Chapter 2. Prehension

308 CONSTRAINTS AND PHASES
the index finger makes contact before the thumb, the glass will rotate
out of the grasp if the direction of the line of pushing (a line parallel to
the motion of the finger, drawn through the contact point) is to the
near side of the center of mass between the glass and the table. If,
instead, the line of pushing is through the center of mass or to the far
side, the glass will translate towards or rotate into the thumb,
respectively.
Hand postures afford different ways to apply forces. Biomaterials
such as bones, muscles, tendons, ligaments, and skin create
limitations on static and dynamic force generation. Chao, Opgrande
and Axmear (1976) argued that typically used postures are ones where
joints are stabilized and where forces can be applied optimally without
undue stress and strain on the ligaments, joint capsules, tendons, and
muscles. An example would be locations where articular surfaces
coadapt, thus permitting increased force generation while decreasing
stress on ligaments and muscles. Results from cadaver studies have
been used to develop and verify computer models of the mechanisms
used by muscles and tendons to generate force across the various
joints for different isometric hand postures (An et al., 1985; Chao et
al., 1976; Cooney & Chao, 1977; Wells, Ranney & Keeler, 1985).
Extrinsic finger flexors (as seen in Appendix A, forearm muscles that
insert into the phalanges by sending long tendons through the wrist
and hand) have a greater mechanical advantage than the extrinsic
extensors (Tubiana, 1981). Within the hand, the intrinsic muscles of
the index finger produce more force in a lateral pinch than in an
enclosing grasp (An et al., 1985). The fingers and thumb have an
average greater strength in a power grasp than in a pulp pinch (An et
al., 1985; Cooney & Chao, 1977), particularly because multiple points
of contact apply a force against the object. However, the size of the
finger separation in the power grasp influences the gripping forces. In
a task involving the isometric grasp of different size cylinders, Amis
(1987) determined that the distal phalanx exerts the largest gripping
(normal) force in all fingers, with a smaller force 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, they tended
to push the object out of the grasp. Other studies have analyzed the
effect of wrist position on available force in precision and power