Stiffness as a control factor for object manipulation

We act on the world by producing forces that move objects. During manipulation, force is exerted with the expectation that an object will move in an intended manner. This prediction is a learned coordination between force and displacement. Mechanically, impedance is a way to describe this coordination. As an efficient control strategy, object interaction could be anticipated by setting impedance before the hand moves the object. We examined this possibility with a paradigm in which subjects moved a handle to a specific target position along a track. The handle was locked in place until the subject exerted enough force to cross a specific threshold; then the handle was abruptly released and could move along the track. We hypothesized that this ballistic-release task would encourage subjects to modify their arm impedance in anticipation of the upcoming movement. If we consider the handle as an object, this paradigm loosely approximates the uncertainty encountered at the end of a reach when contacting a fixed object. We found that one component of arm impedance, stiffness, varied in a way that matched the behavioral demands of the task and we were able to dissociate stiffness from changes in force and displacement. We also found separate components of muscle activity that corresponded to stiffness and to changes in force. Our results show that subjects used a robust and efficient strategy to coordinate force and displacement by modulating muscle activity in a way that was behaviorally relevant in the task.