Zone 2 refers to heart rate training zones. Generally heart rate zones are on a 5 zone scale, 1 being the lowest or easiest and 5 being the highest or hardest. See my post on heart rate zone and how to define them here
There are a lot of confusing terms for those of us that aren’t physiologists or sports scientists. I think of Zone 2 as the aerobic zone, this is where the muscles use mostly fat for energy, and the muscles tire much less easily, meaning you can sustain the level of effort for much longer. Above Zone 2 you move into anaerobic zones, where the muscles use glycogen (from carbs) for energy, the effort produces lactic acid (giving you that burning feeling in your muscles) and can be sustained for much shorter periods.
My training and research helped me understand that zone 2 training should account for around 80% of your training, and of the remaining a significant amount should be at Zone 5. Most professional athletes know and follow this (which I learned through listening to countless hours of podcasts with many of the worlds successful endurance athletes), but most amateur athletes spend most of their time trying in Zones 3 and 4, meaning their easy training isn’t easy enough and isn’t building their aerobic capacity and their hard training is too easy and isn’t improving their ability to transport oxygen around the body.
A model for the 5 heart rate zones is below, where zones 1 and 2 are aerobic and zones 3, 4 and 5 are anaerobic. It’s actually much more complicated and unique to each individual, but a simple rule of thumb is:
- Heart rate zone 1: Active recovery – 50–60% of max heart rate
- Heart rate zone 2: Aerobic – 60–70% of max heart rate
- Heart rate zone 3: Tempo – 70–80% of max heart rate
- Heart rate zone 4: Threshold – 80–90% of max heart rate
- Heart rate zone 5: VO2Max – 90–100% of max heart rate
For me Zone 2 training focuses on the aerobic engine and has a number of benefits for both health and fitness:
- Generates the highest level of fat burning for fuel
- Puts the highest stress on mitochondria and capillaries, encouraging both the increase density increasing your overall aerobic capacity
- Builds endurance, allowing you to perform at a sustained pace for longer