Chapter 2. Prehension

22 WHAT IS PREHENSION?
Schlesinger’s classification incorporated three critical notions:
obiect shape (cylindrical, spherical), of particular hand surfaces (tip,
palmar, lateral), and hand shape (hook, close fist, open fist).
Importantly, he noted how the hand, with its ability to spread the fin-
gers and arch the palm, can grasp arbitrary objects. In the 1919 book,
Schlesinger did not specifically name any of the grasps, other than the
‘fist’; instead, he used object characteristics and tools as methods to
describe the postures. The English terminology used by Taylor and
Schwartz doesn’t capture each posture’s functionality as explicitly as
did Schlesinger. In addition, what has been translated into English
has been only the minimum set of postures that was developed for
unilateral amputees. His complete characterization of human prehen-
sion, as listed in the third column of Table 2.1, included five other
postures, such as the closed fisted cylindrical grasp with the thumb
adducted and the three-jaw chuck, discussed later in this chapter.
An alternative approach was developed at the end of World War II
in order to evaluate the functional loss of hand use due to injuries. As
surgeons in the Army Medical Corps, Slocum and Pratt (1946) identi-
fied three prehensile postures: the grasp, pinch, and hook. The grasp
was the ‘combined action of the fingers against the opposed thumb
and palm of the hand,’ the pinch was ‘apposition of the pad of the
thumb against the pads of the opposing fingers,’ and the hook was a
posture where ‘the fingers are flexed so that their pads lie parallel and
slightly away from the palm’ (Slocum and Pratt, 1946, p. 491).
Simple and elegant, this approach collapses Schlesinger’s many pos-
tures into three functional features of the hand. Using Schlesinger’s
terminology, these can be characterized as the ability to act like a ring,
to act like a pliershippers, and to act like a hook.
2.2.2 Power vs. precision requirements
While Schlesinger’s analysis is extensive and Slocum and Pratt’s
insightful, the major problem with them is that they don’t address the
requirements of the task. For example, people use different hand
postures when picking up a pen to write with it, compared to picking it
up to place it in a container. For a classification that deals with task
features, Napier’s work is more useful.
John Napier published a classic paper in 1956 addressing the
question ‘what is a scientific terminology for describing the move-
ments and functions of the hand as a whole?’ Napier argued that
Slocum and Pratt’s postures (grasp, pinch, hook) were not clearly de-
fined nor comprehensive, and that a classification was needed that was