Chapter 2. Prehension

12 WHAT IS PREHENSION?
prehensile behavior emerges from computations in a multidimensional
constraint space. Chapter 8 reviews prehension, focusing specifically
on these constraints. Classes of constraints which must be included in
a complete model of human grasping are identified, and the levels in
the system at which the constraints are operating are discussed. These
constraints include social, motivational, functional, physical, neural,
anatomical, physiological, evolutionary, and developmental ones.
Some constraints are time varying, others are not. Details of the con-
straints are discussed, thus helping to define their role in the modelling
process. Chapter 9 reviews the problem being solved by the CNS.
Using the black box metaphor, a review is made as to the nature of the
object, the task, the hand, and the controller of prehensile behavior.
Potential applications for the work compiled in this volume are
discussed.
For those readers wishing an introduction or review of topics, we
include Appendices. For functional anatomy of the upper limb of
humans, Appendix A is provided. It includes a description of the
skeletal and joint structures from the shoulder girdle to the fingers and
a review of the degrees of freedom permitted by these joints (e.g., a
hinge joint like the elbow has one degree of freedom, a ball and socket
joint like the shoulder has three degrees of freedom). In addition,
there is a review of the single and multi-joint muscles in the upper
limb, showing their primary actions, the number of joints they cross,
and their peripheral and segmental innervation. Sensations and spinal
innervation of the upper limb are also included. Appendix B includes
summary tables detailing prehensile classification systems, for both
adult and developmental postures. Appendix C provides a straight-
forward tutorial on the computational modelling concepts presented in
this book. It contains definitions of terminology and simple examples
for readers wishing to gain an understanding of the computations.
Finally, Appendix D provides information on a topic closely related to
the human hand: multi-fingered robot and prosthetic hands.
Throughout history, engineers have tried to capture the complexity of
the human hand into some mechanical device. Today, with recent ad-
vances in space-age materials and electronics, sophisticated multi-fin-
gered mechanical hands have been built for use by amputees and on
robot manipulators. This appendix describes some of the systems
being used in the field and at research institutions.
In summary, our overall approach can be stated as follows. After
examining classifications of prehension, we identify unique phases in
prehension. For example, a grasp strategy suitable for the task is
chosen before movement occurs. During movement, the hand is