How dependent are grip force and arm actions during holding an object?

Abstract

In everyday life, when manipulating objects, arm actions and precision grip force are tightly coupled by the central nervous system. To investigate the extent of this neural coupling, 11 subjects were asked to perform tasks that encourage either the coupling (Task 1) or the dissociation (Task 2 and 3) of grip force and arm actions. During Task 1, subjects held a grasping device, with an extra load suspended underneath by a string. Then, using the other hand, subjects were asked to lift or release the suspended load, while maintaining unaffected the posture of the grasping arm. During Task 2, while holding the device, subjects received similar instructions, but this time the extra load was suspended underneath the forearm. During Task 3, subjects were explicitly asked to modulate their grip force without moving the arm. In Task 1, grip force changed in parallel with, or slightly ahead of, changes in load, which is consistent with the view of a feedforward mechanism making grip force largely subordinate to ongoing arm actions. In Task 2, even though subjects had no obvious reasons to modulate their force (i.e. the load of the device was constant) they did so, with a number of features that resemble performance in Task 1. In Task 3, as expected, voluntary modulations in grip force had no effect on arm actions. It is concluded that the neural coupling between arm actions and grip force (1) can possibly lead to clumsy reactions, (2) depends on the focal action, and (3) is only unidirectional.