Chapter 2. Prehension

262 THE PHASES OF PREHENSION
long axis was oriented about the sagittal plane of the finger, and
horizontally on the short axis of the dowel. When the surface area
actually contacted was equated between the two dowels through
experimental manipulation, the differences between the two dowels
disappeared. Thus, dowel shape affected the surface area actually
contacted; the surface area actually used was the important variable,
not shape per se.
6.5 Force Application with Other Oppositions
6.5.1 Measuring forces
It was noted that one of the functions of the hand in prehension is
to apply forces to match the anticipated forces in the task. One
measure is to examine the maximum forces that can be exerted using
the different types of grasps. In their biomechanical studies of
functional strength assessment, Chao and colleagues (Chao, An,
Cooney & Linscheid, 1989) determined the maximum forces that
could be exerted using tip pinch (compressive force in pad opposition
between the tip of the thumb and index), pulp pinch (compressive
force in pad opposition between the pulps of distal phalanx of the
thumb and index), key pinch (side opposition applied by the thumb
pad through the lateral aspect of the distal interphalangeal joint of the
index), and power grip (palm opposition centered on the middle
phalanx of the middle finger, with the thumb wrapped around). Using
specially designed strain gauge instruments, they adjusted for
individual differences in hand size, shapes, deformations, etc, in order
to standardize the results. Table 6.4 summarizes their results, based
on 124 adults, and 32 patients with documented single nerve lesions
of the radial, median and ulnar nerves. Consistent with the
suggestions above, palm opposition generates the greatest gripping
forces, pad oppositions generate the least; and side opposition is
intermediate. Note also that for normals, little difference exists
between the tip pinch and the pulp pinch in the maximum compressive
force.
Information about the maximum grip forces that can be exerted
using the different grasp types is valuable and useful as a way to
identify a physical constraint on a selected posture. However, forces
applied during prehension are usually at levels below maximum grip
force. In addition, the hand’s configuration will factor into this, and
grip force will be affected by the number of fingers participating in the
grasp.