It sounds counterintuitive, but just because you're working out harder, increasing your heart rate and burning calories doesn't mean you're burning fat. Your body's ideal fat-burning zone is only 60 to 70 percent of your maximum heart rate. Work any harder than that, and the percentage of fat calories burned during your workout actually drops.
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Maximum Heart Rate
Your maximum heart rate depends on your age and gender. For women, subtract your age from 226; for example, if you are a 32-year-old woman, 226-32=194. This means 194 beats per minute (bpm) is the highest your heart rate should reach during a workout, even an exceptionally strenuous one. For men, subtract your age from 220. In this case, the max heart rate for a 32-year-old man would be 220-32=188, or 188 bpm. Find your heart rate by placing your index and middle fingers along the side of your neck, just under your jawbone near your glands. Count the number of beats for 15 seconds, then multiply this number times four to get your total beats per minute.
Healthy Heart Zone
The first heart rate level reached during a workout is the healthy heart zone. In this zone, your heart rate should be at 50 to 60 percent of its max. Using our examples from above, a 32-year-old woman would be 97-116 bpm; it would be 94-112 bpm for her identically aged male counterpart. The healthy heart zone is ideal for individuals just starting a workout routine, or for experienced athletes during workout warm-up. In addition to promoting overall wellness – such as lowering blood pressure and cholesterol levels – 85 percent of calories burned while in this zone are fat calories.
The second tier you'll reach during a workout is the fitness zone, also called the fat-burning zone. Just as with the healthy heart zone, 85 percent of calories burned in this zone are also from fat. In this zone, your heart rate should be at 60 to 70 percent of its max. Using the same subjects, 116-135 bpm and 121-131 bpm are recorded, respectively, for a 32-year-old woman and a 32-year-old man. The key difference between the calorie burn between this level and the healthy heart zone is the amount of calories burned. Because you are working at a higher intensity level, you will burn more calories during your workout, provided you are doing the same exercise for the same amount of time.
Aerobic and Anaerobic Zones
The third and fourth levels – called the aerobic zone and anaerobic zones, respectively – are geared towards more serious athletes. These tiers are also referred to as the endurance and performance training zones. The aerobic/endurance zone provides all the benefits of the fitness zone, but also works your cardiovascular and respiratory systems to a greater degree. At 70 to 80 percent of your max heart rate (135-155 bpm for a 32-year-old woman, 131-150 bpm for a 32-year-old man), only half the calories burned will be from fat. However, since a higher intensity workout also means you'll be burning more calories overall, it is possible you could burn as many fat calories during an aerobic workout as during a fitness zone workout – the difference being a lower proportion of fat calories burned in the aerobic workout.
The anaerobic/performance zone also conditions your cardiovascular and respiratory systems, but to a higher degree. The target heart rate for this zone is 80 to 90 percent of your max – 155-174 bpm for a 32-year-old woman and 150-169 bpm for a man of the same age. In this zone, just 15 percent of calories burned are from fat, although the overall number of calories burned in this workout – provided it is the same form of exercise and same duration – is greater than during any of the others. However, since it forces the body to work at such a high intensity, it is harder to work out in this zone for prolonged periods of time.