Chapter 2. Prehension

278 THE PHASES OF PREHENSION
cause rotations into a stable grasp if the forces are not colinear,
creating a stable band inside the friction limits (see Figure 6.24).
Slipping will occur if the force is outside the cone of friction. Each
force makes an angle ai with a normal to the surface. A displacement
that moves the force angles outside the stable region will generate a
restoring force that tends to bring both angles back within the stable
limits. As seen in Figure 6.24a, two fingers initially grasp the object
in a stable grasp. Since the applied force angles ai are parallel to the
surface normal, a1 and a2 are zero. In Figure 6.24b, a third finger
applies a force to the object, causing a disturbance. The disturbance
force is considered to act through a fixed point, here, the second
finger. The forces are no longer perpendicular to the surface, and a1
and 012 increase. The baton rotates about second finger, since a2 is
still within the friction cone. The fist finger is removed, and the
object rotates into a new stable configuration, grasped between the
second and third fingers. Applying finger 1 again, in Figure 6.24d,
will cause a disturbance, and the process will continue, as long as
rotation without slip about the fixed finger is ensured.
6.7 Releasing an Opposition Space
After transporting or manipulating an object, the object is either
dropped, or transferred to some other supporting surface, like a table
top, or another hand. After using an opposition space, the object is
released by extending the fingers and withdrawing the arm.
With respect to letting go of the object, an interesting observation
was made by Wing and Fraser (1983) in their examination of hand
posturing during the preshaping and releasing phases. They examined
a woman with congenital absence of the left forearm and hand; she had
an artificial hand of a design where a harness over the contralateral
shoulder is used to tension the hand against a strong spring that keeps
the fingers (as one unit) and thumb normally closed together.
Shoulder movement provided information about the state of opening
of the hand. In comparing the natural hand of the young woman with
her artificial hand, they had earlier observed that maximum aperture
during the reach was paralleled for both hands (Wing & Fraser,
1983). In comparing maximum pertures prior to grasping and after
releasing, they found that the left, prosthetic hand had a larger aperture
after release than prior to grasp; in contrast, the right, natural hand had
a larger aperture prior to grasp than during release. They suggest that
the main reason for this is that the artificial hand does not have tactile
input from the fingertips during grasping, thus contributing to an