Grip force dynamics in the approach to a collision

This experiment investigated the prediction of load force (LF) in impulsive collisions inferred from an- ticipatory adjustments of grip force (GF) used to stabi- lise a hand-held object. Subjects used a precision grip to hold the object between thumb and index finger of their right hand and used the arm either: (1) to move the ob- ject to produce a collision by hitting the lower end of a pendulum, causing it to swing to one of three target an- gles, or (2) to hold the object still while receiving a colli- sion produced by the experimenter releasing the pendu- lum from one of three angles. Visual feedback of the pendulum's trajectory was available in the production task only. In all conditions, subjects increased GF in ad- vance of the collision. In receiving the collision without advance information, subjects set GF levels to the mid- range of the experienced forces. When subjects pos- sessed knowledge about the maximum angle of pendu- lum swing - either because they were going to produce it or because they were verbally informed - magnitude of the anticipatory-GF magnitude response was scaled to the predicted LF magnitude. Furthermore, GF was scaled to LF with a higher gain when producing compared to receiving the collision. This suggests that updating for- ward models through a semantic route is not as powerful as when the updating is achieved through the more direct route of dynamic exploration.