Chapter 2. Prehension

Chapter 2 - Prehension 17
by Kugler et al. (1982) as follows, “How can the very many degrees
of freedom of the body be regulated in the course of activity by a
minimally intelligent executive intervening minimally?” In separating
the environmentally-defined goal from this motor problem, Schmidt
(1988) pointed out that while it is pretty well clear what the overall
goal is, it is not clear how to coordinate muscular activity to achieve it.
He suggested that a motor goal can be viewed as a spatial-temporal
pattern of action that, when produced accurately, will achieve the envi-
ronmentally defined goal. But, of course, he pointed out that different
patterns of action will result in the same overall goal.
The definition of prehension stressed that ‘functionally effective
forces’ need be applied. In general, the functional demands on a pos-
ture can be summarized as follows:
a) apply forces to match the anticipated forces in the task (stable
grasp)
b) impart motion to the object (manipulate) or transport the object as
necessary
c) gather sensory information about the state of the interaction with
the object during the task in order to ensure grasping and
manipulative stability
For the hand, Qfferent postures present different degrees of available
force, of available motion, and of available sensory information. For
a task, there are a variety of possible postures that could be used. As a
starting point for understanding how the task requirements can be
matched by the functional capabilities of the hand, an examination of
significant taxonomies of hand postures is now made.
2.2 Prehensile Classifications
The question of how the human hand works has been addressed
by engineers who want to duplicate it in mechanical devices, and by
medical scientists and therapists who want to help patients in func-
tional hand usage. For example, a simple taxonomy was developed
by G. Schlesinger in Germany in 1919 as a way to categorize pre-
hensile functionality for prosthetic hands due to injuries from World
War I, as seen in Figure 2.1. Schlesinger’s classification represents
one of many attempts that have been made to classify hand postures by
researchers from different perspectives for medical, clinical, occupa-
tional, and industrial applications. These taxonomies are summarized