Grip-load force coupling: a general control strategy for transporting objects.

The authors examined the coupling of grip force and load force during point-to-point and cyclic arm movements while holding an object in a variety of grips, including 1- and 2-handed grips and "inverted" grips. In all grips, grip force is modulated in phase with fluctuations in load force that are induced by the arm movement. The tight temporal coupling between grip force and load force seen when moving an object held in a precision grip (J. R. Flanagan, J. Tresilian, & A. M. Wing, 1993) is observed in other grips. The control of precision grip force during whole-body jumping movements was also investigated. Grip force was modulated in phase with changes in load force induced by jumping even though the arm's joint angles were fixed. The tight temporal coupling between grip force and load force during object transport reflects a general control strategy that is not specific to any particular grip or mode of transport. Models of the coordination of grasp and transport in prehensile behaviors are discussed.