Chapter 2. Prehension

252 THE PHASES OF PREHENSION
respectively). Thus, the ratio of grip-force/load-force can be lower for
sandpaper than silk for a given weight. The ratio of grip-forceboad-
force always exceeds a slip ratio by some safety margin. The slip ratio
is that ratio of grip-forceboad-force at which the object slips from
grasp as subjects open their fingers, shown in Figure 6.17b. The grip
and load forces and their ratios are shown for the entire duration of the
task in Figure 6.18.
6.4.2 Phases in applying grip and load forces
Westling and Johansson identified a series of distinct phases for
the task of lifting the object from a table, holding it in the air, then
replacing it on the table. From Figure 6.18, they identified the
following events. A preload phase (about 80-100 ms) as the subject
first grips the object (a) during which the grip force increases but the
load force does not. This is followed by a- (b) when the
grip and load forces increase in parallel. When the load force
overcomes the force due to gravity (equal to the weight of the object),
the object starts to move, beginning a transitional phase (c). When the
object is being held in the air, the position of the object and grip and
load forces are nearly constant, comprising a static phase (d). The
replacement phase (e) includes motion of the object back to the table.
After a short delay phase (0, the (g) occurs as the
grip and load forces decrease in parallel. Then, the object is released.
Using a similarly instrumented dowel, many of the Johansson
and Westling results have been replicated in experiments examining
object weight and task requirements (Weir, 1991; Weir & MacKenzie,
1993, in preparation). We did not find a separate preloading phase; at
first contact with the object, our strain gauge transducers register both
grip and load forces. This may be reflecting minor differences in the
object instrumentation or experimental setup. The Weir & MacKenzie
experiments used a strain gauge transducer design similar to that in
Figure 6.16. Our dowels were weighted with buckshot inserts below
the transducing mechanism, whereas Westling and Johansson had
their weights suspended from the instrumented object, below the table
surface. Initially in the loading phase, the load forces were sometimes
negative, indicating that subjects were ‘pushing’ the dowel into the
table surface prior to liftoff, as shown in Figure 6.19. This increased
with object weight such that heavier objects were pushed harder into
the table as forces were first applied, prior to the parallel increase in
grip and load forces. During the loading phase, for heavier compared
to lighter objects, forces were applied for a longer duration, and at a