Chapter 2. Prehension

194 THE PHASES OF PREHENSION
the start of the enclosing phase, they become active to perform their
first function: that of ‘closing the fingers’ around the object. On con-
tact, they begin their second function, that of ‘generating the forces’
against the object. An important correlative study to Smith’s work
would be to look at how these force producers are brought into posi-
tion; i.e., look backwards from contact through enclose through pre-
shape. At some moment in time, the goal of the movements shifts
from ‘closing’ to ‘generating force’, and it would be interesting to
observe this both behaviorally (at peak aperture) and at the EMG level.
In this regard, it is of import that the lumbricals (intrinsic hand
muscles, which serve to flex the metacarpophalangeal joints of the
fingers, but extend the interphalangeal joint; see Appendix A) have
their origins on the four flexor profundus tendons on the palm. They
have been suggested to have an important sensory function, that of
monitoring the rate of finger flexion, or enclosing of the hand during
grasping (Ranney & Wells, 1988). Combined with the myriad of
other sensory information in the multiarticulate hand, the guarded
motion phase should be uniquely detected, and anticipatory of the
force production once contact is made.
5.5.3 Palm-focused model of grasping
The hypothesis that both reaching and grasping movements are
two-phased sets up the question of what is the relationship between
the Moveslow Schema and the Enclose Schema. There has been no
correlation between the start of the slow phase of the arm movement
with the start of the finger enclosing. However, noting that the arm
acts differently when doing simple tasks than when reaching to grasp,
a new model is offered in Figure 5.32. This model focuses on what
the palm is doing during prehension. Instead of the wrist, the palm is
the interface between the transport component and the grasping com-
ponent. Palm orientation is resulting from wrist flexiordextension, ul-
nar/radial deviation, pronation/supination, elbow flexiordextension,
shoulder flexiordextension, abductiordadduction, internaVexternal ro-
tation and pectoral girdle upward/downward rotation, eleva-
tiorddepression, and protraction/retraction. Note that most of the
muscles involved in transporting the arm are involved also in orienting
the palm and thus the fingers; for example, the primary action of the
biceps is to flex the elbow, but the biceps is also a supinator of the
forearm. On the left side of the figure, the Orient Palm Schema
transports and aligns the palm (in contrast to the wrist) relative to the
opposition vector seen in the object. On the right side, the Reshape