Chapter 2. Prehension

40 WHAT IS PREHENSION?
d
Figure 2.9. A virtual finger geometric configuration (li,$i) ap-
plies a force with orientation yi relative to the palm. The origin
of the virtual finger two vector (VF2), on the right, is offset by a
distance d from VF1 (from Iberall, Torras, & MacKenzie, 1990).
one finger. Using a VF2 consisting of the index and middle fingers,
the width would be two. For palm opposition, the grasping surface
patch for VFl is the palm and for VF2, it is typically centered around
the interphalangeal joints of the fingers, although the exact centroid is
dependent on object characteristics. The width of VF2 depends on the
number of real fingers being used as well as the amount of their ab-
duction. In side opposition, the grasping surface patch for VF1 is the
thumb pad and for VF2, it is typically the radial aspect of the index
finger near the interphalangeal joints.
VF vectors are tagged with the orientation yof the grasping sur-
face patch relative to the palm at a given configuration (see Figure
2.9). In the index finger example, as the VF vector changes length
(12) and orientation ($2) during flexion and extension, the pad changes
orientation ("12) relative to the palm. The VF1 configuration (11,@1)
has an orientation (y1) as well. Each VF must also be able to apply
forces and gather sensory information. The amount of force that the
VF can apply depends on the configuration of the finger and the un-
derlying biomechanics. As will be seen in Chapter 6, the largest
forces are available in palm opposition, followed by side opposition