Chapter 2. Prehension

Appendix B - Taxonomies of Prehension 373
B.l). Due to the anatomy of the hand and arm, this posture allows
some manipulation while supplying more strength than two fingered
palmar pinching. Finally, the glabrous skin of our hands, particularly
at the pads, has a rich sensory capability, as outlined in Chapter 6.
Whenever we put a finger against an object, the finger can act as an
antenna to gather sensory information through cutaneous and
proprioceptive mechanoceptors, while applying forces against the
object.
Opposition space provides a framework for organizing and
summarizing these classifications. Oppositions occur between two
virtual fingers (VFs). Palm opposition occurs between the palm
(VF1) and fingers (VF2) generally perpendicular to the palm,
basically describing the way the entire hand ‘grasps’. Pad opposition
occurs generally parallel to the palm as the thumb pad (VF1) and
finger pads (VF2) contact in order to pinch or impart motion to an
object. Side opposition occurs generally transverse to the palm,
bringing the thumb pad (VF1) to oppose the radial side of a finger
(VF2), or adducting the fingers together. A virtual finger can also
oppose a task-related force or torque. For example, a hook grasp (G)
uses the fingers to counteract gravity when holding a suitcase. A
finger placed on a fork or knife to press it harder is another example.
In the next four sections, the postures from Table B.l are
described using the opposition space framework as a guide. In the
final section, a method of extending the table is suggested.
B.l.l Palm oppositions
Palm oppositions have been identified based on four
characteristics:
1) object shape,
2) object orientation,
3) size of hand opening (or object size), and
4) thumb placement.
In the open fist (A) and closed fist (B) cylindrical grasps, the thumb
is wrapped around the object or around the back of the fingers. The
object is generally cylindrically shaped, and it is held transverse to
the palm. The spherical grasp (C) has been noted in two situations.
The first is for regularly shaped objects with radial symmetry, such
as spheres. However, Schlesinger noted that one of the important