Chapter 2. Prehension

Chapter 2 - Prehension 31
applied in opposition to each other against the object’s surfaces.
Iberall, Bingham, and Arbib (1986) used the term oppositions to de-
scribe three basic directions (or primitives) along which the human
hand can apply forces. A prehensile posture then consists of combi-
nations of these primitives. They are as follows (see Figure 2.5):
1) PAD OPPOSITION: occurs between hand surfaces along a direc-
tion generally parallel to the palm. This usually occurs between
volar surfaces of the fingers and thumb, near or on the pads. An
example is holding a needle or small ball.
2) PALM OPPOSITION: occurs between hand surfaces along a direc-
tion generally perpendicular to the palm. Grasping a large ham-
mer or screwdriver are examples of palm opposition.
3) SIDE OPPOSITION: occurs between hand surfaces along a direc-
tion generally transverse to the palm. As an example, one holds
a key between the volar surface of the thumb and the radial sides
of the fingers. Of course, it can occur between the sides of the
fingers, as in holding a cigarette.
2.3.2 Virtual fingers
In observing how subjects grasped different sized mugs (Figure
2.6), Arbib, Iberall, and Lyons (1985) noted that different numbers of
fiigers were used depending on the length of the mug handle. Yet, the
task remained basically the same: a finger was placed on top of the
handle, one or more fingers were placed inside the handle, and if
available, fingers were placed against the outside of the handle. They
suggested that each of these functions were being performed by a Vir-
tual finger (VF) as the method of applying the force. A virtual finger
is an abstract representation, a functional unit, for a collection of indi-
vidual fingers and hand surfaces applying an oppositional force. Real
fingers group together into a virtual finger to apply a force or torque
opposing other VFs or task torques. Figure 2.6 shows how the num-
ber of real fingers used can be a function of an object property, such
as handle size. For a teacup with a very small handle (Figure
2.6a),only one finger fits inside the handle, providing an upward
force from within the handle. Here, the thumb, as virtual finger 1
stable either.
80pposition used here should not be confused with the term ‘thumb opposition’,
which describes a complex movement of the thumb’s six degrees of freedom as it
rotates into opposition to the finger pads (see Appendix A).