Chapter 2. Prehension

Chapter 6. During Contact
“Andjinally, the act of grasping an object includes more than the
accurate direction of the movement on to the target: the pressure
of the grasp must neither crush an egg nor allow it to slip from
the fingers.”
-- C. G. Phillips (1986, p. 6)
203
After initial contact, the hand captures the hammer, establishing a
stable grasp. The hand must maintain this stable grasp. Otherwise,
the object will fall out of the hand. Of course, as Fearing (1986)
noted, skills such as baton twirling go beyond this restriction. Here,
the skill is to move quickly between points of stability. Creating a
stable grasp means taking into account the complex interactions
between object and hand surfaces, and taking into account a variety of
forces and torques. The hand must also be able to resist small external
perturbations, and be able to generate restoring torques and forces to
ensure manipulative stability. While all this is occurring, the hammer
is lifted and transported to a location above the shelf and then placed
on the shelf.
Several phases are involved in prehensile tasks that involve stable
grasping. On contact with the object, the fingers are pressed against
it, stabilizing the object in the grasp. If the initial point of contact is
not the most appropriate, the fingers follow the contour of the object
until reaching a goal location. Stabilization can potentially be broken
down further into separate and distinct temporal components, as will
be seen below. Once a stable grasp is achieved, it must be maintained
during subsequent manipulations. These include imparting motion,
such as lifting the object, moving it around, using it in task contexts,
and placing the object on some support surface. Beyond these are
more complex uses of objects, such as manipulating pens, musical,
surgical or dental instruments, other tools and utensils. In this
chapter, these phases are considered as a person creates, maintains,
and then releases a stable grasp.
Making contact with an object involves a collision between two
systems and, if errors are made, can have potentially disastrous
effects. Creating, maintaining, and releasing a stable grasp is a
complex interaction between object surfaces and the skin, and involves
a variety of forces and torques. The hand is both an input and an
output device. As an input device, it gathers sensory information