Initiation and development of fingertip forces during whole-hand grasping

The present study examined the initiation of digit contact and fingertip fo rce development during whole-hand grasping. Sixteen healthy subjects graspe d an object instrumented with force transducers at each digit and lifted it 10 cm. The grip (normal) and load (tangential) forces and the position of the object were recorded. Twenty-five lifts were performed with various obj ect weights (300 g, 600 g, 900 g) and surface textures (sandpaper and rayon ). Despite the large number of degrees of freedom, grip initiation with an object using the whole hand was characterized by stereotypical contact patt erns, which are idiosyncratic to each subject across all object weights and textures. However, in spite of the initial asymmetric control, the forces were mainly synchronized by the occurrence of the peak grip and load force rates. The contribution of each digit to the total grip force decreased fro m radial to ulnar digits. The final force distribution was generally establ ished already at the onset of load forces. Only subtle adjustments were see n thereafter, suggesting a fairly fixed force distribution pattern througho ut the grasp. The findings suggest that, despite the large number of degree s of freedom in terms of contact initiation and force distribution in whole -hand grasping: (1) subjects employ preferred movement patterns to establis h object contact with their digits, and (2) synchronize the subsequent forc e develop, ment and temporal coordination of the task. Thus while the compl exity of the task requires control mechanisms beyond those seen in two-fing er precision grasping, there are strategies to simplify the complex task of the initiation and development of fingertip forces in whole-hand grasping.