This article is from the Ergonomics and Typing Injury FAQ, by Dan Wallach firstname.lastname@example.org with numerous contributions by others.
A. Standing with the arms at your sides, palms facing forward, "flexion"
is folding of any joint of the body so that the angle between the parts
decreases in the forward direction, except at the knee and toes. Returning
he joint to its straight position requires "extension".
A joint which continues its extension past its straight posture is in
"hyperextension". This occurs in the hand and wrist when you pull the
Standing with your arms at your sides, palms facing forward, "pronation"
is the turning of your hand so that you thumb points toward your leg.
"Suppination" is the opposite movement.