Right now at age 19 Iam 5'9 145, but at 13 I was 5'1 155. Back then my doctor kept telling me to loose weight. I just hadn't really hit any growth spurts yet....but anyway, he said one of the best execises to burn fat is to walk up and down steps for 20 to 30 mins. How true is this? I always heard it could damage your knees