Chapter 2. Prehension

Chapter 2 - Prehension 33
Figure 2.6 Grasping different sized mugs. The hand initially is in
a resting state, on the right side of the figure. As it reaches to
grasp the cup, different numbers of fingers are used to be inserted
into the handle. The three forces needed in this task are being
supplied by three virtual fingers. Virtual finger 1 (VF1) is placed
on top of the handle. The number of real fingers mapped into
virtual finger two (VF2) depends here on the size of the handle. A
third virtual finger (VF3) forms by remaining fingers to counteract
the moment of the mug (from Arbib, Iberall & Lyons, 1985;
reprinted by permission).
(VF1) applies a force in opposition to the index finger (VF2). For a
coffee mug (Figure 2.6b), two fingers fit within the handle to form
VF2, while Figure 2.6~ demonstrates the case where VF2 is
comprised of three fingers.
Two virtual fingers apply forces in opposition to each other, and
the direction of these forces is relevant in order to clarify the type or
types of oppositions. A hand coordinate frame can be placed on the
palm for specifying these directions (see inset, Figure 2.7). Pad
opposition (Figure 2.7a) occurs along the x axis between the thumb as
VF1 and one or more fingers as VF2. Palm opposition (Figure 2.7b)
occurs along the z axis between the palm as VF1 and the fingers as
VF2. Side opposition (Figure 2.7~) occurs along the y axis between
the thumb as VF1 and the index finger as VF2. Side opposition can
also occur between two fingers.