Chapter 2. Prehension

Chapter 2 - Prehension 35
In terms of the prehensile classifications, palm opposition is used
in the coal hammer grasp, cylindrical grasp, and spherical grasp,
where the palm (VF1) is in opposition to the fingers and thumb (VF2).
Pad opposition is used in precision grasps such as palmar pinch and
tip prehension, where the thumb (VF1) is in opposition to one or more
fingers (VF2). Finally, side opposition is used in lateral prehension,
between the thumb (VF1) and the radial side of the fingers (VF2). It
is also used in the adduction grip, where one digit (VF1) is in opposi-
tion to another digit (VF2).
In addition to VF1 and VF2, which apply forces in opposition to
each other, a virtual finger can apply a force to counteract a task-re-
lated force or torque. This is called a virtual finger three (VF3), and it
is seen in the gravity-dependent grasps. For example, a waiter hold-
ing a tray on the flat of the hand in a platform grip is using the whole
hand as a VF3. Holding a suitcase in a hook grip is using the four
fingers as VF3. A third virtual figer can also counteract a torque that
arises in the task. For example, in the mug task (Figure 2.6), some
fingers were pressed against the outside of the handle opposing the
mug as it rotates into the hand. Since these fingers act as a virtual fin-
ger that counteracts a task-related torque, they are VF3.
2.3.3 Combined oppositions
Napier stressed that grasp postures showed combinations of
power and precision, especially because tasks usually had both as-
pects, saying that ‘although in most prehensile activities either preci-
sion or power is the dominant characteristic, the two concepts are not
mutually exclusive’ (Napier, 1956, p. 906). He pointed out that some
postures exhibit both power and precision characteristics in what he
called a combined giD: a precision grip (pad opposition) in the radial
fingers can work in combination with a power grip (palm opposition)
in the ulnar fingers. Patkin (198 1) used a similar term, double griD.
An example of a combined grip would be tying a knot in a cord.
Tension is maintained on the cord (power activities) by the ring and
little fingers, while manipulation of the knot (precision activities) is
performed by the thumb, index, and middle fingers. Another example
from surgery is where instruments are tucked into the palm by the ring
and little fingers9 (palm opposition) so that the radial digits can per-
form some other task (sensing tissue thickness or using pad opposi-
tion).
%atkin (198 1) called this the ulnar storam.