Zone 2 training: your real heart rate and what your blood shows

Zone 2 training is endurance work at roughly 60 to 70 percent of your maximum heart rate: hard enough to stimulate fat oxidation and your mitochondria, easy enough to keep talking. For many trained adults that sits somewhere between 120 and 145 beats per minute.

That number is rarely the same as the one on your watch.

The zone your smartwatch shows almost always comes from the rule of thumb 220 minus your age. In individual athletes that formula can be off by 10 to 20 beats, and then you spend months training slightly too hard without noticing. Below is how to find your own zone, and which blood values show whether it is working.

What is zone 2 training exactly?

Zone 2 is the highest intensity at which your body still clears lactate as fast as it produces it. Your lactate stays around 2 mmol/l instead of climbing. Physiologically, this is where your aerobic base is built.

The idea is not new. Research on training intensity distribution in well-trained endurance athletes has shown for years that roughly 75 to 80 percent of their training time sits at low intensity, with only a small share genuinely hard.

That distribution is called polarized training. A meta-analysis of randomized studies found polarized distribution outperformed training that hovers around threshold.

Zone 2 is not the zone that makes you fast. It is the zone that makes the rest possible.

How do you calculate your zone 2 heart rate?

Start with an estimate and correct it by feel. The 220 minus age formula gives you a starting point: at 40 it estimates your maximum at 180 and your zone 2 at roughly 108 to 126 beats.

Use that as a departure point, not as truth.

The practical check is the talk test. In zone 2 you can speak full sentences without gasping, but singing no longer works. As soon as your sentences shorten, you are too high and should ease back.

A more precise variant uses your heart rate reserve: subtract your resting heart rate from your estimated maximum, take 60 to 70 percent of that, then add your resting heart rate back. With a maximum of 180 and a resting rate of 50 you land around 128 to 141.

Which heart rate zones are there?

Most watches and training apps use five zones, expressed as a percentage of your maximum heart rate. They differ in what they stimulate physiologically and in how long you can hold them.

Zone 3 is the trap. It feels productive and it is just too easy to be genuinely hard and just too hard to be genuinely easy, which is how many recreational athletes accidentally spend their whole week there.

Why does zone 2 feel so slow?

Because it is. Anyone used to pushing steadily often has to give up a minute or more per kilometre in zone 2, and for runners that sometimes means walking the hills.

That is not a sign you are doing it wrong.

The pace at which you run zone 2 is itself the measurement: as your aerobic base grows, you go faster at the same heart rate. That is the progress you track, not your heart rate.

What does zone 2 training change in your blood?

Low intensity endurance work mainly acts on how your body handles fuel. That is exactly what a blood test can show and your watch cannot. The changes are slow: expect eight to twelve weeks before a repeat measurement says anything useful.

Your triglycerides and HbA1c are the two most useful here, because both summarise a period rather than a moment. For the insulin side, the background sits in fasting insulin and HOMA-IR.

Watch the other direction too. High endurance volume can slowly drain your iron stores, especially in runners, and you notice that in your recovery before you notice it in your heart rate.

How much zone 2 do you need per week?

The Dutch Health Council advises adults at least 150 minutes of moderate intensity activity per week, and zone 2 falls squarely within that. Athletes chasing training adaptation usually go higher, often three to four sessions of 45 to 90 minutes.

More important than the total is the distribution. If you keep hard sessions hard and easy sessions genuinely easy, your body does something different than when everything lands in the middle.

One long session does more than three short ones. Below 30 minutes little of what you are after happens.

Where do you start?

Lock one session this week to the talk test and ignore your pace completely. Note your average heart rate and your average speed, then repeat the same route in eight weeks.

If you are faster at the same heart rate, it is working.

If you want the metabolic side visible, our insulin resistance test with HOMA-IR gives you glucose and insulin with an assessment by a BIG-registered doctor. For the wider context, the guide to healthy ageing as an athlete is there, and the difference between your threshold and your VO2max sits in lactate threshold or VO2max.

If you are unsure whether the rest of your protocol holds up, see which biohacks survive the research.

References

1. Stöggl T, Sperlich B. The training intensity distribution among well-trained and elite endurance athletes. Frontiers in Physiology. 2015. PMID 26578968.
2. Rosenblat MA, Perrotta AS, Vicenzino B. Polarized vs. threshold training intensity distribution on endurance sport performance: a systematic review and meta-analysis of randomized controlled trials. Journal of Strength and Conditioning Research. 2019. PMID 29863593.
3. Comparison of polarized versus other types of endurance training intensity distribution on athletes endurance performance: a systematic review with meta-analysis. Sports Medicine. 2024. PMID 38717713.
4. Casado A, González-Mohíno F, González-Ravé JM, et al. Does lactate-guided threshold interval training within a high-volume low-intensity approach represent the next step in the evolution of distance running training? International Journal of Environmental Research and Public Health. 2023. PMID 36900796.
5. Dutch Health Council (Gezondheidsraad). Physical activity guidelines. Accessed 2026.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article provides general information and is not a substitute for medical advice. Discuss your results with your doctor before making treatment decisions.