Does cardio burn muscle?

Alfreda Smith asked:
People have been telling me that when you excercise too fast that all you are doing is burning muscle and not fat. I do step aerobics at a fast pace three times a week. If you work over your maximum heart rate do you burn muscle? And how do you know your heart rate with those 6 and 10 second counts aerobics instructors usually do?

Larry DeLuca, EdM, CSCS

answered:

Lots of questions...

1. You'll be burning a combination of fats, carbohydrates, and proteins all the time, whether you're working out, sleeping, or anything in between.

   The key factor in determining whether or not you lose primarily body fat or primarily muscle is the amount of deficit (the difference between the calories you take in and the calories you expend). Keeping your deficit to around 250-1000 Calories per day will result in a weight loss of about 1/2 to 2 lbs. per week, primarily from fat. Larger deficits will result in a shift toward losing more muscle.

2. Don't worry about how hard you exercise. The total number of calories expended (see above) is the most important factor. Harder exercise allows you to expend more calories more quickly, but you can also exercise at a lower intensity for longer if you prefer.

3. No, you won't burn muscle if you work above your target heart rate. However, you may get fatigued quickly and will have to slow down and catch your breath.

4. You can figure out your training heart rate range by doing the following:
   (220 - age - resting HR) * Intensity + resting HR

   The intensity should range from .50 to .85.

   If your instructor counts for 6 seconds, divide the numbers by 10 to get your training range for a 6-second count. If your instructor counts for 10 seconds, divide the heart rate numbers by 6. For example, if you are training at a heart rate of 150 BPM, a 6-second count would be 15 beats, whereas a 10-second count would be 25 beats.