Chapter 2. Prehension

42 WHAT IS PREHENSION?
quired opposing forces, virtual fingers and state variables allows for
modelling and control of natural, prosthetic and robotic hands.
2.5 Summary
Beginning with an overview of prehension and grasp classifica-
tions, the essence of these typologies has been searched. In so doing,
the importance of more detailed information about prehensile postures
was identified, based on the hand, object, and task characteristics.
The black box introduced in Chapter 1 mapping object and task char-
acteristics to prehensile behaviors can be looked at with a finer level of
detail (see Figure 2.10). Objects come in a variety of shapes, sizes
and surface characteristics, and tasks have constraints on their spatial
degrees of freedom. In order to achieve the task goals with these ob-
jects, the hand is used to apply forces, impart motions as necessary,
and gather sensory information. Prehensile postures can be described
either using the terminology of the classifications listed in Table 2.1,
or else in opposition space terms. Schlesinger identified hand surfaces
and shapes that combine with object characteristics to name possible
ways that the hand, in effect, creates tools for prehension. He pointed
out how the hand can grasp arbitrary objects. Napier introduced the
notion of task features, noting that the power and precision
requirements of tasks could be met by the power and precision
capabilities of the human hand. Landsmeer focused on the dynamic
aspects of grasping and contrasted precision handling with power
grasp, noting the manipulative options are afforded by opposing the
pads of the thumb and fingers. Kapandji gave examples, such as
turning, squeezing, and cutting, and Elliott and Connolly provided a
vocabulary to expand on this notion (twiddling, rocking, rolling,
stepping, sliding). Other researchers, studying the hand from
anatomical, occupational, disability, and robotics perspectives, have
identified unique sensorimotor features of the hand that are exhibited
in the link grip, the hook grip, the three-jaw chuck, the dynamic tri-
pod, the adduction grasp, and the finger-as-antenna. The Opposition
Space model suggests that, in all these postures, the hand is applying
oppositional forces against task forces and torques and/or around an
object along three general directions, either alone or in combinations.
This extensive multidisciplinary research provides insights into the
functionality of the human hand, based on three issues: applying
forces to match the anticipated forces in the task, imparting motion to
the object as necessary, and gathering sensory information about the
state of the interaction with the object.