Grip force regulates hand impedance to optimize object stability in high impact loads

Anticipatory grip force adjustments are a prime example of the predictive nature of motor control. An object held in precision grip is stabilized by fine adjustments of the grip force against changes in tangential load force arising from inertia during acceleration and deceleration. When an object is subject to sudden impact loads, prediction becomes critical as the time available for sensory feedback is very short. Here, we investigated the control of grip force when participants performed a targeted tapping task with a hand-held object. During the initial transport phase of the movement, load force varied smoothly with acceleration. In contrast, in the collision, load forces sharply increased to very large values. In the transport phase, grip force and load force were coupled in phase, as expected. However, in the collision, grip force did not parallel load force. Rather, it exhibited a stereotyped profile with maximum ∼65 ms after peak load at contact. By using catch trials and a virtual environment, we demonstrate that this peak of grip force is pre-programmed. This observation is validated across experimental manipulations involving different target stiffness and directions of movement. We suggest that the central nervous system optimizes stability in object manipulation-as in catching-by regulating mechanical parameters including stiffness and damping through grip force. This study provides novel insights about how the brain coordinates grip force in manipulation involving an object interacting with the environment.