Y N Turrell | PubMed Health

PubMed2001

Estimating the minimum grip force required when grasping objects under impulsive loading conditions.

As an aid to studying the efficiency of grip force scaling in the context of collisions, we present a simple cost-effective approach to estimating the slip ratio--that is, the minimum grip-to-load-force ratio needed to prevent object slippage. The grip apparatus comprises a sturdy load cell to measure grip force and two linear potentiometers to provide detailed description of finger movements. The slip ratio was estimated by plotting the magnitude of finger movement against the grip-to-load-force ratio at the time of impact. The slip ratio was dependent on the direction of loading, which stresses the importance of estimating slip ratios in a context similar to that of the experiment in which the efficiency of subjects' behavior is to be assessed.

Adult↗

PubMed1999

Grip force dynamics in the approach to a collision.

This experiment investigated the prediction of load force (LF) in impulsive collisions inferred from anticipatory adjustments of grip force (GF) used to stabilise 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 object to produce a collision by hitting the lower end of a pendulum, causing it to swing to one of three target angles, or (2) to hold the object still while receiving a collision produced by the experimenter releasing the pendulum 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 advance of the collision. In receiving the collision without advance information, subjects set GF levels to the mid-range of the experienced forces. When subjects possessed knowledge about the maximum angle of pendulum 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 forward models through a semantic route is not as powerful as when the updating is achieved through the more direct route of dynamic exploration.

Adolescent↗