Control of Grasp Stability when Humans Lift Objects with Different Surface Curvatures. PER JENMALM, ANTONY W. GOODWIN, ROLAND S. JOHANSSON. Department of Physiology, Ume† University, S-901 87 Ume†, Sweden; and Department of Anatomy and Cell Biology, University of Melbourne, Parkville Victoria 3052, Australia.
APStracts 4:369N, 1997.
ABSTRACT
In previous investigations of the control of grasp stability, humans manipulated test objects with flat grasp surfaces. The surfaces of most objects that we handle in every-day activities, however, are curved. In the present study, we examined the influence of surface curvature on the fingertip forces used when humans lifted and held objects of various weights. Subjects grasped the test object between the thumb and the index finger. The matching pair of grasped surfaces were spherically curved with one of 6 different curvatures (concave with radius 20 or 40 mm; flat; convex with radius 20, 10 or 5 mm) and the object had one of 5 different weights ranging from 168 to 705 gm. The grip force used by subjects (force along the axis between the 2 grasped surfaces) increased with increasing weight of the object but was modified inconsistently and incompletely by surface curvature. Similarly, the duration and rate of force generation, when the grip and load forces increased isometrically in the load phase prior to object lift-off, were not influenced by surface curvature. In contrast, surface curvature did affect the minimum grip forces required to prevent frictional slips (the slip force). The slip force was smaller for larger curvatures (both concave and convex) than for flatter surfaces. Therefore, the force safety margin against slips (difference between the employed grip force and the slip force) was higher for the higher curvatures. We conclude that surface curvature has little influence on grip force regulation during this type of manipulation; the moderate changes in slip force resulting from changes in curvature are not fully compensated for by changes in grip force. 

Received 18 September 1997; accepted in final form 2 December 1997.
APS Manuscript Number J777-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 January 1998