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The laws of fat loss are relatively simple: (1) Work out longer and you'll burn more calories and lose more fat, and (2) work out more often and you’ll burn more calories and lose more fat. Following this line of reasoning it would make sense that; (3) if you work out harder you’ll also burn more calories. This is true, but only up to a point. As you push harder and harder, a level will be reached where if the intensity goes any higher, you won't be able to sustain the activity long enough to burn much fat. You’ll become fatigued before you can finish your workout. Therefore, selecting an intensity that’s not too light but not too hard is critical. This is known as your "training zone," "target zone," or “fat-burning zone.” This optimal zone for fat burning and cardiovascular conditioning is generally between 60% and 80% of your functional capacity or 70-85% of your age predicted maximum heart rate.
A long workout (30 minutes or more continuously) with an extremely low intensity won’t burn enough calories to have any impact on fat loss. There’s a minimum intensity threshold you must cross to get maximum benefit from each workout and you’ll learn about this “fat burning intensity zone”