Tip · Movement

Zone 2 cardio 2–4× per week

Q: How do I recognize Zone 2 without a lab test?

With a rule of thumb, only very roughly. In a measurement of 50 cyclists, the common Zone 2 markers scattered with coefficients of variation between 6 and 29 percent, with fixed percentages of maximum heart rate among the least reliable. The first ventilatory threshold and the maximal fat oxidation point agreed best, and both require a measurement. Cardiac rehabilitation guides make do with the Borg scale and the talk test when cardiopulmonary exercise testing is not available.

Q: Is the rule of 60 to 70 percent of maximum heart rate correct?

Not for many people. Thresholds derived from a maximum heart rate calculated with an age formula consistently underestimated the lactate thresholds, with systematic bias, in a study of 58 runners. Thresholds derived from a measured maximum heart rate were still too low, though less so. The formula is therefore an approximation whose direction of error is known.

Q: Do I really need a lactate test?

If it needs to be precise, yes. The scatter of the alternatives is too large to be argued away. If it is only about training easily on a regular basis at all, the criterion that you could still talk while doing it is sufficient as a working basis. It just does not guarantee that you hit the intended intensity.

Q: How often and for how long?

The long-term study Generation 100 trained moderately twice a week for five years, the comparison arms of several intervention studies three times a week for twelve weeks. A meta-analysis on muscle biopsies included interventions lasting two weeks or longer and names training duration and modality as possible influencing factors. An optimal dose for Zone 2 cannot be derived from this.

Q: Is cycling or walking better?

For the cardiovascular system, both are useful, and on the bike the intensity can be held more precisely. For bone strength the difference is large: the DVO S3 guideline on fracture prevention explicitly calls for weight-bearing loading and lists cycling and swimming as not belonging to this category. Anyone who wants both combines them.

Q: Why do professionals train so much at an easy pace?

Because this intensity can be sustained in large volumes. World-class runners complete 11 to 14 sessions per week and cover at least 80 percent of their total volume over the training year at low intensity. That is an argument about the tolerable total amount and comes from competitive sport, not from a health study.

Easy endurance training builds mitochondria. That Zone 2 of all intensities is the optimum for this is a popular exaggeration.

Easy endurance training is undisputedly worthwhile. The difficult part is not the training but the zone: the common rules of thumb miss the intended intensity in many people. This page therefore deals with intensity control — how to find the zone and where the approximations fail.

In short

Moderate continuous training works: in a meta-analysis of 14 biopsy studies with 184 participants, mitochondrial density (p < 0.00001) and VO2max (p < 0.0001) rose significantly. The zone itself, by contrast, is a measurement problem. In a measurement of 50 cyclists, the coefficients of variation of the common Zone 2 markers ranged between 6 % and 29 %; fixed percentages of maximum heart rate were among the least reliable. Only the first ventilatory threshold and the maximal fat oxidation point agreed well — both require a measurement. If it needs to be precise, there is no way around lactate testing or cardiopulmonary exercise testing; if it is only about regular easy training, the talk test is sufficient as a working basis.

What Zone 2 means physiologically

Below the first threshold, the muscle works predominantly oxidatively. The stimulus for adaptation comes from the repeated, long duration of contraction and not from a high metabolic disturbance. The intended boundary is precisely the one at which fat oxidation reaches its maximum and ventilation begins to rise disproportionately for the first time.

Exactly these two markers agree in measurement: the first ventilatory threshold and the maximal fat oxidation point matched well in 50 cyclists. Everything else that is commonly used as a Zone 2 marker scatters more widely.

Why the rules of thumb miss

The cyclist measurement is the study that tests the approximations: 50 participants, 30 men and 20 women, one ramp test and one step test each in the laboratory, measuring power, heart rate, blood lactate, ventilation and substrate use. The coefficients of variation of the derived Zone 2 markers ranged between 6 % and 29 %. Fixed percentages of maximum heart rate and lactate thresholds showed the largest individual differences.

On top of that comes a second, systematic error. In a treadmill test with 58 participants, thresholds derived from a theoretically calculated maximum heart rate consistently underestimated the lactate thresholds, with a clear bias. Thresholds from the measured maximum heart rate also underestimated, but agreed better. If you use heart rate at all, then start from a measured rather than a calculated maximum heart rate.

What the type of exercise also determines

Cycling, rowing and brisk walking keep the intensity more stable than running, because the power output can be set directly. For bone, however, this is the weaker choice: the DVO S3 guideline on fracture prevention calls for weight-bearing loading for bone strength, with recommendation grade A and evidence level 1a, and explicitly lists cycling and swimming as activities that do not fall into this category. Anyone who does Zone 2 only on the bike does not cover bone as well.

