Zone 2 is easy aerobic effort: the intensity at which you can still hold a conversation. In a five-zone model based on maximum heart rate, it is 60 to 70% of your maximum.
Why runners care about it
Easy running builds your aerobic base with little fatigue, so you can do a lot of it and still recover for the hard sessions. Studies of endurance athletes by Stephen Seiler and others found that most of their training time is spent at low intensity, with a small share done hard and very little in between.
The common mistake is the opposite: easy days drift too fast, hard days end up too slow, and everything lands in the middle.
A caution about the numbers
“Zone 2” is not one fixed thing. Five-zone models based on maximum heart rate, models based on lactate threshold and three-zone models used in research all draw the lines differently. Percentages of maximum heart rate are a starting point. If you know your own thresholds from a test, use those.
In Kiiro
Kiiro uses five heart-rate zones, by default at 60, 70, 80 and 90% of your maximum heart rate, and you can set your own boundaries. For every workout it shows your time in zones, and the Intensity view shows how your recent training splits between low, medium and high. See heart rate zones in Kiiro.
More: How does Apple Watch set my heart rate zones, and can I change them?
Related terms
- Resting heart rate: Resting heart rate is the number of times your heart beats in one minute when you are calm and not moving. A lower value usually means a fitter heart.
- VO2 max: VO2 max is the amount of oxygen your body can use when you exercise your hardest. It shows how fit your heart and lungs are.
- Training load: Training load is how much strain your workouts put on your body. It combines how long and how hard you trained into one number.
