Grip and manipulation forces are controlled independently in a coupled bimanual task.

Background: Grasping and manipulating objects requires humans to adapt both grip and manipulation forces. When handling an object with both hands, the additional degrees of freedom introduce more levels to the redundancy of the object manipulation since we can distribute the contribution of the grip and manipulation forces between hands.

Methods: In this study, we investigated the forces produced by both hands during coupled bimanual manipulation of a needle object in a virtual environment. The task objective was to puncture a virtual tissue, modeled as a linear spring, and stop immediately after, with the hands arranged in front and back positions in the movement direction.

Results: We show that during tissue interaction, grip forces are modulated consistently between front and back hands across participants, but manipulation forces are not. That is, the back hand consistently produced excessive grip force compared to the front hand regardless of hand configuration, while manipulation force distribution between the two hands was variable. After the tissue puncture, we again observed consistent grip force behavior during the reactive response to the force drop following the puncture. The grip force signal exhibited a consistent temporal profile in both the front and back hands with amplitude modulation according to the tissue stiffness in the front hand.

Conclusions: Overall, our results support the idea of distinct control mechanisms for grip and manipulation forces which rely on hand position rather than hand dominance.