Chapter 2. Prehension

16 WHAT IS PREHENSION?
(1988) referred to the task goal as the ‘environmentally defined goal’,
distinguishing it from the muscular activity needed to produce the de-
sired environmental outcome. A fundamental functional constraint is
that the object not be dropped. This means that one key goal in most
prehensile tasks is the establishment and maintenance of a stable
grasp. The posture used by the hand during the task must be able to
overcome potential perturbations or take advantage of the anticipated
forces acting on the object. These include gravity and, depending on
the actual task, there are likely additional inertial and/or coupling
forces. Another functional constraint in some tasks is the ability to
manipulate the object. In order to accomplish the goal of the task, the
object is transported or motion is imparted to the object, with potential
instabilities occurring along the way. The shaping of the hand prior to
contact reflects these anticipated task requirements.
Underlying these functional constraints are physical constraints,
including: properties of the objects; forces like gravity and friction; and
properties of the arm and hand. For the object, these include such
properties as surface compliance, shape, texture, temperature, size,
etc. (see Chapter 4 for those object properties assessed by vision; and
Chapter 6 for those assessed by touch). Object size and shape can de-
limit the number of fingers potentially contacting a surface; the avail-
ability of object surfaces will constrain the orientation and potential
contact locations of the hand. With respect to forces, gravity is a con-
stant. In contrast, friction arises from the interaction between the hand
and the object. Physical constraints also include properties of the arm
and hand. Postures are created by the muscles of the hand directing
the bones into some configuration, based on the motion capabilities of
the various joints2. Each joint provides one or more motion capability
(each capability for motion is called a degree of freedom, df). Degrees
of freedom refer to the independent states of a system or the number of
values free to vary independently. In the upper limb there are close to
40 df, of which about 30 df are in the hand and wrist. The degrees of
freedom problem in motor control refers to the problem of coordina-
tion and control by the central nervous system, and has been phrased
2For those readers wishing an introduction or review of upper limb functional
anatomy of humans, see Appendix A. It includes a review of the degrees of
freedom permitted by the joint structures of the upper limb (e.g., a hinge joint like
the elbow has one degree of freedom, a ball and socket joint like the glenohumeral
joint of the shoulder has three degrees of freedom). As well, there is a review of
the pattern of single and multijoint muscles in the upper limb, and the peripheral
and segmental innervation of these muscles by the nervous system.