Michael Evans 
The Myths on Exercise [EPUB ebook] 

To preface this article, let me start out by saying that the no. 1 rule is the same for everyone. Every person is an individual and needs to learn their own bodys signals that exercise may be too much or too little.
I am also not a doctor and make no claims medically. You should know that when it comes to your health, medicine and doctors have their place. It is just not always the first place, and many times, its the last place.
Also if you have already had surgeries (i.e., neck or spine surgeries, pins, screws, prosthetics, pacemakers, or any other foreign material in your body), you will need to take extra precautions while exercising. Once the mechanics of the human body have been altered, it can no longer function completely, as it was intended to. However, this does not mean its okay to park yourself on a couch and never do anything about your fitness level.
Fitness is not about vanity. Your body was never intended to be dormant. It needs to move to survive.