Fig. 1.

A, Instrumented object.1, Exchangeable grip surface for thumb (grasp with right hand); grip surface for finger hidden from view. Load cells in the object transduced the forces normal (grip) and vertical tangential (load) to the surfaces separately for the finger and thumb.2, Handle attached to exchangeable mass, not in subjects’ view. 3, Ultrasonic transmitter for vertical position sensing. 4, Rigid shrouds to avoid finger and thumb pad placement on far edges of the grip surfaces.B, Slip at the thumb during a single trial, as indicated from object acceleration, precipitous unloading at the thumb grip surface, and increased load at the finger. Notice the slip-triggered increase in grip force ∼100 msec after slip onset. The grip and load forces at the slipping surface on acceleration onset were used to estimate the inverse coefficient of friction. C, Examples of signals recorded during a single trial from a young adult and measurements taken for analysis. Interval a(preload) began on first evidence of digit contact with either grip surface and ended with the rise in load force. Intervalb (loading) began on the rise in load force and ended on vertical motion of the object. Interval c indicates the time from object set-down to the beginning of the precipitous fall in grip force that marked the onset of grasp release.