Chapter 2. Prehension

Chapter 2. Prehension
“Although in most prehensile activities either precision or power
is the dominant characteristic, the two concepts are not mutually
exclusive.
-- J. R. Napier (1956, p. 906)
2.1 Overview of Prehension
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Prehension involves the taking hold of an object for a purpose
such as manipulating it, transporting it, or feeling it. One might also
grasp for the purpose of transporting or rotating one’s whole body
about the object, as in climbing or gymnastics. On the kinetic level,
prehension entails applying forces during interaction with an object.
Stable grasping adds the requirement that forces are applied by hand
surfaces in opposition to other hand surfaces or external objects in or-
der to overcome slight perturbations. On the kinematic level, prehen-
sion involves the orienting and posturing of the hand and fingers, with
the appropriate transportation of the limb to the correct location in
space. In this book, prehension is defined as the application offinc-
tionally effective forces by the hand to an object for a task, given nu-
merous constraints. This definition places an emphasis on the task
and functional aspects of the problem. The words ‘functionally effec-
tive’ are used to highlight the fact that the forces must be applied
within the functional constraints of the task; i.e., while forces can be
used to effect a stable grasp and impart motions as needed in a task,
there are functionally specific demands on how this is accomplished,
such as the precision requirements or stability needs. As well, consid-
erations address how the hand is postured to apply these forces, and
how the dynamic postures used for applying these forces are set up as
the hand reaches out toward the object.
The human hand has a variety of ways to grasp objects stably.
But there are constraints on the ways that the hand can be postured, as
well as on the potential success of a chosen posture. These include
both functional constraints and physical constraints’. Functional con-
straints are not so much a property of the object to be grasped, but
more a property of how the object will be used within the task. The
task goals and subgoals determine functional constraints. Schmidt
konstraints are examined in more detail in Chapter 8.