I know people are longing to lose fat or weight but hardly heard of anyone wanting to lose muscle. Aerobics helps in burning muscle mass. Cut down on your protein intake as protein helps build muscle. You need to seek help from your gym instructor on what are the exercises you need to do to lose muscle mass. He will also advise on how many repetitions and weights should be used. Speak to you dietician as well, you don’t want to stop the intake of proteins completely. Your dietician will give you a complete diet chart accordingly.