Chapter 2. Prehension

310 CONSTRAINTS AND PHASES
While their existence is more an aspect of the hand’s anatomy (see
Section 6.1), their use in a prehensile posture is such that they can
automatically supply a force sufficient to counteract small amplitude
perturbations of the object, useful in stable grasping (along the lines
of a ‘soft finger’ as a contact type that resists moments about the
contact normal (Salisbury & Craig, 1982). The lubricant is greasy,
having good adhesive qualities at low shear velocities, enhanced by
the hills and valleys of the ridged surface extending the total shearing
surface area (Moore, 1975). At high shear velocities, friction is
reduced, thus minimizing wear and tear of the hand surface. Because
ridges are concentric in orientation, there will always be some
perpendicular to the force exerted (as opposed to in the palm where
this is not true). However, due to biomechanical properties, skin
responds in nonlinear ways to loads (Wilkes, Brown, & Wildnauer,
1973). As the load increases, collagen fibers in the dermis reorient
from an irregularly arranged pattern to one orientated in the direction
of the applied load. As more fibers become aligned and thus resist
extension along their length, the skin becomes stiffer, reducing its
ability to comply with the object.
Object properties and the object’s relationship to the environment
can affect the chosen posture. Jeannerod (1981) made a distinction
between intrinsic and extrinsic properties of objects. Extrinsic
properties are those spatially related to the person (i.e., location,
distance, angle, etc.) and would include support constraints, such as a
table supporting a surface, and other obstacle constraints (forcing one
to reach around another object). Lederman and Klatzky (1987) have
shown how subjects can extract intrinsic object properties relevant to
the task. These include surface properties (texture, hardness, and
temperature), structural properties (shape, volume, weight), and
functional properties (part motion, specific function). Before making
contact with an object, people can perceive many object properties
visually (see Klatzky & Lederman, 1990). In a study where subjects
had to use a precision grip to grasp one of ten different sized wooden
disks, Marteniuk, Leavitt, MacKenzie, and Athenes (1990) showed
that the maximum aperture reached during preshaping between the
index finger and thumb closely correlated with disk size. In terms of
the constraints that object properties impose on prehension, examples
are numerous: object shape can delimit grasp postures and the number
of fingers potentially contacting a surface; the weight of the object can
determine the type and strength of grip; and the availability of an object
surface will constrain the orientation of the reach and potential contact
locations.