Your Fitbit Cardio Fitness score is an estimate of your Vo2max, which is a measure of how efficiently your body uses oxygen when you're working out hard. It is estimated using your age, gender, weight, resting heart rate, and heart rate during a run. Higher numbers are better. The test was developed by researchers at Stanford University. It's not exactly clear why or how it works, but it's believed to be related to the amount of fat in your body. The study that developed this test also found that people with more fat have higher cardiac frequencies at rest and while running, which seems to indicate they use more oxygen than people with less fat. Fitbit says their training programs are designed based on how you fitbit cardio fitness score you. They recommend that you increase your score by doing more intense workouts and reducing your time between exercises. For example, if you want to become a better runner, you should do some speed work followed by a long walk or slow jog. When you come back from your break, you should still be feeling tired so you need to stop over-doing it and reduce your intensity a little. Soon you will find yourself getting fitter even without trying. Here are some global scores for different countries: France, Germany, United Kingdom, Australia, India, China, Brazil, Mexico, South Africa.