Chapter 2. Prehension

Chapter 2 - Prehension 45
mation (due to the large number of mechanoreceptors in the finger
pads and interphalangeal joints). Although it reduces the number of
fingers available for applying forces, the extended finger in itself can
apply a limited force, provide direction and impart some motion as
necessary for a VF3. Importantly, it can be used in combined grasps,
as seen in extending the index finger on a screwdriver or knife, or
placing the thumb on the side of a beer mug.
The key to understanding the versatility of the human hand is not
so much that it can create any one of these postures, but that it can also
do these in combinationsll. While using the dynamic capability and
sensitivity of the three radial digits in pad or pad and side opposition,
the other two fingers can still oppose the palm, grasping an object in
palm opposition. The surgical techniques described by Patkin are
some examples of this ability of the human hand to hold (and even
manipulate) more than one object at a time. An antenna can be formed
by the extended index finger or adducted thumb that acts both as a re-
ceiver of sensory information and a transmitter of forces as needed.
At the same time, the other digits can create the stable grasp, as seen in
Figure 2.2 and Figure 2.4. Examples of functional combinations were
seen: in grasping a mug where side opposition combines with VF3; in
the internal precision grip where palm opposition combines with side
opposition and a VF3 as antenna; and a double grip, where side op-
position occurs between the index and middle fingers and pad opposi-
tion is used by the other digits. Napier’s power grasp, in fact, com-
bines palm opposition with side opposition. An example of using
these multiple functional features to hold more than one object at a time
was seen in grasping chopsticks. Ultimately, the versatility of the
human hand stems from what Napier pointed out in 1956, that preci-
sion and power are not mutually exclusive. The human hand (and
brain!) can resolve these multiple task components and in doing so,
find a set of oppositional forces that are functionally effective for satis-
fying the competing task requirements for arbitrary objects. And this
is true whether the task is to hold one or even 10 oddly-shaped objects
at a time, and whether the task is to do either one or many things with
them!
For Napier, the terms power and precision grips were intended to
be used to describe the whole hand in the dynamic as well as the static
sense, just as the terms flexion and extension can describe movement
or the static posture about a joint. However, he suggested that, “it is
llIn Appendix B, a table listing most of the possible combinations of oppositions
and virtual finger mappings can be found.