Chapter 2. Prehension

Chapter 6 - During Contact 271
very small objects, where side opposition is used and movement is
generated by the index finger and while the thumb remains stationary.
In twiddling, this is reversed (thumb moves, index stationary) so that
alternating between pad and side opposition is exhibited. The rock
movement is for larger objects, such as turning jar lids or coins. The
squeeze motion, occuring in the x axis, does not cause movement of
the opposition vector; instead, it changes the length of it, as in
squeezing a syringe or a rubber ball. Finally, regrasping can occur if
the fingers must be repositioned to repeat the movement in a sequential
or phased way.
Elliott and Connolly (1984) also observed dynamic combinations
of oppositions. The ulnar fingers could affix the object, or part of the
object, in the hand, in palm opposition, while the radial fingers could
perform a manipulation using pad opposition. An example is holding
a string and tying a knot in it. Another combination is what they call
the ‘solo’ use of a finger: an object, such as a cigarette lighter, is held
in palm opposition, while the thumb, as a virtual finger three, strikes
the lighter. One interesting pattern of dynamic movements that they
observed is the palmar slide (see Table 6.6). Using palm opposition to
hold part of the object, translation of the other part of the object occurs
in the x-y plane using either pad opposition or a combination of pad
and side opposition.
Table 6.6 could be viewed from a different perspective, so that
the focus is not on the movement names, but instead on the
functionality. This is similar to what was done in Appendix B for
static postures. Besides the oppositions used and virtual finger
mappings seen in the appendix, the dynamic function table for human
hands would include the type of opposition vector transformation,
direction of movement, and relative use of fingers, as outlined in Table
6.6. These, along with the length of the opposition vector, are a
dynamic way to relate the motions needed in the task to the motions
available to the hand, demonstrating Napier’s conjecture that power
and precision task requirements are met by the hand’s power and
precision capabilities (Napier, 1956).
Johansson and colleagues (Johansson et al., 1992a,b,c) made a
contrast beween two types of purposeful grasping activities,
manipulating ‘passive’ objects (to move or reposition mechanically
predictable, stable objects like a mug) or ‘active’ objects (to manipulate
mechanically unpredictable objects, like my dog’s leash, when the dog
is attached). They suggested that for mechanically predictable objects,
we rely on sensorimotor memories, and motor output is based on
anticipatory parameter control, i.e., we are using robust but flexible