Chapter 2. Prehension

Chapter 6 - During Contact 261
suggested that the automatic regulatory processes to maintain a safety
margin (a grip forcefload force ratio slightly above that for the slip
ratio) interfered with the drive to decrease the grip force voluntarily.
Even during the period of releasing forces, slip responses were seen.
Interestingly, in contrast to ordinary slip responses, these slip
responses did not result in maintained change of the force balance.
“Thus the voluntary command to separate the fingers appeared to
selectively suppress one component of the motor response, that is, the
maintained, memory-based upgrading of the grip force. That an early
brief force response still remained suggests that two different CNS
mechanisms mediate the motor response to slips” (Johansson &
Westling, 1990, p 707).
The magnet phenomenon could be also reflecting the fact that
adherence to papillary ridges increases as grip force decreases. The
lubricant secreted through the tops of the papillary ridges is greasy,
having high adhesive qualities at low shear velocities and lower
adhesive qualities at high shear velocities (Moore 1975). Past
researchers had attributed the magnet effect to vibration effects on
muscle spindles’ afferent activity. Torebjork, Hagbarth & Eklund
(1978) showed that the magnet reaction, in which the fingers tend to
adhere to a vibrating object and there is difficulty in loosening the grip,
is attributed to a reflexive response to the excitation of the
mechanoreceptors in the fingers, not to vibration of the muscle
spindles in the fiiger flexor muscles.
6.4.4 Object shape and contacting area: Geometry of
contact
In comparing pad opposition using different shaped dowels, Weir
(1991) noted differences between a dowel instrumented for
transducing grasping forces, with flat, parallel square gripping plates,
and a cylindrical dowel. Both dowels had the same available surface
area for grasping and 1.5 cm between the grasping surfaces, i.e., the
magnitude of the opposition vector was 1.5 cm. An analysis of the
elliptical fingerprints made by the thumb and index finger in pad
opposition yielded striking differences between the two dowels.
When grasping the flat, parallel plates, the surface area contacted was
substantially greater than when grasping the cylindrical object.
Further, the orientation of the elliptical contacts made by the thumb
and index finger differed. For the cylindrical dowel, the long axis of
the ellipse was oriented about the transverse plane of the finger and
long axis of the dowel; in contrast, for the parallel plated dowel, the