This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
Flexibility is defined by Gummerson as "the absolute range of movement in a
joint or series of joints that is attainable in a momentary effort with the
help of a partner or a piece of equipment." This definition tells us that
flexibility is not something general but is specific to a particular joint
or set of joints. In other words, it is a myth that some people are
innately flexible throughout their entire body. Being flexible in one
particular area or joint does not necessarily imply being flexible in
another. Being "loose" in the upper body does not mean you will have a
"loose" lower body. Furthermore, according to `SynerStretch', flexibility
in a joint is also "specific to the action performed at the joint (the
ability to do front splits doesn't imply the ability to do side splits even
though both actions occur at the hip)."