It is convenient to analyze digital mechanics
in terms of flexion-extension variations in the
digits, thumb postures, and variations in the
radioulnar axis.
INDIVIDUATION OF DIGITAL FLEXION-EXTENSION
Insertion of flexor and extensor muscle systems
into several major segments along the
proximal-distal axis provides a variety of
flexion-extension patterns in the digits. In
Figure 7, the essential components are shown
schematically for digits I and III. With these
attachments, fixation of carpal and metacarpal
segments by cocontraction of flexor and extensor
carpi muscles provides a firm base for
independent movements and fixations of the
phalangeal segments. Individual flexions of the
second and terminal phalanges stem from
separate flexor muscle groups. Such flexor
groups, inserted distally, can also cause complete
cylindrical-grasp prehension by "rolling