Chapter 2. Prehension

44 WHAT IS PREHENSION?
sion; i.e., imparting motion to the object by the fingers. A combined
case of pad and side opposition occurs when the thumb opposes the
index and middle fingers. The unique relationship between the thumb,
index, and middle fingers creates a powerful tripod, much like a three-
jaw chuck of a drill, for both stabilizing an object and manipulating it.
For longer objects, resting the object against the cleft of the finger
adds a fourth surface for counteracting task torques. This in effect is a
third virtual finger, because it is not opposing another virtual finger.
In palm opposition, the hand can match or create larger anticipated
forces while still ensuring a stable grasp, using the arm and wrist to
provide grosser motions. Two aspects to note for palm opposition
that have caused much speculation in taxonomies are the thumb posi-
tion and the shape of hand. In terms of position, the thumb can either
contribute to the opposition (Napier’s coal hammer grasp) or act as an
antenna (Schlesinger’s closed fist with adducted thumb). In terms of
hand shaping, the palm and fingers can wrap around objects symmet-
rical about an axis (Cutkosky’s prismatic grasps and Schlesinger’s
cylindrical grasp) or the palm can arch around objects with radial
symmetry (spherical or circular grasp).
In side opposition, the thumb pad is brought against the object in
opposition to the radial side of a finger. As a bridge between power
and precision grasps, this posture offers a medium range of forces
while still offering some availability of sensory information due to the
thumb pad being in contact with the object and some ability to impart
motions to the object (as in turning a key). Mentioned by a majority of
the classifiers, the functionality of side opposition again rests in the
anatomy of the hand: pronation and supination at the wrist can be used
to rotate the object being held in a secure grip by the thumb and radial
side of the index finger. A special case of side opposition is seen in
the adduction grip that allows the same object manipulation (pronation
and supination of the wrist) at the expense of applying forces and
gathering sensory information (no distal pads or extrinsic hand mus-
cles are used to create the oppositional force). However, motion can
actually be imparted to the object, since finger flexion and extension
can be used.
A third virtual finger can apply a force against gravity, can impart
motion mostly with the arm or wrist, and can gather sensory informa-
tion using sensors in the hand surfaces to determine the state of the
object. Most taxonomists mention the hook grasp, but others like
Kapandji and Cutkosky generalize the hook to the notion of gravity-
dependent grasps. Extending a finger (usually the index) as an an-
tenna is an excellent way to enhance the availability of sensory infor-