Tangential Torque Effects on the Control of Grip Forces when Holding
Objects with a Precision Grip
HIROSHI KINOSHITA2, LARS BÄCKSTRM1, J. RANDALL FLANAGAN3, AND ROLAND S.
JOHANSSON1
Roland S. Johansson, Department of Physiology, University of Umeå, S-901 87
Umeå, Sweden Fax: +46 90 166683, Email: Roland.S.Johansson@physiol.umu.se
APStracts 4:0081N, 1997.
ABSTRACT
When we manipulate small objects our finger tips are generally subjected to
tangential torques about the axis normal to the grasp surface in addition to
linear forces tangential to the grasp surface. Tangential torques can arise
because the normal force is distributed across the contact area, rather than
focused at a point. We investigated the effects of tangential torques and
tangential forces on the minimum normal forces required to prevent slips (slip
force) and on the normal forces actually employed by subjects to hold an
object in a stationary position using the tips of the index finger and thumb.
By changing the location of the object’s center of gravity in relation to the
grasp surface various levels of tangential torque (0 to 50 Nmm) were created
while the subject counteracted object rotation. Tangential force (0 to 3.4 N)
was varied by changing the weight of the object. The flat grasp surfaces were
covered with rayon, suede or sandpaper, providing differences in friction in
relation to the skin. Under zero tangential force both the employed normal
force and the slip force increased in proportion to tangential torque with a
slope that reflected the current frictional condition. Likewise, with pure
tangential force these forces increased in proportion to tangential force. The
effects of combined tangential torques and tangential forces on the slip force
were primarily additive but there was a significant interaction of these
variables. Specifically, the increase in slip force for a given increment in
torque decreases as a function of tangential force. A mathematical model was
developed that successfully predicted slip force from tangential torque,
tangential force and an estimate of coefficient of static friction in the
digit-surface interface. The effects of combined tangential torques and forces
on the employed normal force showed the same pattern as the effects on the
slip force. The safety margin against frictional slips, measured as the
difference between the employed normal force and the slip force, was
relatively small and constant across all tangential force and torque levels
except at small torques (<10 Nmm). There was no difference in safety margin
between the digits. In conclusion, tangential torque strongly influences the
normal force required for grasp stability. When controlling normal force
people take into account, in a precise fashion, the slip force reflecting both
tangential force and tangential torque and their interaction, as well as the
current frictional condition in the object-digit interface.
Received 1996 October 28; accepted in final form 1997 May 14.
APS Manuscript Number J854-6
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 11 June 1997