What are heart rate training zones?

In one line

Heart rate training zones split exercise intensity into a few bands according to how close your heart rate is to your maximum heart rate.

What are heart rate training zones?
The 'Zone 2' and 'Zone 4' on your smartwatch
When you exercise with a smartwatch, words like 'Zone 2' or 'Zone 4' appear next to your heart rate. These are heart rate training zones. Here is what they are based on and how they are divided, step by step.
In one line
Bands of exercise intensity set by heart rate
Heart rate training zones split how hard you are exercising into a few bands, based on your heart rate during exercise. The bands are usually set as a percentage of your 'maximum heart rate', your heart rate at all-out effort. Each band is called a 'zone', as in 'Zone 1' and 'Zone 2'.
What heart rate is
How many times the heart beats in 1 minute
Heart rate is the number of times the heart beats in 1 minute. You can count your pulse with fingers on your wrist or neck, and a smartwatch measures it by shining light into your wrist. At rest, an adult's heart usually beats 60 to 100 times a minute.
Why heart rate?
The harder you move, the faster the heart beats
The harder you exercise, the more oxygen your muscles use. So the heart beats faster to send more oxygen-carrying blood. That is why heart rate during exercise is used to judge how hard the exercise is right now.
Maximum heart rate
The highest value reached at all-out effort
Maximum heart rate is the highest your heart rate goes during all-out exercise. It drops with age, so it is often estimated as '220 minus your age'. By this formula, a 30-year-old's maximum heart rate is 190.
Which percentages?
Moderate and vigorous intensity
The American Heart Association counts 50 to 70% of maximum heart rate as moderate intensity and 70 to 85% as vigorous intensity. At moderate intensity you can talk but not sing; at vigorous intensity you have to stop for breath after a few words.
Five bands
The most used scheme is Zones 1 to 5
There are many ways to split zones, but among endurance exercisers and coaches the five-band scheme (Zones 1 to 5) was used most. The lowest band is easy enough for recovery, and the second band, Zone 2, is an easy pace you can keep up for a long time. The middle band is moderate, and the top two are hard and very hard.
What sets the borders
Two points set by lactic acid
Even when zones are split by percentages, the anchor points behind them come from changes in the body. The marker is 'lactic acid', a substance made when glucose is split. The two markers are the point where blood lactic acid starts to rise as intensity goes up, and a higher point where it starts to keep piling up. In the five-band scheme, up to Zone 2 is below the first point, and the middle band is between the two.
Common misconception
Is '220 minus age' my real maximum heart rate?
No. The formula is an estimate by age based on many people's values. Compared with adults' measured maximum heart rates, it was close on average, but for individuals the gap was as large as more than 20 beats per minute. The exact value is measured with an exercise test that gets harder step by step.
Summary
Bands of intensity measured against maximum heart rate
Heart rate training zones split exercise intensity into bands by what percentage of your maximum heart rate you are at. '220 minus age' is only an estimate, so it differs from each person's real maximum heart rate. Related terms: lactate threshold, VO2max (maximal oxygen uptake).