I was recommended to use a trackball mouse after visiting my physio with muscle pains in my upper right arm. This may well have been due to constant use of a normal mouse at my PC every day. Tried a friend's trackball mouse with the ball on top, operated with a finger, and did not find it too comfortable. Saw the M570, and although not actually confimed in the product description, assumed the ball was operated by the thumb. I was immediately comfortable using it, and although stange at first I am rapidly becoming very adept at controlling the cursor with my thumb. It is really comfortable and I wish I had tried this before! It is very noticeable that my right arm is not having to move all the time, and I feel that this will be great for my muscle problem. I assume this will also be the case if you suffer from RSI. A bonus is that the wireless USB dongle is tiny and unobtrusive on my laptop. Great product!