Quote from:
Learning the Classic Guitar, Part I  Aaron Shearer
      Mel Bay Publications, Inc. #4 Industrial Drive, Pacific, MO 63069-0066
             Toll Free 1-800-325-9518 FAX (314)257-5062

page 10             
----------------------
The Four Principles of Efficient Muscle Function are as follows - each 
functions interdependently with the others:

Muscular Alignment: Muscles function most efficiently only when 
naturally aligned with their base and joint attachments.  Natural
alignment provides the most direct pull of the muscles which control
your back, wrist, and finger joints.

Midrange Function of Joints: Muscles function most efficiently only when 
the joints they control are operated within their midrange of movement.
Midrange positioning and movement provides optimum leverage to the
muscles involved.

Uniform Direction of Joint Movement: Muscles function most efficiently 
only when all three joints of a finger or the thumb are either flexed
or extended together.  In contrast to flexing one joint while extending
another, simultaneous extension or flexion simplifies coordination of
the muscles.

Follow-Through: Muscles function most efficiently only when there is
sufficient follow-through to avoid a build-up of counterproductive 
tension.  Sufficient follow-through means that, once a movement has
been initiated, no intentional restraint is applied to the movement.

Another important consideration, though not involving the mechanics of
movement, strongely influences muscle function:

Muscles function most efficiently only when the mind is free of anxiety.
---------------------
end of quote