An understanding of abnormal finger motion depends on a basic knowledge
of normal functional anatomy, which can be found in the contributions made by
Eyler and Markee (1954), Kaplan (1953), Milford (1968), Stack (1962), Tubiana
and Valeillin (1963). It is then possible to appreciate that the finger is a combination
of two biarticular, bimuscular systems: a proximal system consisting of the
metacarpophalangeal joint and the proximal interphalangeal joint, and a distal
system consisting of the proximal interphalangeal and the distal interphalangeal
joints.
In such a biarticular bimuscular system four combinations of joint positions
are found: Extension-extension, extension-flexion. flexion-flexion, and flexion-extension
(Stack, 1962).
Movement being a synonym for a change of position, twelve different motions
are possible: eight single actions involving only one joint, and four double motions
involving two joints