Chapter 2. Prehension

Chapter 6 - During Contact 249
the grip force, with a constant ratio of grip-forceboad-force for a given
surface texture. With heavier objects, lifting forces were applied for a
longer duration, and at a higher rate. In contrast, manipulations of
surface texture affected only the grip force, not the load force, thus
different ratios of grip-force/load-force were obtained for a given
weight (Johansson & Westling, 1984a, 1990; Westling, 1986).
Thus, the surface texture manipulations affecting the frictional
conditions in relation to the skin created a need for subjects to grip
more forcefully a more slippery object, in order to achieve the same
load force. These effects are shown in Figure 6.17.
Westling and Johansson (1984) showed how:
“the static grip force is adapted to the friction between skin and
object as well as to the weight of the object so that the employed
grip force is greater by a relatively small safety margin than the
minimal force required to prevent slip. The adaptation to the
frictional condition appears to be dependent on signals in
afferents terminating in the fingers, most likely tactile afferents,
since it was impaired during blockage of the afferent signals in
the digital nerves.”
The coefficient of static friction with respect to the skin of sandpaper is
higher than of silk (mean values reported by Johansson & Westling,
1984b, are 1.21, 0.68 and 0.35 for sandpaper, suede and silk
standing on the table. To reliably define the point in time when
the object took off from the table and when it made contact again,
the contact with the table was electrically detected. B. and C.
Illustration of the arrangement of the strain gauges and levers
constituting the force transducers. Three groups of four strain
gauges (with length = L) were glued to both sides of three levers
(nonfilled in fig.). Each group constituted a full electric bridge
measuring the load force and the two grip forces, respectively. B.
The object with unloaded transducers rests on the table. C. Loaded
transducers while the object is lifted. The initial length of the
strain gauges are labelled L, and the lengths after loading L + Ln;
and L - Ln (n=l, 2 and 3 for the three transducers respectively).
Two of the strain gauges in each group were lengthened whereas
two were shortened. Because the load force bridge was balanced
when the object freely rested on the table, all load forces at the
touched surfaces were reliably measured (from Westling, 1986;
reprinted by permission from Johansson & Westling, 1990).