Well, according to Dr. Sarno, and the doctors who practice TMS, the condition is universal. All human beings suffer from it, at least after a certain age.
For some it's just a headache when stressed, for others it's a full blown migraine or fibromyalgia.
It boils down to emotional health - healthy mind in a healthy body. The people who suffer the worst symptoms of TMS (which is just a sugarcoated way of saying psychosomatic pain) have a certain personality type - perfectionists, hard on themselves. Paradoxically, this fits the personality type of a lot of runners.
The main idea is that the ego puts a lot of pressure on the id (your inner child), which in turn reacts with rage. The attempt by the ego to supress the rage causes the pain.
It's a subconscious and primitve interaction between different layers of our brain.
Read "The Divided Mind" by Dr. Sarno. Out of all his books, I found this one to be the most enlightening, because it is written by 7 different doctors, some of whom are extremely intelligent and eloquent writers.
There are many eye-opening passages in the book, but one of the most memorable is one discussing the increased incidence of high-blood pressure in people who suppress their rage. At first it's a consciouss decision, but after years of repetition it becomes unconscious. They no longer feel anger/rage like a normal person would.
However, the rage still gets generated, and because the stress is not released it builds up in the brain - which in turn causes physical symptoms.
Please read it with an open mind. I have not yet posted my Amazon review of the books, but I'll say this:
Aknowledging that it's even POSSIBLE that you may have TMS is a hard pill to swallow.
1)It means, for one thing, that you are not fully in control of your own brain, which is unsettling (again, Dr. Sarno explains that this is just a knee-jerk reaction, as TMS is universal, and part of the human condition. It's just how the different lairs of our mind interact).
2)It means that there is no quick fix, and nobody can help you. Only you can help yourself.
3)It seems far fetched, and unlikely, and the realm of pseudo-science. The fact that it's not accepted by modern mainstream medicine doesn't help.
However, please consider the stakes with an open mind. It's your life, and your quality of life. Suppose the premise is wrong - you just wasted $5 on a book, and a day of reading.
Suppose it is correct - and as someone who has almost fully recovered from it, I can only cringe at the thought of countless people who will never even hear about TMS - and you ignore the diagnosis; well, then you will be in chronic pain for the rest of your life.
Here is the link that finally convinced me to try reading the book. Read the comments after the article as well.
http://aaroniba.net/articles/tmp/how-i-cured-my-rsi-pain.html
And here is a link to Harvard University's RSI (Repetitive Strain Injury) Action group. Some Harvard students who recovered feel so strongly about it, that they now give lectures on the subject.
http://www.rsi.deas.harvard.edu/mb_what_is.html
At the very least, it's worth being open minded and reading a book on the subject.
One last thing: the pain of TMS "moves" when being actively challenged whether through meditation or psychotherapy. Your knee may suddenly be ok, but now your back will hurt for no reason, etc. That's one way to figure out that it truely is.