Chapter 2. Prehension

2.2.1 Hands as tools
Chapter 2 - Prehension
Schlesinger (1919) put forth a now classic taxonomy that was de-
veloped to capture the versatility of human hands for designing func-
tionally-effective prosthetic hands (see Figure 2.1). He used tools and
fasteners to describe the special purpose nature of various postures.
For example, simple, regularly shaped objects, symmetrical about an
axis internal to the object, could be held by a pincer or palmar grip,
acting much as a set of pliers or as a set of nippers for smaller objects.
A hook grip could be used for suitcase handles and a ring or cylindri-
cal grip could be used for holding a hammer.
Seeing the limitations of sensor-based prosthetic design in his
time, Schlesinger focused on determining what specific functionality
was needed for grasping and holding various objects (e.g., book
page, matchbox, pen, etc.). From this analysiss, Schlesinger devised
a minimum set of six grasp postures (see Figure 2.1):
a) An open fist grip for tools, such as for a hollow cylinder and
beer mug. This is the ring function Schlesinger described, and
the posture has been called cvlindrical prehension (see, for example,
Taylor and Schwartz, 1955). It can also be a closed fist
grip for thin objects, such as a shaft.
b) A posture for spherical objects, such as a ball. The fingers can
spread, while the palm can be arched. This is the most adaptable
posture and is called spherical prehension.
c) A posture for flat, thick objects. Movement is provided by fingers
for grasping objects such as a large box and a matchbox.
This is the pliers function; this posture has been called palmar
prehension .
d) A posture for sharp, small objects, such as a needle, pen, small
bearing, and book page. Again, movement is provided by the
fingers. This is the nippers function and the posture is called ~
prehension .
A posture for thin, flat objects, such as a piece of paper. In this
case, movement is provided by the thumb. This posture is called
lateral prehension.
A posture for heavy loads, such as suitcases. This is the hook
function, and the posture is called hook prehension.
SSchlesinger sometimes arbitrarily discarded functionality because it was
‘uneconomical’, as in grasping a fie!
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