The third major posture to avoid is called 'pronation'. Pronating
the hand simply means turning it so the palm faces downward.
The bones in the forearm are actually straight and essentially
parallel when the hands are slightly 'supine', with palms
(You don't have to remember the terms to understand
Observe how 'pronating' the hands (turning the palm downward)
causes the bones in the forearm to cross and 'scissor' the
soft tissues in the arm. It forces blood vessels and nerves
between two hard objects (your bones) and then puts a crushing
pressure on them! Do you see how this could be harmful over
long periods of time?
Now, as you sit down at a conventional flat keyboard, what
movements do you have to perform to put your fingers on the
Pronation, Deviation and Extension... all at the same time!
The only ergonomic keyboards that will help
prevent injury are those that cannot be adjusted improperly,
and which eliminate significant amounts of extension, deviation,
and pronation of the wrist. Anything else is a waste of your
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