This article is from the Ergonomics and Typing Injury FAQ, by Dan Wallach firstname.lastname@example.org with numerous contributions by others.
Here's the theory.
Normally, your muscles and tendons get blood through capillaries which pass
among the muscle fibers. When you tense a muscle, you restrict the blood flow.
By the time you're exerting 50% of your full power, you're completely
restricting your blood flow.
Without fresh blood, your muscles use stored energy until they run out, then
they switch to anaerobic (without oxygen) metabolism, which generates nasty
by-products like lactic acid, which cause pain.
Once one muscle hurts, all its neighbors tense up, perhaps to relieve the load.
This makes sense for your normal sort of injury, but it only makes things worse
with repetitive motion. More tension means less blood flow, and the cycle
Another by-product of the lack of blood flow is tingling and numbness from your
nerves. They need blood too.
Anyway, when you're typing too much, you're never really giving a change for
the blood to get back where it belongs, because your muscles never relax enough
to let the blood through. Stress, poor posture, and poor ergonomics, only make