Chapter 2. Prehension

344 CONSTRAINTS AND PHASES
scientific visualization and human hand animation. The role of vision
and haptics will be critical for applications to virtual reality. For
engineers, challenges exist for telerobotics and telemanipulation. For
the medical professions, understanding the normal hand is relevant for
applications to rehabilitation, surgery and microsurgery, functional
electrical stimulation, hand therapy, and prosthetics.
9.4 Summary
What do hands do? The human hand interacts with the world in
complex ways. When an object is grasped, it is not only grasped for
itself; it is grasped for a reason. A person grasps an object to do
something with it. Because we generally use our one dominant hand
to perform this task with this given object, we feel that our hands and
arms are always doing the same thing--reaching to grasp the object.
However, the data explored in this book show that, depending on the
task and object, there seem to be different control mechanisms. For
pointing, which is a task performed in free space and basically with
our arm, one controller is used. For grasping a pen to write with it,
which is an oscillatory task that involves first a free space movement
and then the grasp, lift, and oscillations, another controller is used.
For grasping the pen to hand it to someone, a third controller is used.
It is, as Greene (1972) said, as if we had a collection of virtual arms.
Our brains are complex systems, containing many redundancies.
While the motoneurons are final common pathways on the way to our
hands, many parallel systems are involved in selecting the right
combination of control strategies to allow us to perform sophisticated,
versatile, goal-directed behaviors. Evolutionary specializations have
enhanced the human hand’s ability (through control by the CNS) to
perform a wide variety of prehensile tasks, equipping the hand as both
a precision and power device. Within the complex interaction of
physical constraints, active and passive sensorimotor features, goals
and motivations, prehensile postures are chosen that will apply
functionally effective forces to an object. The term ‘functionally
effective’ is used in order to make explicit the notion of task goals
(i.e., applying forces, imparting motions, gathering sensory
information), creating constraints on the methods by which a human
hand is shaped into a force-applying and sensory-gathering posture.
The opposition space model focuses on human hand functionality
in terms of task requirements and explicit parameters. By comparing
hand functionality and task requirements, the effectiveness of a
posture can be determined for a given task. The model avoids the