I started doing cardio to slim down. I'm 5'11 and was about 205-210 lbs. I started doing a whole lot of cardio, about an hour to an hour and a half for six days a week a few months ago. I slimmed down to 187 lbs, and I still have a little ways to go before I get my body fat percentage down to where I need it to be. My question is this: How do I know if the amount of cardio I'm doing is counter productive? I've cut back to 1/2 an hour to an hour for my cardio each day, and am only doing it five days a week, but I want to make sure that I don't loe any of my muscle mass. How do I know?