Chapter 2. Prehension

Chapter 6 - During Contact 279
indeterminate sense of their position and loading. Given Johansson &
Westling's reports on the role of FA11 afferents signalling contact with
the table at release, this is likely so. The dowel was unstable and
could easily be knocked over. Thus, the individual opened the hand
wider on release, relying more on visual guidance in withdrawing the
prosthetic hand than the natural hand.
6.8 Summary of Using an Opposition Space
We defined prehension as the application of functionally effective
forces to an object for a task, given numerous constraints. Figure
6.25 shows the sensorimotor features of the hand considered in this
chapter, along with relevant task and object properties for establishing
and maintaining stable grasp, and moving between points of stability
in manipulation. A distinction was made between active touch and
active manipulation; sensory information is gathered for both. Both
the process and the product of force generation were examined during
compliant motion phases, noting how the system responds to various
task demands and levels of perturbations, from microslips to external
interference.
The hand was considered as both an input and an output device.
We discussed how the hand gathers sensory information about the
object, the state of interaction with the object for a task, or the task
outcomes, through proprioceptors (skin, muscle and joint receptors)
and exteroceptors (skin receptors and vision). In this regard, skin is a
critical organ providing housing for muscles, mechanoreceptors and
eccrine sweat glands, essential for establishing stable grasp. Palmar
creases, or flexure lines, reflect axes of movement. Some
characteristics of skin relevant to force generation and adhesion
include: characteristics of epidermis, dermis and their interface;
papillary ridges; eccrine glands; sensory receptors; and their
innervation. Papillary ridges extend over grasping surfaces that
comply with the environment, as evidenced by human hands, gorilla
knuckle pads and prehensile monkey tails. They act like ridges on
automobile tires, to increase grip and facilitate weight bearing by
increasing surface area. The concentric arrangement of the ridges
makes some asperities always perpendicular to shearing forces.
Moberg (1962) noted that a patient with impaired sensibility in the
median nerve region has difficulties with manipulations using pad
opposition, like buttoning and unbuttoning; in contrast, patients with
impaired eccrine functions (dry or very wet hands) more often have
difficulties with tools using palm opposition - the subsequent loss of