Thanks to Romain for pointing out to me this very interesting article on the possible causes of RSI:
Could it be the light action and short travel of modern keyboards and mice that cause the trouble? Dysponetic activity (inappropriate and misdirected as well as unconscious muscle bracing) is implicated in the aetiology of RSI. How much of this is due to the need to support the weight and muscle tone of one’s fingers to avoid inadvertent key presses?
As somebody suffering from the problem, I must say that I find this theory pretty convincing. The discussion is interesting too.