Chapter 2. Prehension

Chapter 9 - Reevaluation and Future Directions 335
receptors respond to contact. But recent evidence shows that the
reverse is also true. The muscles (somatically innervated sensorimotor
system) work in parallel with the eccrine sweat glands (autonomically
innervated sudomotor system). Biomechanicians have measured
aspects of hand postures, such as maximum grip strength, mechanical
advantages of tendons, effect of wrist orientation, torques at the
joints, and direction of frictional and shearing forces for various object
sizes. See (Amis, 1987: Chao et al., 1989; Hazelton et al., 1975).
As described in Chapter 2, taxonomies of prehensile capabilities
have been developed that offer insights into the complexity of human
prehension. Across this diverse set, themes are repeated, suggesting
the possibility of a unifying view. Hand surfaces, hand shapes,
functions, and object characteristics are possible identifying features
for taxonomies (Cutkosky, 1989; Jacobson & Sperling, 1976;
Kamakura et al., 1980; Kapandji, 1982; Schlesinger, 1919; Taylor,
1948). Before actual tools were constructed, our hands were the tools,
and prehensile postures can be described in those terms (Schlesinger,
19 19). Power versus precision capabilities have been noted (Napier,
1956). Power grasps create a method for fixing the object in the hand,
while precision grasping offers manipulative abilities by opposing the
pads of the thumb and fingers (Landsmeer, 1962; Napier, 1956). The
three jaw chuck (Schlesinger, 1919) and dynamic tripod (Elliott and
Connolly, 1984; Kroemer, 1986), between the thumb, index, and
middle fingers, are precision postures that allow fine manipulations.
Other sensorimotor features of the hand are exhibited in the lateral
pinch (Skerik et al., 1971), the hook grip (Napier, 1956), the
adduction grasp (Kamakura et al., 1980), and the finger touch
(Kroemer, 1956). This latter notion we call a ‘finger-as-antenna’,
highlighting the hand’s sensorimotor capabilities. Importantly, power
and precision capabilities can be exhibited in one posture, as when
tying knots (Napier, 1956).
In all these postures, the hand is applying forces to the object in
three directions with respect to the palm. We call the participating hand
surfaces grasping surface patches on virtual fingers. A virtual finger is
the palm or else a collection of one or more real fingers. Pad
opposition occurs between the finger and thumb pads along an axis
generally parallel to the palm. Due to the nature of the sensorimotor
features of the hand, it provides the ability to comply with small
forces, impart small motions, and gather precise sensory information
to match the manipulative requirements of the task. Palm opposition
occurs between the fingers and palm along an axis generally
perpendicular to the palm. It matches larger anticipated forces, gathers