Programmed and triggered actions to rapid load changes during precision grip

A test object (grip apparatus) was held at its upper part using a precision grip. Small balls were dropped into a target cup at the bottom of the apparatus. The grip force, the load force (vertical lifting force) and the vertical movement were measured. Electromyographic activity (e.m.g.) was recorded from four antagonist pairs of hand/arm muscles primarily influencing the grip force or the load force. The balls were dropped either by the subject during a bimanual task, or unexpectedly by the experimenter. When the subject dropped the ball, preparatory actions occurred before the rapid increase in the vertical load caused by the impact. These actions appeared ca. 150 ms prior to the impact and involved a grip force increase and a lifting movement of the grip apparatus. The e.m.g. activity increased in all eight of the hand and arm muscles, indicating a general stiffening of the hand/arm system prior to the impact. Furthermore, the preparatory actions were programmed adequately for the size of the load force step at the impact, i.e. an adequate safety margin to prevent slips was preserved during the critical period of the impact. Thus, variations in this step caused by changes in (i) the weight of ball, (ii) the weight of the grip apparatus and (iii) the length of the drop were adequately taken into account during the programming of these actions. In addition, the frictional condition between the skin and the grip surface was also taken into account. The relevant sensory information apparently was obtained during the handling of the ball and the grip apparatus prior to the drop. There were also task-related automatic muscle responses triggered by the impact. These responses, which also served to stiffen the hand/arm system, were most pronounced during unexpected load changes, but they appeared too late to prevent slips. However, if no overall slip occurred, the triggered responses were functional in the sense that they helped to quickly restore the safety margin and the vertical position of the object.