My joints are slightly hypermobile at the best of times - that is, they tend to bend the wrong way unpredictably - and some of them are worse than others. This means that if I overuse my body, the tendons in various joints get stretched too much and this leads to pain. If I ignore the pain and keep going, the tendons will start "snapping" (not literally snapping - but they will go from having been over-elasticy to being almost inelastic and not allowing any movement) and the muscles will go trembly and/or spasmy. So, if my thumbs start to hurt I need to stop writing/typing. If I carry on anyway, within an hour I physically won't be able to write or type anymore.
And you'd be amazed how many normal life activities require opposable thumbs. Even eating and drinking, let alone reading or using a computer.
After a few days of rest, I can write again or use the computer a bit, but now after an hour of mainly reading with occasional clicking, my elbows, wrists and thumbs all feel like crap. So now I have to stop even though I've hardly read anything that's gone on here and am feeling isolated... really got to do something about getting a more ergonomic keyboard, but that's difficult to combine with a laptop. Hrm.
If I owe you email, it will be written when I am physically capable of writing email. Yep.