Chapter 2. Prehension

Chapter 2 - Prehension 23
based on both a functional and anatomical description of the human
hand in prehension. Major points that Napier made include the following:
1. Influences on the posture chosen for the grasp come from multi-
ple sources. These include object shape, size, weight, surface
characteristics (texture, temperature, and dryness), and human
motivations (fear, hunger, distaste, etc.). He stressed, however,
that the most important influence on the chosen posture is the
goal of the task; i.e., the intended activity.
2. He described prehension as the ‘application of a system of forces
in a given direction’ (Napier, 1956, p. 906), and argued that
prehensile tasks can be resolved into power and precision re-
quirements (with one fundamental requirement being a secure
grasp). Power reauirements relate to the ability to apply forces
and resist arbitrary forces that may be applied to the object; re-
cision reauirements involve small adjustments of posture in or-
der to control the direction in which force is being applied.
3. Napier’s insight was to suggest that the power and precision re-
auirements of tasks could be met by the power and precision ca-
pabilities of the human hand. The power requirements of a task
could be met by the hand’s ability to oppose arbitrary forces, es-
pecially when the thumb is used to reinforce the action of the
fingers. Meeting the precision requirements of a task could be
accomplished by placing the skin receptors in contact with the
object, providing the nervous system with cutaneous information
for making the fine adjustments.
Napier’s classification is presented in Figure 2.2. Anatomically,
Napier defined the power mip as follows (see Figure 2.2a and refer to
Appendix A for assistance with anatomical terminology). The thumb
is in the plane of the palm; its metacarpophalangeal and car-
pometacarpal joints are adducted. The fingers are flexed, laterally ro-
tated, and inclined towards the ulnar side of the hand. The fingers flex
in opposition to the palm, with the degree of flexion depending on
object dimensions. The wrist is positioned with ulnar deviation, neu-
tral between extension and flexion. An element of precision in the
power grip depends on thumb placement, ranging from some preci-
sion when it is adducted and able to contact the object and gather sen-
sory information (Figure 2.2a) to no precision (and maximum power)
when the thumb is abducted as in the ‘coal hammer’ grip (Figure
2.2c).