VARIATIONS IN THE RADIOULNAR AXIS OF THE
HAND
A third principal mode of variation concerns
cross-hand alignments. Thu s the metacarpophalangeal
joints ma y be drawn into line, and
with abducted thumb a flat-hand position is
assumed. At the other extreme, the han d is
Fig. 12. Effect of forearm-hand angle upon wrist flexion and extension forces
and upon prehension forces. Above, relationship between forearm-hand angle and
maximum forces of wrist flexion and extension measured at the carpometacarpal
joint. Heavy lines, flexion (volar flexion); light lines, extension (dorsal flexion).
Solid lines, averages; dotted lines, standard deviations. Unpublished data, UCLA,
15 male subjects. Below, relationship between forearm-hand angle and maximum
prehension force measured between thumb and opposing index and middle fingers
grasping a 1/2-inch block. Right hand, eight normal male subjects. Solid line, average;
dotted lines, standard deviations From a UC report (14).
cupped for spherical prehension (Fig. 13) as
the opponens muscles of thumb and little
finger, aided by other adductors and flexors,
act to pull these digits toward each other.
Similar alignment occurs when a fist is made.
HAND MOVEMENTS
The large number of muscles and joints of
the hand obviously provides the equipment for
numerous and varied patterns of movement.
Not so evident, but equally important in determining
complexity and dexterity of motion,
are the large areas of the cerebral cortex given
over to the coordination of motion and sensation
in the hand. Thus, in the motor cortex the
area devoted to the hands approximately