extension. In tip prehension,
the action of
muscles upon carpal
and metacarpal bones
is similar, but distributed
flexion in all phalangeal
segments indicates
predominant
flexor activity.
THUMB VERSATILITY
PATTERNS
The versatility of the
thumb lies, first, in the
variety of its flexionextension
patterns and,
second, in the adjustable,
rotatory plane in
which flexion-extension
can take place. The
first of these is directly
analogous to the digital
system for the other
four fingers, in that for
any given metacarpal
position there are numerous
possible positions
of the phalanges.
The second effect is
due to the relative
mobility of the carpometacarpal
joint, which
allows the thumb to act in any plane necessary
to oppose the digits. The principal oppositions
are semidirect, as seen in palmar,
tip, and spherical prehensions. Actually, in
these cases the plane of the thumb action is
inclined 45 to 60 deg. to the palmar plane. In
lateral prehension, the plane is approximately
parallel to the palmar plane.