Chapter 2. Prehension

neuropathic
(lesions)
ulnar (n=14)
median (n=13)
radial (-5)
normal (n4)
Chapter 6 - During Contact
power grasp tip pinch pulp pinch key pinch
18.5 2.8 2.4 4.9
20.1 4.2 4.2 7.6
14.0 3.2 4.0 7.1
34.6 5.6 5.1 9.8
normal
I h=60 n=124 h=60 - n=84
males I 40 (9) 6 (1) ., 6 (1) ., 11 (2)
females I 23 (7j 5 (1) 5 (1) 8 (i)'
power grasp tip pinch pulp pinch key pinch
- .
263
Another physical aspect of applying forces is the direction of the
force application. Using a task involving an isometric palm opposition
on cylinders, Amis (1987) studied the effect of object size on force
application. Amis computed the total normal force (gripping) applied
by the four fingers and also the shearing force (pushing or pulling) at
the phalanges. He determined that the distal phalanx exerts the largest
gripping force in all fingers. A smaller force was produced by the
middle and proximal phalanges. Summing up these forces within a
finger, the total normal force was largest for the smallest objects, and
then decreased as the object size increased. Shearing forces at the
distal and proximal phalanges for the index, middle, and ring fingers
tended to pull the object into the grasp for smaller objects. As the
object got larger, shearing forces on the middle and proximal
phalanges tended to zero out, while at the distal phalanges, shearing
forces tended to push the object out of the grasp.
Other object properties have an effect on force generation as well.
Cochran and Riley (1986) found that handle shape affects the force
exerted: higher forces were seen with irregularly shaped handles
(rectangular and triangular) than with uniformly shaped ones
(circular).
Napier pointed out that the wrist posture is different in a power
grasp vs a precision grasp. In the power grasp, the wrist is positioned