CONTROL OF GRASP STABILITY WHEN HUMANS LIFT OBJECTS WITH DIFFERENT SURFACE CURVATURES

In previous investigations of the control of grasp stability, humans m anipulated test objects with flat grasp surfaces. The surfaces of most objects that we handle in everyday activities, however, are curved. I n the present study, we examined the influence of surface curvature on the fingertip forces used when humans lifted and held objects of vari ous weights. Subjects grasped the test object between the thumb and th e index finger. The matching pair of grasped surfaces were spherically curved with one of six different curvatures (concave with radius 20 o r 40 mm; flat; convex with radius 20, 10, or 5 mm) and the object had one of five different weights ranging from 168 to 705 g. The grip forc e used by subjects (force along the axis between the 2 grasped surface s) increased with increasing weight of the object but was modified inc onsistently and incompletely by surface curvature. Similarly, the dura tion and rate of force generation, when the grip and load forces incre ased isometrically in the load phase before object lift-off, were not influenced by surface curvature. In contrast, surface curvature did af fect the minimum grip forces required to prevent frictional slips (the slip force). The slip force was smaller for larger curvatures (both c oncave and convex) than for flatter surfaces. Therefore the force safe ty margin against slips (difference between the employed grip force an d the slip force) was higher for the higher curvatures. We conclude th at surface curvature has little influence on grip force regulation dur ing this type of manipulation; the moderate changes in slip force resu lting from changes in curvature are not fully compensated for by chang es in grip force.