Chapter 2. Prehension

170 THE PHASES OF PREHENSION
150 T
I
130 J;
1 I
,
50 1
0
-a- Collective
grasp
_o_ Independent
grasp
1 .o 2.5 4.0 5.5 7.0
Dowel size (cm)
Figure 5.22 Peak aperture as a function of dowel size (1.0, 2.5,
4.0, 5.5 and 7.0 cm in diameter) when grasping with pad
opposition, using independent fingers or with palm opposition,
using collective fingers. In all cases, maximum aperture is greater
when using the fingers collectively than when using the fingers
independently, in pad opposition. Note the greater sensitivity to
object size with independent finger movements in pad opposition,
compared to collective finger movements in palm opposition
(Adapted from Sivak, 1989; reprinted by permission).
Marteniuk et a1.(1990). She used wooden dowels 1.0, 2.5, 4.0, 5.5
and 7.0 cm in diameter. Movement time and transport parameters did
not show a clear monotonic change with dowel diameter. Rather, the
4 cm dowel seemed “easier” to grasp, had shorter movement times,
higher peak velocities, and less time spent in deceleration. This result
suggested that the 4.0 cm dowel could accommodate both independent
and collective grasps quite easily. Shown in Figure 5.22, we see that
earlier results were replicated, aperture was wider with palm
opposition (collective grasp) than with pad opposition (independent
finger grasp); further, with pad opposition there was a monotonic in-
crease in maximum aperture with dowel size. In contrast, there ap-
peared to be two groupings of maximum aperture with collective fin-
ger use in palm opposition: one, smaller, for the 1.0 and 2.5 cm dow-
els, and a second, much larger maximum aperture for the 4.0,5.5 and
7.0 cm dowels. The second set may be reflecting a ceiling effect due
to biomechanical constraints of the hand, ie., the hand was unable to
open wider! Thus palm opposition, with contact on the palm, and fin-
1