Chapter 2. Prehension

326 CONSTRAINTS AND PHASES
8.6 Summary
The versatile performance seen in human prehension emerges
within a large multidimensional constraint space. There is a complex
interaction between movement goals, object properties, environmental
characteristics, and the performer’s biological structure and
experience. Even deeper is the evolutionary pressures on the hand and
why it has taken on the form that it has, and what developmental needs
constrain it further. The ultimate issue in this complex interaction is to
match the task requirements (which one could argue incorporate a
wide range of needs from social to motivational to informational to
functional) to the capabilities of the hand (its sensorimotor capacity)
given the physics of compliant motion. Functional goals can range
from ‘palm a coin to hide it from view’ to ‘don’t drop the object’ or
‘move quickly and accurately.’ Force production is affected by the
chosen posture, the wrist angle, the mechanical properties of skin, the
use of the fatty ridged pads, and the generation of sweat. Features of
the hand contributing to its use in prehension include its numerous
degrees of freedom, the thumb’s mobility and placement, the mobility
of the 5th carpometacarpal joint, the pads, and the placement and
response characteristics of cutaneous and proprioceptors. Neural
contributions specific to prehension include the pyramidal tract for
fractionated finger movements, multiple representations of the hand
and wrist in motor, premotor, and somatosensory cortex, grasp reflex,
a grip force reflex, and contextual neural responses. The laws of
physics that influence the interaction include stability, laws of motion,
kinema tics, dynamics.
One way to view the control of the hand is as a three-tiered model.
This looks at how the sensorimotor level (anatomy and physiology of
the limb, the peripheral nerves, and kinesthetic and cutaneous
receptors) constrains the hand as a biomechanical device (forces and
torques acting on the bones and joints in a given posture) which in
turn constrains possible opposition space choices. In addition,
identifying a desired opposition space, given functional constraints
such as object properties and task requirements, sets up a goal
posture. The goal posture can hopefully be met by some real posture
without undue stresses and strains on the joints and bones that can in
turn be met by the anatomical constraints. And this mapping can be
done for the multiple phases of prehension. Separating the goals from
the contraints serves two purposes. First, it identifies the direction of
information flow, as goals are sent down and constraints are sent up.
Secondly, it presents a method for mapping a device independent hand