What is well supported

Robust, first, is the training effect itself: 14 studies with 184 participants, all with a muscle biopsy from the vastus lateralis before and after at least two weeks of moderate continuous training, show a significant increase in mitochondrial density and VO2max as well as a modest increase in citrate synthase (p = 0.05) and mitofusin 2 (p = 0.01). Robust, second, is the scatter of the markers, with coefficients of variation between 6 % and 29 %. And robust, third, is that easy training makes up the largest share for those who make a living from it: world-class runners complete 11 to 14 sessions per week, marathon runners 160 to 220 km and track runners 130 to 190 km in the middle preparation phase — and at least 80 % of total running volume over the training year at low intensity.

What the studies show

Meixner 2025: the study that tests the rules of thumb

50 cyclists, 30 men and 20 women, completed a ramp test and a step test in the laboratory. From power, heart rate, blood lactate, ventilation and substrate use, the common Zone 2 markers were derived and their variability determined. Depending on the parameter, the coefficients of variation ranged between 6 % and 29 %. The authors argue for individual targets based on the first ventilatory threshold and the maximal fat oxidation point. Limitation: these are cyclists, not untrained people, and it is a cross-sectional measurement, not a training study.

Kanniainen 2025: why the age formula is not enough

58 participants completed a stepwise incremental treadmill test. Thresholds from a new method based on the dynamic correlations of RR intervals were compared with the lactate thresholds and with thresholds from theoretical and measured maximum heart rate. Thresholds from the theoretical maximum heart rate consistently underestimated the lactate thresholds and showed a systematic bias. It is a methods study, not a training study.

Generation 100: the long-term study with the hard endpoint

1,567 people born between 1936 and 1942, mean age 72.8 ± 2.1 years, five years, randomized to moderate continuous training twice a week at around 70 % of peak heart rate (n = 387), high-intensity interval training at around 90 % (n = 400) or following the national physical activity recommendations (n = 780). The primary endpoint, all-cause mortality, was not met. For the continuous training group, mortality, at an observed 4.7 %, was 1.2 percentage points higher than in the control group (hazard ratio 1.24, 95 % CI 0.73–2.10) — an interval that clearly includes 1 and thus demonstrates neither harm nor benefit.

Where the data stop

The scatter of the markers was measured in 50 cyclists, not in untrained or older people — that is, not in those who read this tip. How large the scatter is there, nobody knows. The certainty of evidence for the training effect itself is also weak: the meta-analysis of 14 studies with 184 participants explicitly rates its certainty of evidence as very low to low and names training duration, modality and sex as sources of heterogeneity.

How to do it

The gold standard is cardiopulmonary exercise testing with determination of the first ventilatory threshold, or lactate testing. In cardiac rehabilitation, prescription based on ventilatory thresholds is the preferred method; where it is not available, a review on intensity prescription recommends combining an objective test without gas analysis — at least one functional test such as the 6-minute walk test — with subjective methods such as the Borg scale and the talk test. On dose: Generation 100 trained twice a week for five years at around 70 % of peak heart rate. The comparison protocol of a well-known intervention study was moderate continuous training at 70 % of the highest measured heart rate, three times a week for twelve weeks. The meta-analysis included interventions lasting two weeks or longer. A dose-finding study that tests frequencies against each other does not exist for Zone 2.

Safety

For healthy adults there are no specific contraindications. In coronary heart disease the risk is low, but not zero: in the largest count from training practice, with 4,846 coronary patients and 175,820 training hours, one fatal cardiac arrest occurred during moderate training, that is, one event per 129,456 hours — counted in supervised rehabilitation. Under beta blockers, control via a percentage of maximum heart rate does not work, because the latter is lowered by the medication. For vertebral fractures and for knee or hip replacements, the DVO S3 guideline lists impact loading as contraindicated, so cycling and rowing rather than running. And anyone with osteoporosis should know that precisely these knee-friendly forms do little for bone. This is information, not a treatment recommendation.

BK-Score Supported, with caveats

Human evidence		6
Mechanism		7
Safety data		9
Hype gap		4
Track record of use		9

That endurance training increases mitochondrial density goes back to Holloszy 1967 – a rat study; the human data are more recent, and the latest meta-analysis itself rates its certainty as very low to low. That Zone 2 intensity of all things represents the maximum is a popular exaggeration – in direct comparison, high-intensity training often does just as well for mitochondrial biogenesis.

What is rated is the state of knowledge, not the effect. “Safety data 9” means well studied – not harmless.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)

Frequently asked questions about Zone 2 cardio 2–4× per week

How do I recognize Zone 2 without a lab test?