What’s actually happening in your body when you run? And does it make a difference whether you jog along easily or push to your limit? Yes – and that’s exactly what heart rate zones are for. We explain what’s behind the numbers and why the right zone makes all the difference.
What are heart rate zones?
The five-zone model is today the most common way to classify training intensity based on heart rate. The foundation is two lactate thresholds – LT1 and LT2 – the points at which the body measurably switches to a different energy system. The idea of using these thresholds as zone boundaries comes from the sports physiology of the 1970s and 80s. Norwegian sports physiologist Stephen Seiler later systematically incorporated them into his studies on the training distribution of elite athletes, contributing significantly to their widespread use today.
To calculate the zones, you need your own maximum heart rate – most reliably determined through a field test or a VO₂max test.
Note: if you’ve been using the formula “220 minus age” up until now – it was always intended as a rough population estimate and is too inaccurate for individual training management.
Zone 1 — Recovery
Zone 1 is easy jogging where you can chat effortlessly. The body works almost exclusively on fat as an energy source, and lactate levels stay at resting values. What happens here is essentially active recovery: the increased blood flow carries metabolic waste products away from the muscles without creating new load. You could theoretically run in Zone 1 for hours – the next day you’ll barely feel it.
Typical use: warm-up, cool-down, recovery runs between hard sessions.
Zone 2 — Aerobic Base
Zone 2 is the foundation zone par excellence. Below the first lactate threshold, the body works predominantly aerobically (i.e. energy is produced using oxygen). Blood lactate concentration stays low, and energy comes primarily from fat burning.
Regular training in Zone 2 stimulates the body to build more mitochondria (the power plants of muscle cells, which produce energy from oxygen and nutrients). At the same time, the number of small blood vessels in the muscles increases, improving oxygen delivery. Both processes take months, but they are the foundation of every endurance performance.
An important caveat: Zone 2 is not the only way to build mitochondria. More recent research shows that higher intensities also trigger strong adaptations. The real strength of Zone 2 lies elsewhere – in the combination of good adaptation quality and low recovery costs. This allows large training volumes without overloading the body.
Typical use: long jog, endurance run 1.
Zone 3 — Tempo
In Zone 3, the body uses a mix of fat and carbohydrates. Lactate begins to accumulate noticeably but is still broken down sufficiently. Conversation becomes harder – short sentences are still possible. Sustained sessions in this zone improve the lactate threshold and train the body to work efficiently at higher paces. In running.COACH, endurance run 2 sessions are typically placed in this zone.
Typical use: endurance run 2, tempo runs.
Zone 4 — Threshold
Zone 4 is the hardest intensity that can still be maintained for a certain period of time. Conversation breaks down, single words are still possible.
Above the second lactate threshold, lactate accumulates faster than it can be broken down. The body now burns primarily carbohydrates, with the anaerobic energy system contributing substantially. The most important adaptation: regular threshold training shifts LT2 upward – you’ll eventually be able to run faster before lactate starts to accumulate. This is a key mechanism behind improved race performance at distances from 10 km to half marathon.
Typical use: tempo runs, intervals, race pace.
Zone 5 — VO₂max
Zone 5 is the maximum zone. Speaking is barely possible, all focus goes to the effort. Physiologically, the body has exceeded its aerobic capacity limit. The cardiovascular system can no longer deliver oxygen fast enough, anaerobic glycolysis and the phosphagen system take over. In this zone, you can run for a maximum of 2–3 minutes.
The training stimulus is exceptional: Zone 5 is the most effective zone for improving VO₂max – that is, maximum oxygen uptake and therefore endurance capacity. Studies show improvements of 5–15% over a training block.
Zone 5 sessions should be used sparingly.
Typical use: VO₂max intervals (400–1000 m), sprints.
How much time in each zone?
The exact distribution depends on the training phase. In a base phase, Zones 1 and 2 form the foundation – supplemented by targeted sessions in Zones 4 and 5. Closer to competition, the balance shifts: sessions in Zones 3, 4 and 5 increase, while Zones 1 and 2 remain as recovery and stability anchors.
Conclusion
Heart rate zones are not an end in themselves, but a tool. Those who understand what happens in the body in each zone train more purposefully and progress faster.
The good news: if you train with running.COACH, you don’t need to worry about the distribution. The app calculates the zones individually and manages training automatically – based on your fitness level, training phase and goal. You focus on running, running.COACH takes care of the rest.