Chapter 2. Prehension

264 THE PHASES OF PREHENSION
with ulnar deviation, neutral between extension and flexion, whereas
in a precision grasp, the wrist is dorsiflexed, positioned between ulnar
and radial deviation. Because the extrinsic muscles send tendons
through the wrist, the amount of force that the fingers are able to apply
is greater in wrist extension and/or ulnar deviation than in other
positions. In addition, there are differences between the fingers in
their potential contribution to force application. For a hook grip,
Hazelton et al. (1975) noted that the relative amount of force available
at each finger remains constant across wrist positions (25% at the
index, 33.5% at the long finger, 25% at the ring finger, and 16.5% at
the little finger). For the power grasp, Amis (1987) found the mean
contributions to the overall grasp force to be 30%, 30%, 22%, and
18% for the index, long, ring and little fingers, respectively.
The human hand can impart arbitrary forces and torques using
pad opposition in the tripod (Drucker & Hollerbach, 1987). Subjects
grasped a medium sized sphere using the thumb, index and middle
fingers. The sphere was instrumented with a sensor capable of
measuring forces and torques along six axes. Receiving constant
feedback about actual forces being applied, subjects were asked to
manipulate the sphere in arbitrary directions in order to achieve a
target. Results indicated that subjects could control all force and
torque components separately.
6.5.2 Analytic posture measures
The analysis of a grasp in the field of robotics uses formal quality
measures. A formal quality measure is a quantification of some aspect
of a grasp posture. Cutkosky and Howe (1990) informally described
these measures as follows:
Compliance: what is the effective compliance (the inverse of
stiffness) of the grasped object with respect to the hand?
Connectivity: how many degrees of freedom are there between the
grasped object and the hand?
Force closure: assuming that external forces act to maintain contact
between the fingers and object, is the object unable to move
without slipping when the fingers are ‘locked’?
Form closure: can external forces and moments be applied from
any direction without moving the object when the fingers are
locked?
Grasp isotropy: does the grasp configuration permit the finger
joints to accurately apply forces and moments to the object?