Ok, young man I am here to tell you that knee popping has nothing to do with masterbation. You stated that you hardly exercise, so thats your issue. Your body can and will become stiff and illnesses can accur at any age if you do not exercise. As for masterbating, I would talk to a doctor, if you can not control your hormones and you feel you have to masterbate regularly. As young as you are you should get involved in some type of sport or activity that you enjoy, to keep your mind on other things besides masterbation. You seem like you might have to much free time on your hands. Try to stay busy, your just a kid, so go play ball with your friends or something instead of being alone and doing that.