Chapter 2. Prehension

Chapter 2 - Prehension 25
uses of the hand. Ultimately, he was able to sense that even though
tasks are complicated and full of variety, there are two fundamental
qualities to tasks--their precision and power requirements--and he was
able to recognize that the human hand could match these.
In the robotics literature, Cutkosky and colleagues (Cutkosky,
1989; Cutkosky and Howe, 1990; Cutkosky and Wright, 1986b) ex-
tended the Napier precisiordpower dichotomy by further classifying
power grasping into nine subordinate types and precision grasping
into seven subordinate types using a set of grasp attributes (see Figure
2.3). In power grasps, the emphasis is on stability, a grasp attribute
defined as the ability to resist external forces without slipping and re-
turn to nominal position, and security, or the ability to resist slipping.
In precision grasps, the emphasis is on dexterity, which is defined as
how accurately fingers can impart larger motions or forces, and sensi-
tivity, or how accurately fingers can sense small changes in force and
position. In addition to grasp attributes, Cutkosky and Howe use ob-
ject characteristics to refine postures in a style similar to, but more
precise than, Schlesinger. Prismatic grasps are wrap postures (i.e.,
cylindrical grasps) that enclose a tool with a long symmetry. A circu-
lar posture, on the other hand, is used for objects with radial symme-
try (i.e., spherical grasp).
2.2.3 Precision handling and dynamic grasps
Identifying postures is useful for describing a static view of a hand
posture. However, many grips6 are associated with actions.
Landsmeer (1962) extended Napier’s dichotomy, rethinking the pre-
cision grip as precision handling, thus highlighting the dynamic aspect
of the movements during fine translations, rotations and complex ma-
nipulations. Power grips immobilize the object so that there is a def-
inite static phase once the object has been grasped, whereas precision
handling does not have a definite static phase. Rather, Landsmeer
suggested that grasping the object in opposition between the thumb
and finger pads enables greater variety of movements, because the fin-
gers can impart those movements. Kapandji (1982, p. 272) called
these dvnamic nips since ‘the hand can act while grasping’, and gave
numerous examples, such as top turning, lighting cigarettes, pushing
6We note that others have made a distinction between the words ‘grip’ and ‘grasp’;
for example, Malek (1981) suggested a grip is a static posture and a grasp is the
dynamic unfoldmg of a posture. Here, for simplicity, the two words are used
interchangeably.