Chapter 2. Prehension

312 CONSTRAINTS AND PHASES
for a given object. In the next subsection, the constraints explicitly
imposed by the sensorimotor system are discussed.
8.2 Sensorimotor constraints
The hand of Homo supiens sapiens represents millions of years
of evolutionary pressures, changing it from a tool of tree-dwellers into
what it is today. Holder (1983) noted, from studying 145 limb
skeletons of amphibians, reptiles, birds, and mammals, that the
overall pattern of skeleton and muscles has varied little in 350 million
years, even though the limbs have become adapted for different
functions. LeGros-Clark (1959) noted the development of the
pentadactyl (five-fingered) hand, the replacement of claws by nails,
and the specialization of the finger pads. Other signs of refined
functionality have been noted as in tendon insertions (Abbott, 1970),
the remodeling of joints (Lewis, 1977; Marzke, 1983), and intradigital
proportions between phalanges (Musgrave, 197 1). A functional axis
of a primate hand can be distinguished by various criteria
(Rabischong, 1981) such as along the longest finger or at the insertion
of the dorsal interossei muscles (intrinsic hand muscles that abduct the
fingers). In the human hand, two dorsal interossei insert into the
middle finger making it the functional axis of the hand. However, it is
interesting to note that the head of the 4th metacarpal (see Appendix
A.2), unlike its counterparts in the other fingers, is symmetrical,
suggesting instead that the ring finger may be the functional axis of the
hand (Dubousset, 198 1). These, and other characteristics of hand
structure, such as the presence of five fingers, are evolutionary
constraints in the sense that they have modified the organization of
prehension over the eons (see LeGros-Clark, 1959 and Napier, 1962
for further reading). Today, as one moment in time, they have left the
human hand with a variety of highly asymmetrical anatomical and
physiological features and constraints.
The anatomy and physiology of the hand and arm create structural
limitations on possible joint configurations, movements, directions
and extents. In the robotics literature, the directions and extents of
movements are referred to as the ‘workspace’. In the human literature,
the region of extrapersonal space where prehension may occur has
been termed ‘grasping space’ (Grusser, 1986). Grasping space is a
function of motion about the shoulder and about the elbow and wrist
joints of the arm, given that the body is stationary.
Although the hand has over 25 degrees of freedom, many are
coupled by the nature of the ligamentous structure and location of