Sometimes the spirit is willing, but the knees are weak. If you have knees that hold you back from running, jumping, etc... this will be helpful for you. Here are some 3 exercises we do in boot camp that will burn fat, work your core, get you a great workout in a short amount of time, and won't hurt your knees.  http://www.youtube.com/watch?v=ZhykYaAARWU