THE STABILITY OF PRECISION GRIP FORCES DURING CYCLIC ARM MOVEMENTS WITH A HAND-HELD LOAD

In this paper we examine the coordination of grip force and load durin g brisk cyclic arm movements with a hand-held object under a range of conditions. We show that, regardless of the surface texture of the obj ect or movement frequency, grip force is modulated in parallel with lo ad. Thus, the tight coupling between grip force and load observed in s hort-duration tasks such as lifting or point-to-point movements is als o seen in longer-duration cyclic movements. Moreover, the gain of the relation between grip force and load remains essentially constant over time. Across conditions, we find a dissociation between the gain of t he relation between grip force and load and the grip force offset. Wit h a more slippery surface texture both the gain and offset increase, w hereas increases in frequency lead to an increase in the offset but a decrease in gain. This suggests that these two parameters are under in dependent high-level control. We also observe that when subjects were instructed to maintain a high-baseline grip force during the movement, grip force was still modulated with load even though an increase in g rip was not necessary to prevent slip. This suggests that there is an obligatory coupling between grip force and load. This coupling might b e subserved by low-level mechanisms not under high-level control.