Chapter 2. Prehension

Chapter 6 - During Contact 269
an unloaded limb, i.e., shape invariance in the velocity profile was
observed. This finding has extended to transporting grasped objects
of different weights. For pad opposition, investigating the effects of
object weight (66, 155, and 423 g cylinders, of constant size and other
visible characteristics) on kinematics and grip forces in a grasp to place
task, Weir (1991; Weir & MacKenzie, 1993) found that during the
dowel transport phase, maximum resultant velocity varied inversely
with dowel weight, but the transport trajectories showed shape
invariance, i.e., were scalable over time. In addition, a surprising, but
robust kinematic finding, demonstrated by all subjects tested, was that
during dowel transport, although the finger pads remained in contact
with the gripping plates of a force transducer dowel, the aperture
between thumb and index markers decreased substantially (about 15
mm), presumably reflecting finger posturing for force application and
propulsion of the dowel forward towards the target location.
With respect to the forces applied during transport of a stably
grasped object, Weir (1991) found that peak grip forces were higher
when subjects transported the dowel to a target location 20 cm further
forward in front of body midline (6. 0 N) than when they lifted the
dowel to a 4 cm height (4.5 N). Peak load forces did not vary as a
function of task requirements to lift or place, only as a function of
object weight, replicating Westling and Johansson (1984; Johansson
& Westling 1988a). In transporting an object, Flanagan, Tresilian and
Wing (1993) concluded that the programming of grip force is part and
parcel of the process of planning the loaded arm movement. Flanagan
et al. studied point-to-point and cyclic movements of varying rates and
directions and reported a tight coupling between grip and load forces;
grip forces were modulated in phase with load force during
movements with grasped objects. Flanagan et al. noted that the load
forces varied both with the gravitational and inertial components.
In dynamic grasping four forms of manipulation can be
considered:
1) FIXED CONTACTS - contacting locations between hand and
object remain constant as motion is imparted to the object using
coordinated manipulation.
2) ROLLING CONTACTS - contacting locations between hand and
object roll past one another while imparting rolling motion to the
object. Fingers can roll on object or object can roll in fingers.
3) SLIDING CONTACTS - contacting locations slide relative to
one another imparting sliding motion to the object.
4) REPOSITIONING OR REGRASPING - motion is imparted to
object by relocating the hand’s contacting points on the object.