Zone 2 training

Intensity zones divide the range between rest and maximal effort into bands. They can be defined in three ways. The talk test uses speech: at an easy pace conversation is effortless, at a moderate pace full sentences are possible but singing is not, and at a vigorous pace only a few words can be spoken between breaths. Heart-rate zones use a percentage of maximum heart rate. Laboratory zones use blood lactate or breathing: as effort rises there is a first threshold, where lactate begins to rise and breathing deepens, and a second threshold, beyond which lactate accumulates quickly and the effort cannot be sustained for long.

Zone 2 lies at or just below the first threshold. For comparison, the Generation 100 trial in older adults set moderate continuous training at about 70% of peak heart rate and interval training at about 90% [7]. In absolute terms, moderate activity is usually defined as about 3 METs (three times resting oxygen use) and vigorous activity as about 6 METs. In a laboratory study of 76 adults aged 21 to 40, a walking cadence of about 100 steps a minute marked moderate intensity and about 130 steps a minute vigorous intensity [4].

The zone labelled Zone 2 on consumer sports watches is usually a percentage band of an estimated maximum heart rate rather than a measured lactate threshold. Different manufacturers and coaching systems draw the zone boundaries differently.

During low- and moderate-intensity exercise, skeletal muscle derives a large share of its ATP from mitochondrial beta-oxidation of fatty acids, which requires oxygen and produces little lactate. As intensity rises, glycolytic flux increases, carbohydrate oxidation rises, lactate production exceeds clearance and blood lactate concentration climbs. Proponents of Zone 2 training argue that sustained work at the upper end of the fat-dominant range is an efficient stimulus for mitochondrial biogenesis and for metabolic flexibility, the capacity to shift between fat and carbohydrate as fuel.

The best-known supporting data come from a laboratory comparison of professional endurance athletes, moderately active adults and people with metabolic syndrome, using indirect calorimetry and blood lactate measurements. Fat oxidation was significantly higher and blood lactate significantly lower in the athletes than in the other two groups. Within each group, fat oxidation and lactate were strongly inversely correlated (r = -0.97 in athletes, -0.98 in moderately active adults and -0.92 in metabolic syndrome), and the correlation across all participants was r = -0.76 [1]. The authors proposed lactate and fat oxidation during exercise as an indirect measure of mitochondrial function.

The study was a cross-sectional comparison of groups that differ in genetics, training history and body composition. It did not test whether Zone 2 training causes these differences in untrained people, and it measured no health outcomes. Specific popular prescriptions, such as an exact lactate target or a fixed number of weekly hours, are extrapolated from the training of elite endurance athletes.

Large mortality studies have measured self-reported moderate-to-vigorous leisure activity rather than training zones. The largest pooled analysis combined six US and European studies of 661,137 adults, with 116,686 deaths over a median of 14.2 years. Results are reported as hazard ratios (the risk in one group relative to another; 0.80 means 20% lower). Compared with no leisure activity, doing some but less than the recommended minimum of 7.5 MET-hours a week (about 150 minutes of moderate activity) was associated with 20% lower mortality (hazard ratio 0.80, 95% CI 0.78 to 0.82); one to two times the minimum with 31% lower (0.69, 0.67 to 0.70); two to three times with 37% lower (0.63, 0.62 to 0.65); and three to five times with 39% lower (0.61, 0.59 to 0.62), where the curve levelled off. There was no evidence of harm at ten or more times the minimum (0.69, 0.59 to 0.78) [2].

A companion pooled analysis of six studies with 654,827 adults and 82,465 deaths expressed the same kind of data as life expectancy. Activity equivalent to up to 75 minutes of brisk walking a week was associated with 1.8 more years of life expectancy after age 40 (95% CI 1.6 to 2.0) than no leisure activity, and activity equivalent to 450 minutes or more of brisk walking a week with 4.5 more years (4.3 to 4.7) [5]. Physical activity in both analyses was self-reported.

The World Health Organization physical activity guidelines recommend that all adults do 150 to 300 minutes of moderate-intensity or 75 to 150 minutes of vigorous-intensity aerobic activity a week, or an equivalent combination, together with regular muscle-strengthening activity [3]. Continuous moderate sessions are one of the most common ways the moderate target is met. The wider relationship between activity and lifespan is covered under exercise and longevity.

The Nurses’ Health Study and the Health Professionals Follow-up Study followed 116,221 US adults for 30 years, re-measuring leisure activity up to 15 times, with 47,596 deaths [6]. In analyses adjusting moderate and vigorous activity for each other, meeting the vigorous guideline of 75 to 149 minutes a week was associated with 19% lower all-cause mortality than no vigorous activity (hazard ratio 0.81, 95% CI 0.76 to 0.87), and meeting the moderate guideline of 150 to 299 minutes a week with a similar 19 to 25% lower risk of all-cause, cardiovascular and non-cardiovascular death [6].

The near-maximum association with lower mortality was reached at about 150 to 300 minutes a week of vigorous activity, 300 to 600 minutes a week of moderate activity, or an equivalent combination. Among participants doing less than 300 minutes a week of moderate activity, additional vigorous activity was associated with lower mortality; among those already doing 300 minutes or more, it was not [6]. This pattern is consistent with the guideline equivalence of one vigorous minute to about two moderate minutes [3].

Vigorous training does raise fitness more per minute. Across 24 meta-analyses of trials, high-intensity interval training increased VO2 max by 0.52 to 3.76 mL/kg/min more than moderate continuous training, though the included reviews were of mixed quality [8]. The comparison is covered in more detail under high-intensity interval training and VO2 max.

No outcome trial has compared Zone 2 training as a specific prescription with other moderate exercise, or with the same total work performed at mixed intensities, for mortality or disease incidence. Claims that Zone 2 training uniquely restores mitochondrial function, reverses metabolic disease or is the most effective form of exercise for longevity go beyond the available data. The lactate thresholds come from laboratory and athlete physiology [1], and the mortality figures come from observational studies of self-reported moderate activity [2, 6].

The observational studies share the limitations of all such research: activity was self-reported, active people differ from inactive people in ways that statistical adjustment only partly captures, and people with undiagnosed illness tend to be less active. The evidence supports steady moderate aerobic exercise, including brisk walking, as one form of the activity associated with lower mortality, but not a distinct benefit of Zone 2 over other moderate activity.