Chapter 2. Prehension

30 WHAT IS PREHENSION?
suggested that the hook grip, or the claw, is a gravitv dependent
grasp. Very interestingly, he identified other ways the hand can use
gravity, including: cupping the hands into the shape of a spoon; and
flattening out the hand as a platform, as a server does in holding a
tray. The gravity dependent postures do not involve two hand sur-
faces opposing each other. Instead, a hand surface is opposing a task-
related force or torque.
2.2.7 The finger as antenna
Various researchers have noted postures where the index finger is
used effectively much as an antenna listening in on the state of the ob-
ject or in on the interaction of the tool with the environment. For ex-
ample, when a surgeon holds a scalpel, the index finger is placed
along the edge while the object in turn is grasped by the other fingers
within the palm (see Figure 2.4~). This posture has been called the in-
ternal precision mip [Patkin, 198 1) and the directional or centralized
gr& (Kapandji, 1982). It has been observed in the grasping of a
screwdriver, a conductor’s baton, and a knife. Kamakura et al.
(1980) suggested it was a variation of the power grip and called it a
power mip--index extension. Another example offered by Patkin is
the way surgeons use the index finger for sensing tissue thickness,
while using their ulnar fingers and thumb to manipulate the forceps.
However, just as an antenna both receives and transmits, so can
the index finger. Besides sensing information about the forces acting
on the object, the index finger can impart fine motions to it. It can also
counteract larger forces, as in stabbing a piece of meat with a fork or
cutting it with a knife. This notion of finger-as-antenna can be gen-
eralized to any finger or fingers. The thumb, in effect, could be per-
forming this very function in Napier’s power grasp (depicted in Figure
2.2a).
2.3 An Opposition Space Classification
2.3.1 Types of oppositions
Another way to analyze what the hand is doing in prehension is
to focus on the fact that a posture involves at least two forces7 being
7~his statement can be qualified in two ways. ~irst, two forces don’t make a grasp
necessarily stable, as is discussed in Chapter 6. Secondly, one force can be used to
balance an object, opposing gravity directly. This palming or balancing is not