In Zone 2, you mainly use fat for fuel. Training in Zone 2 is great for your mitochondria and your metabolic flexibility.
Your Heart-Fit Score is not a heart rate zone. It is the maximum amount of energy that your body can use during exercise.
It measures your aerobic fitness and is highly correlated with longevity.
Your heart fitness inevitably declines with age. So, the higher your score when you are a young adult—the more reserves you will have as you get older.
Along with your Vitality Score, research has shown that your Heart-Fit Score may be one of the single best predictors of lifespan and healthspan.
