Chapter 2. Prehension

414 A ppen die es
automatic lock which prevents the device from opening further when
outside forces act on the thumb. When the user places tension on the
line, the lock releases; when tension is relaxed, the lock engages. The
CAPP provides pad opposition, with VF1 being the lower thinner jaw
and VF2 the upper larger jaw. A notch in the more proximal end of
the larger jaw provides a way to turn the larger jaw into a VF3, one
that opposes gravity in holding an object hooked in the notch.
Useful for more rugged activities, the Otto Bock Svstem Electric
Greifer has two equal sized thick jaws with small talons on the ends
that can be adjusted in order to improve the angle of approach. These
flat tip grasping surfaces of the tips remain parallel throughout range
of opening and provide high pinch force for a secure grip (Sears et al.,
1989). The System Electric Greifer has one degree of freedom. In
terms of oppositions, it provides pad opposition, with VF1 being one
jaw and VF2 being the other one. Using both jaws together as VF3, a
suitcase can be carried.
TRS (Boulder, Colorado) has a family of designs that are neither
in the shape of split hooks or hands. In all these devices, two rounded
and scalloped virtual fingers provide either pad opposition. Children
are capable of gripping as hard or harder than with their natural hands.
Tasks they can do include swinging, climbing, riding a bike, tie
shoes, swing a bat, etc. The upper jaw hooks around so that it can be
used as a VF3.
D.2.6 Research examples
Table D.5 provides a list of hands currently being developed at
several universities and laboratories.
Kenworthy (1974) developed a design for a hand that is based on
an observation that in the normal hand, the fingers exhibit a low level
of mobility in comparison to the thumb. His design allowed objects,
such as pens and utensils. The index and middle fingers were
constructed in the form of a rigid hook, while the ring and little fingers
were cosmetic only. Three point contact was achieved using the
thumb, middle, and index fingers, with the middle finger slightly more
flexed than the index finger. A fourth contact point could occur on the
proximal phalanx of the index finger, thus creating the writing or
external precision grip, as described in Chapter 2. The fingers
themselves did not move, while thumb movement occurred in a plane
perpendicular to the fingers. External precision is a combination of
pad and side opposition, where pad opposition occurs between the
thumb (VF1) and index finger (VF2), while side opposition occurs