Zone 2 training is performed at an intensity where you are primarily using fat as a fuel source through oxidative phosphorylation in the mitochondria. This builds **Mitochondrial Density**—the engine of your metabolic health.
#### The Mitochondrial Gap
Sedentary life and high-sugar diets lead to “mitochondrial dysfunction.” Zone 2 reverses this by forcing your cells to become more efficient at clearing lactate and burning fatty acids.
#### The Talking Protocol
1. **The Talk Test:** You should be able to speak in full sentences but sound slightly strained. If you can sing, you’re in Zone 1. If you can’t speak, you’re in Zone 3+.
2. **The 150 Minute Target:** Aim for 150–200 minutes of Zone 2 per week.
3. **Monitor Your Heart:** Find your specific Zone 2 range using our [Heart Rate Zones Calculator](/heart-rate-calculator/).
**Reference:** *San-Millán, I., & Brooks, G. A. (2019). Assessment of Metabolic Flexibility in Professional Athletes and Sedentary Individuals. Sports Medicine.*