Chapter 2. Prehension

306 CONSTRAINTS AND PHASES
Table 8.1 Sources of constraints on grasping. Movement
constraints and goals act to limit the way the human hand can be
controlled in prehension (from Iberall & MacKenzie, 1990;
reprinted by permission.
-
Group Level of Analysis Examples
High
Level
-
?hysical
~ ~~
Sensori-
motor
Social/Cultural
Motivational
Informational
Functional
Object Properties
Biomechanicall
Mechanical
Neural
Perceptual
Anatomical/
Physiological
Evolutionary1
Developmental
don't stick out elbows; stick out little finger
thirst; anger
convey affection, anger
don't drop object; manipulate object; 'move
as quickly and as accurately as possible'
intrinsic (texture, surface length, weight, etc);
extrinsic (location, distance, environment, etc)
kinematics; dynamics; limitations on force
genemtion due to bones, muscles, tendons,
ligaments, skin; effect and use of pads
temporal and spatial limitations on CNS;
pyramidal tract needed for fractionated finger
movements; sensory info needed to sustain
movement ; sensory information needed to
preshape hand; tonic vibration reflex
types, locations, and response characteristics
of receptors; numerous tactile receptors in
pulps with small receptive fields
structural limitations on movements, directions,
and extents; length of phalanges; additional
muscles in index and little finger; pads;
anatomical couplings
evolutionary pressures; five fingers; pyramidal
tract develops in about eighth month
performer's biological structure and experience, it can be argued that
there are different levels of analysis within this constraint space.
These constraints, or levels of analyses, are grouped here into three
broad categories, as seen in Table 8.1. This list expands and
reorganizes the constraints listed in MacKenzie and Marteniuk (1985)
and Marteniuk et al. (1987). The key point is that the list brings in the
notion that higher level goals work together with harder constraints, as
well as suggests ways that a roboticist's view of prehension can be
integrated with a kinesiologist's view. For example, when a roboticist
studies the constitution of a stable grasp, she is detailing the
constraints imposed by an implicit functional goal 'to not drop the