Chapter 2. Prehension

Chapter 5 - Movement Before Contact 151
MacKenzie, and Leavitt (199O), they suggested their pattern of results
showed a strong functional linkage between the grasp and reach
components.
Examining the variability of grasp movements as a function of
movement speed and practice, the most striking finding was the in-
variance of the location of fingertip contact on the thumb (Darling,
Cole & Abbs, 1988). Also, the movements of the thumb and index
were not coplanar; the index finger was elevated .2 - .8 radians relative
to the plane of the thumb tip movement. In contrast to arm move-
ments, variability of end position of finger and thumb joint end posi-
tions did not increase with increases in movement speed. This was
apparently due to an offsetting of the positional increases in variability
during acceleration by the decreases in positional variability during de-
celeration. Rapid movements (100 ms duration) had bell-shaped sin-
gle peaked velocity profiles, and the slower movements (200,400 ms
durations) had multiple peaks in the associated velocity profiles of
joint angles of the thumb and index finger.
Wing and Fraser (1983) noted also the constant location of con-
tact on the finger pads and suggested that the visual monitoring of the
thumb allows it to be directed toward the object. This is a driving
constraint in these types of movements, and therefore arm and hand
control are not separable processes. Referring to the opposition space
view presented in Chapter 4, with respect to directing the approach
vector towards the opposition vector, these data might suggest that the
the approach vector is directed to the opposition vector in the object at
the location where the thumb makes contact. However, using
graphics animation of WATSMART data for grasping, visualization of
the approaching hand from the perspective of the object suggested that
instead of being aligned along VF1 (the thumb), the approach vector
from the hand to the object was aligned between VF1 and VF2
(Forsey & MacKenzie, unpublished).
Accessibility constraints can also affect the transporting of the
hand to the object. MacKenzie, Sivak, and Iberall(l989) performed a
screwdriver experiment, where subjects were instructed to reach and
grasp a screwdriver, make 3-4 turns on a screw, and then replace the
screwdriver. Four different sized screwdrivers were used, and the
task (tighten a screw into a board) varied as well, depending on the
size of the screws and whether or not the screwdriver handle was ac-
cessible. During the reaching to grasp the screwdriver, the peak speed
of the wrist increased with screwdriver size. The accessibility of the
screwdriver handle affected both the grasp and transport components:
when the handle was accessible, grasping movements were slower,