Chapter 2. Prehension

Chapter 8 - Constraints on Human Prehension 321
e) amount of force available from the VF (mean, maximum,
minimum)
f) amount of sensory information available at grasping surface
patch (innervation density of cutaneous mechanoreceptors)
VF vectors are tagged with the orientation of the grasping surface
patch at a given configuration. For example, in pad opposition, the
grasping surface patches are the finger and thumb pads. As the VF
vectors change length and orientation during flexion and extension,
the pads are changing orientation relative to the palm. The grasping
surface patch size depends on the number of real fingers being used in
the VF and the amount of their abduction, as well as on the
configuration. In terms of state variables, an opposition spaces is
defined by the following state varibles:
a) the number and type of oppositions (pad, palm, and/or side)
being used,
b) the virtual to real finger mapping (i.e., which fingers),
c) VF state variables for each VF in each opposition being used
(see above).
Of course, this goal posture only makes sense within the constraints of
the hand itself.
Using Marr’s approach (see Chapter 1) as a guiding principle, the
Opposition Space level can be re-represented or mapped into a
Biomechanical level (Figure 8.1). At this level, opposition space
parameters are re-represented in terms of the forces and torques acting
at the joints and contact points. Inverse kinematic and dynamic
equations translate VF parameters into real joint angles and torques.
For example, if one objective of the movement is to ‘not drop the
object’, the opposition space chosen for the grasp must effect a stable
grasp, taking into account the active and passive components of force
generation by the muscles, tendons, and skin surfaces. In addition,
the chosen posture must relate to stress and strain on biomaterials and
take into account mechanical advantages. Using pad opposition, the
enhanced frictional component of the finger pulps helps in reducing
the amount of active force. However, if the object is heavy and/or if
there is a low coefficient of friction between the skin and object
surface, the posture involves a different opposition space, one where
active available forces are larger. A palm opposition, having multiple
phalangeal contact points and greater strength, would be used. As
well, the palm, as a large virtual finger, brings in a torquing