursts of up to a minute and a half. Over a whole program, all three raised mitochondrial content by about a quarter, 23% for the easier training and 27% for each of the harder kinds, a difference too small to separate from chance. The gains were not influenced by age, sex, menopause, disease, or the amount of muscle mass engaged. What did matter was how often people trained and where they started. The size of the change is largely determined by the initial fitness level, with greater changes observed in individuals with lower initial fitness.

Efficiency was another matter. Measured per hour of exercise, sprint intervals built mitochondria about four times as fast as the easier training, and high-intensity training was a little under twice as fast. The authors’ summary gives each side its due: “a high exercise intensity will compensate for a low training volume in exercise training interventions with a short duration, but a larger exercise training volume is necessary to promote further increments in mitochondrial content over time”. Sprint training reached a ceiling within a few weeks. Easier training kept producing gains for longer.

On capillaries the easier training held up better. All three kinds added capillaries around each muscle fiber by similar amounts, and the density of capillaries rose most with continuous low-to-moderate exercise.

One limit on this evidence bears directly on zone 2. The review’s easier category was wider than zone 2. It took in any continuous training below the second ventilatory threshold, a higher marker than the first, so it mixed true zone 2 with moderately hard steady efforts. No pooled analysis has isolated zone 2 on its own.

Is zone 2 training the best intensity for health?

The experts who defined zone 2 did not claim so. Sitko’s panel expected a wide range of benefits from it and added that these expected adaptations might not be unique to zone 2 and could also be induced with sessions performed at slightly higher and lower intensities.

The Queen’s and McMaster group went further. For members of the public, they concluded, the evidence favors including harder efforts, particularly in the context of lower training volumes. Someone exercising two or three hours a week gets more adaptation from those hours if some of them are demanding.

Whole-body fitness shows the same thing. A meta-analysis of controlled trials compared steady endurance training with high-intensity interval training (HIT) for their effect on VO2 max, the standard measure of aerobic fitness. Its authors found that endurance training and HIT both elicit large improvements in the VO2max of healthy, young to middle-aged adults, with a modest edge to the intervals.

Easy exercise remains a sound choice. It is easier to recover from, less likely to put people off, and can be done for longer. The dispute is over the word “optimal”.

Polarized training in athletes: what trials found

Among athletes, the 80-to-20 pattern is known as polarized training, and it has been tested against other ways of dividing up effort. Pedro Silva Oliveira and colleagues at the University of Porto pooled those trials. Seventeen studies met the inclusion criteria, with 437 participants in all. Polarized training gave a slightly larger gain in peak oxygen uptake, and the advantage appeared particularly in shorter duration interventions and highly trained athletes. For performance in time trials it was no better than the alternatives.

That is modest support for the way elite athletes train, in elite athletes. It says little about a recreational exerciser.

What exercise guidelines say, and where zone 2 training fits

Public health guidelines are written in broader terms. The World Health Organization asks adults for a weekly total of moderate or vigorous aerobic activity and does not prescribe a zone. Its guidelines also recommend regular muscle-strengthening activity for all age groups and carry the reminder that some physical activity is better than none.

Easy, steady exercise of the zone 2 kind counts toward that weekly total like any other aerobic activity. The guidelines make no claim that it does something other intensities cannot.

What intensity is suitable for someone with a heart or lung condition is a separate matter that their doctor assesses.

No health trials yet: what zone 2 training research has not tested

The claims that made zone 2 famous are about health and long life. The studies are about muscle. Mitochondria and capillaries are measured in small samples of muscle taken before and after a training program, and nobody has shown that building more of them by one method or another changes who develops diabetes or heart disease.

No randomized trial has assigned members of the public to zone 2 or to another kind of exercise and followed them for health outcomes. The comparison that exists pools training categories broader than zone 2. And the precise threshold varies from person to person, which makes a watch’s “zone 2” an approximation.

Easy endurance exercise has solid evidence behind it as exercise, and the claim that zone 2 is uniquely beneficial does not.