RSI used to be a very rare condition, in the last 20 years or so, it has become progressively more prevalent, and is very strongly linked to the increase in the use of computers. People at highest risk are those whose jobs require them to use a computer for a significant number of hours in the day. The exact number of hours at which someone becomes a high risk is unclear, but I have noticed a definite prevalence in certain industries and almost all of them are computer software related (engineers, programmers, developers etc). This is not to say that people in other fields don't suffer it, it is just people who sit in front of a computer typing and using the mouse all day seem more susceptible.
In very simple terms, RSI is the over use of a few motions of some muscles, and the lack of use of other motions in the same muscles. Muscles have evolved to be used through many rotation axis, and to be used for both macro and micro movement. Using a computer generally only uses one axis, and only a very limited set of micro movements. The resulting problems usually start at a level of minor discomfort or stiffness, and if left unchecked, will escalate to pains. For some people it can go as far as "pins and needles" and even going as far as numbness over time. In Short, it is pretty unpleasant!
This type of injury is not what people usually associate with the word injury. It is much more insidious and tends to sneak up on people because it is a progression of symptoms rather than a rapid onset. This can be viewed either positively or negatively. I tend to take the positive approach, which is that if it is a slow progression or symptoms, mostly related to limited movement, for many people there is a fairly simple way to undo and reverse the problems, by using the muscles differently, and assisting the tendons to release and stretch.
One of the most interesting things I have found about this is that while people experience the worst symptoms in the lower part of the arm (wrist, hand and fingers) the most effective methods of holistic treatment are often around the shoulders. This is not really surprising since there is a major "nerve bridge" in the shoulder-blade/collar-bone area. The muscles around and underneath the shoulder blades are are pretty notorious for being tight without people realising it. Often this is because they feel the tension in the upper shoulders and neck or slightly lower down in the back.
The suggestions I have for this category of problem, I have found are useful for stopping it from getting any worse, and depending on how far along the problem is, they may help reverse it.
This is really really important, and anyone with the type of problem I am referring to in this post, will probably find that the stretches I describe are uncomfortable. It is also really important to remember, never stretch too far, as a wonderful yoga teacher I know puts it, "the edge feels the same for everyone" I will add to that statement, it looks different. Please allow the sensations to guide your stretching. You want that "oohh, I certainly feel that, but it doesn't hurt" feeling. You will probably find your 2 arms have different amount of stretch, it is the feeling that will guide you to the correct "edge" Everything I share here, I do in good spirit, please use good sense, and remember you are responsible for you your actions and any impact on your body and how you use what other people and they have found it helpful.
Hand Down (do both hands separately for about 20 seconds, 2 or 3 times)
Stretch your arm out in from of you, palm down, allow the hand to flop down. Use your other hand to gently pull the whole hand a little closer/down. Now if you are able, holding the hand still with your 2nd hand, rotate your arm back and forth a little, it will be most obvious at the elbow that the elbow is rotating, but it will include the whole arm and include even some of the shoulder muscles.
Fingers backwards (do both hands separately for about 20 seconds, 2 or 3 times)
Hold your arm out in front of you, palm facing forwards, and fingers to the sky, now use the other hand to gently hold the top of the fingers and pull them backwards, for some people simply the action of getting their fingers to point upwards will be uncomfortable. It is important to use the other hand to "pull" the fingers back as it actually allows certain muscles to relax, that would be working if the fingers were not being "held" by the other hand.
Hands over your head (do 10-15 times SLOWLY)
A very simple and underrated shoulder release stretch. Move very slowly when you do this one.
Stand with your hands by you sides. Keeping your arms fairly straight, raise your arms in front of you, until your hands are above your head with your arms by your ears. Now slowly lower your arms out to the sides (the way a birds wings would move on the downward stroke of flying). I have had people who have found this made them a little light headed the first few times, please use your discretion as to whether it is safe to do this one for you personally.