Chapter 2. Prehension

24 WHAT IS PREHENSION?
A. POWER GRASP B. PRECISION GRASP C. COAL HAMMER
Figure 2.2 Napier’s classification of prehensile postures. He
focused on the goal of the task, i.e., whether precision or power
was required, and argued that the hand could form into either a: A.
power grasp or B. precision grasp to match these task
requirements. C. A type of power grasp, called a coal hammer
grip, occurs when the thumb is abducted and brought onto the dor-
sal surface of the fingers (from Napier, 1956; reprinted by permis-
sion).
The (see Figure 2.2b), according to Napier, occurs
when the thumb is abducted and medially rotated at the metacarpopha-
langeal and the carpometacarpal joints. The fingers are flexed and ab-
ducted at the metacarpophalangeal joints, producing a degree of axial
rotation in the digits. The wrist is dorsiflexed (extended), positioned
between ulnar and radial deviation. The object is pinched between the
flexor aspects of the fingers and the opposing thumb, specifically be-
tween the thumb and index finger, which are best adapted to perform-
ing fine control and thus more useful with smaller objects.
Napier’s insights stemmed from his research into the evolution of
the hand. In comparing the human hand to the hand of other primates,
he was able to recognize its simplicity and general purpose nature.
Arguing that the human hand is highly unspecialized, he pointed to the
dead ends that extreme specialization can lead to, such as the potto
(stubbed index; specialized for f ii gripping), the gibbon (long fin-
gers, short thumb; specialized for brachiation), and the aye-aye
(extremely skinny middle finger; specialized for scooping insects from
holes). Extreme specialization for one purpose limits other possible