Impact of the surface slipperiness of grasped objects on their subsequent acceleration

Seven subjects were asked to reach and grasp an object between the thumb an d index finger, lift it about 30 cm high and 25 cm forward from one table t o another, at their preferred speed. The perpendicular grip force and the t angential load force applied to the contact surface were digitized at 500 H z and stored on a laboratory computer. The trajectory of the wrist and of t he object was recorded using four infrared cameras tracking the movement of reflective markers attached to the distal styloid process of the radius an d on the top of the object. The aim of this study was to demonstrate the in fluence of low friction (i.e. surface slipperiness) on the acceleration of the wrist. Friction was reduced by coating the smooth brass grasping surfac e with talc. The seven subjects had skin to surface coefficients of frictio n which ranged from 0.52-1.18 for dry brass and 0.24-0.34 for talc-coated b rass. Two weights (418 and 1070 g) were used with each surface. The results indicated that with the slippery surface the necessary higher grip force/l oad force ratio was produced by an increase in the grip force and by a decr ease in the wrist acceleration and a consequent reduction in the load force . This strategy was observed for both weights over a range of grip strength s between 21-98% of the individual's maximum voluntary contraction (MVC). T his implies that even with adequate grip force reserves the reduction in ac celeration is an acceptable and probable alternative solution to the force control problem. Our results also suggested that the loading rate and the o bject acceleration were planned and controlled together which emphasizes th e role played by a predictive mechanism in organizing the kinematics of mov ements involving hand-held objects. This study shows that friction of the g rasping surface not only affects the prehensile force dynamics: but it also influences the kinematics of the entire upper limb. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.